Strategies For Improving Drought Resistance In Grain Legumes

G. V. Subbarao, C. Johansen, A. E. Slinkard,' R. C. Nageswara Rao, N. P Saxena, and Y: S. Chauhan International Crops Research Institute for the Semi-And Troptcs (ICRISAT), ICRISAl Asia Center. Patancheru, 502 324, Andhra Pradesh, lnd~a

* A E Sllnkard. Crop Dc\elr~pmenr Crl~trr. I ln i~cr \ i l ) ill Si~\i.itrhcu,;tn. S;i\A,~t~ron. C;tnad.i S7K O W 0

Reterw: Dr. R. J. Lawn. Dlvislon 01 Tropical Crops and Pastures. CSIRO. 306 Carmody Road, St. Lucla Queenaland 4067, Australia

ABSTRACT: Thi, rcblcu d ~ $ t ~ l l \ recent ~nlt~rniati i~n OII d~~iught rc\~\t~ince character~\tici of graln legumrr u ~ t h a vlru tnu.ird de\clop~ng appropriate gcnetlc cnhanccment \tretegle* li,r uater-i~m~tcd enrlronmcnl\ Flrat. the p<~\\ihle adaptation\ that allow grain legumes io hrtter cope w~th drought itre<< ate \urnmartzed 11 Ir \uggc*ted Ihdl there ;!re ci~nr~derahlr gain\ to he mild? In lncreailng yield iirid )~eld \t;~hil~ty In en\lronmrliti chi~Ti~cter~/.ed hy tcini~ni~l drtrught itre\\ h) further explo~l~ng drcught cxapr. hy \horten~ng crop duration Many Iralt\ conferring dehydrat~on a\otdancr and drhydrat~on tt~iernncc itre ;~~a~l;~hle. hut ~ntcg~ltcd trailr. ehprervng at a higher lebel ~ r i< r rpn~ /s l~<m, are uggcrted IO hr morr u\elul in crop tmpro\enient program, Po\\~hle grnenc In~prirvcmenl \tralcplei are c~utl~ncd, rangtnp lrt~ti i clnp~ticai ~eiecl~iln ior yleld in drcrughted envlronmentr to a phy\ioli,gicai genetic approach. I t I* wgge*tcd thal ~n vieu of recent ad\ances In undcr\tand~ng drought rc\i\t;incc nicchanlama. the latter \tratcgy I \ hec(~ming more Ieavhle It Ir concluded that use of'th~r rcce~itly derived Lnouledge In a \yitemallc manner can lead to \~pnificant g;ltn\ In y~cid and y~cld \rahility of llic world', tnuj<lr grdn legume\, ar they are milinl) grout1 land n ~ i I conti~~uc lo he grown) under rain-fed ctlnditt~rn~

KEY WORDS: drought exape, dehydration a\rridancc, dchydrarlon tolerencc, ro111 trait*,

osmotic adjustment, lntegratcd trail\. \ymhli>l~c nllrugen fixatliln. drought weening. physioiagl- cal idct~type. hreedlng rtrategy.

L INTRODUCTION

The advantages o f including legumes ln cropping systems have long been recognized (Nutman, 1987). The prime advantage is their ability to fix atmospheric nitrogen and thus positively contribute to the nitrogen (N) bal- ance o f the cropping system. However, i t must be recognized that such contributions may be o f lesser significance for grain le-

gumes than for forage legumes hecause o f their high N harvest index (H I ) and often pc~or nodulation (Hoshikawa, 1991). Other positive effects of legumes come from their ability to break disease cycles, improve soil physical conditions, encourage myconhizae. and mobilize normally unavailable soil phos-

phorus sources (Hoshikawa. 1991). I n developing countries at least, the ever-

increasing demand for cereal grain mitigates

0735-2689195/%.50 0 I995 by CRC Press. Inc

against the use of grain legumes in better endowed agricultural lands and often rel- egates them to less favorable, usually rain- fed, environtnents (Saxena et al., 1993). Demand for grain legumes is increasing but economlc5 of production still do not en- courage their cultivation on the more pro- ductive soils (Saxena el al.. 1993) Many of the hiotic and abiotic $tre$te5 faced by grain legumes (Johansen et al.. 1994) contrlhute to the large yield gap between potentla1 yields and realized y~elds ar reflected in national production \tatistic\ (Table I ). Drought i$ the malor ahiotlc stre\\ it] many

pans of the world (Johansen et al.. 1994). Constraint analysis can attribute large yield and production lossec to moisture deficit. but hopefully these lossea can to some ex- tent he alleviated through appropriate re- search (Table I ). Improvements in grain legume productivity in drought environ- ment\ \hould he considered not only in terms of ~ncreased gram y~eld, hut also In im- pro\.ements In roil physical, chem~cal. and hiolog~cal factor\ as mentioned earlier. The\e improvement\ will further enhance the growth envirt~nment for non-legume\ in the cropping sy\tem.

TABLE 1 Potential and Realized Graln Yields; Global Production Losses due to Drought, and Recoverable Yield through Successful Drought Research Estimated for the ICRISAT Mandate Legumes

Potentla1 yleld (t ha ') Reg~onal mean y~elds (1 ha I ) '

Asla Afr~ca World

Reg~onal mean productlon ('000 I)* Asla Afrlca World

Global product~on (t) losses due to drought ('000 t)!

Production recoverable from drought through crop Improvement ('000 t)'

Chickpea Pigeonpea Groundnut

Dry gram yleld for chickpea and plgeonpea and dry pod y~eld for groundnut. Winter-sown chickpea In Syria, 198311984 (Singh, 1987). From three harvests of short-duratlonpigeonpea wlthin 217 d In 198211983 (Chauhan et al.. 1987). Irrigated crop in Zimbabwe (Meterler Kamp, 1967). Chickpea and groundnut data for 1991 fmm FA0 (1992) and pigeonpea data for 1992 courtesy of E. A. Kueneman, FAO. Rome. Calculeted according to the procedure followed for the ICRISAT Medium Term Plan 1994 to 1998, based on 1991 global productlon data for ch~ckpea and groundnut and 1992 data for pigeonpea. Losses estimated according to irrigation responses recorded in different regions and recoverable production estimated amording to genetic advance in drought environments predicted by incorporating known sources of drought resis- tance and by exploiting drought escape.

This review prov~des a status repon on our understanding of drought response In grain legumes and summarizes current re- search on the enhancement of t h e ~ r grtiwth and yielding ahi l~t) in ~ater- l imited envi- ronment\. We f t rua our attention on the grain legumes within our direct research experi- ence. namely. chickpea tCi1.r~ orirre~l~oni . lentil i k n s cul~nurr.\ J , plgeonpea r Cujlirr~rs c,u/urrJ, and groundnut (Ar(1il71s /t!p[~,qut,oi. In addttlon. we a l w draw on rele\ant ex- amples from the other gram legumes. mainl! soyhei~n lGlv(.ir~t, I I I ~ I \ J . coupea 1 \'igrtii ~rngnic~rlufui, black g u m ( \! r~trrngo), and munghean iV r(idrornJ Ne\erthcles\. we reter to othrr crop plants to present nur o\er- ail approach on \ariou\ a\pects related to drought reslrtance when relevant ex i~~np les from legume\ are not axailahle.

1t 15 recogn~zsd that pcnetlc ~n~prc i \ e - nient ir only one component of'an integrated approach tv s rah i l~z~np and ~ n t p r o ~ i n g crop production In drought envir~~nmcnt\ . Thii review k ~ c u s e i mainly on the phys~olr~fical mechan~sms that inlluence the pedr~rnmance of g r a ~ n legume, under moisture deficit. e \aluat~ng the \cope for thetr genetic mil- nipulation and dircugiing their relative im- portance in various production environments. Field screening methodologter and selection crltena based on yield and yield-derived in- dices are discussed in the context of genetic improvement strategies. The problem\ asio- ciated with y~eld-bared cr~teria and the phi- losophy hehind a trait-based approach are discussed to establish a more analytical ap- proach. We provide a conceptual framework for the integration of physiological mecha- nisms into genetic improvement programs for the development of drought-resistant grain legumes, based on a sound understanding of the biological defense mechanisms involved in adaptation to drought stress. We consider the strategies used previously to achieve progress in drought environments. propose lmprovementc, and attempt to assess the po- tential impacts of current research endeavors.

I I . DEFINING TARGET DROUGHT ENVIRONMENTS

For the purpose\ of this review, we de- fine drought stress In the agronomic sense. ~ I Z , a reduction in grain yield attributable to plant water defic~t . Grain legumes depen- dent on current ra~nfall are prone to interm~t- tent drought \tress during the vegetative or rcprr~duct~ve growth per~od: the crop'\ re- covery from the drought is determined hy whsequent ru~nfnll. Termtnal drought stress. which occura during the pod-filling phnse of crops, is common and a comrnon yield re- ducer for crops grciwing with current rainfall (Nagerwara Kao et al., 1Y85a.h) but is even rniire c r ~ t ~ c a l for crop\ prnwn dur~ng a post- ralny season and reliant on \tored roll Inolsture

Thur, a first \tep In designing strategies to alleviate drought stre.;\ 1s characteriza- tion of the drought pattern of the target cn\ironment. Thir itep ha\ often been ad- dressed ~nadcquately rn drought research program\. rna~nly because ol' the complex- ~ t y of the task. However, t h ~ i complexity has heen reduced in recent years uith the development of characterization tools such as $oil hater balance models (used a \ iuh- routines of crop growth models) and gen- graphrc inlirmation systeni5 (CIS) to assist in spatlal visualization of'the drought pnlb- lem. Varlahil~ty in \oil moisture defic~t must he considered ober years for the entire crop- ping iea\on. Thir knowledge permitv esti- mation of long-term crop losses due to drought \tress and the potential gains from alleviating drought stress through genetic and management options (Tahle I ).

Ill. ADAPTATIONS TO COPE WITH MOISTURE DEFICITS

Crop plants have evol\,ed various mecha- nisms to cope with the drought stresc pat- terns under which they naturally evolved or

were domesticated. Thus, landraces are well adapted to local environmental conditions and have evolved a range of morphological, phenological, and physiological mechanism to efficiently utilize the available prtduction environment (White. 3988; Ashraf and Karim, I09 I ). This local adaptation becomes a dominant force and presents a major dit'fi- culty in evaluating drought re\isknce of a wide range 111' genetic material at any one location (Whrte. IYKXJ. Al\o. wild relatiber may posres\ certain tralts that may he rel- evant 111 drought reststance mechanism\ of cultivated legumes (Parrons and Howe. 19x4: Castonguay and Markhart. 199 1 ). For in- stance, V unaurrulortr ssp. I IIIRUI~.U/U~~I 1s . well adapted ti) hot. semi-arid envircmments (Rachie and Roherts. 197.1). This subspecie6 prehurnahly originated In Africa and man> landraccs are Iound in the \emi-arrd and humrd ~ o n c of West Africa (Steele. 1976). Alw. \~ihspecrcr in the wlld, weedy. and alien gene p~xll\. such as V. irr~,qurr.irlufl~ rrp. d(~!iinrlriutru, are nattve ti1 the sabanna 01' Wert Afrlcit and Ethiopia, and can easil) he cro\ed with cultivated cowpca (Rowal, 1975: Stecle. 19761. P/I~I.IPo/N.\ 111 uf i /ol l i~, a wlld rclat~ve 01' P. \,ulanrir, has a higher osmotic ad jus t~nen~ ahillty than lhc cultivated hean (Parsons ;rnd Howe. 19x31. whlch could he tranjferrcd to the culti\,alt.d hei~n through ,

inter\peciflc hyhridilation. From an ecological pc~spective. \u r~ iba l

and perpetualton ofthe grnonlt' plays o d ~ m l - nant role in the adaptatton o f a given landrace to a region. Consequently, these landraces usually have low yield potential, despite thelr superrcir local adiiplati\in (Fischer and Maurer. 1978). Also, many of these landraces do not fit into present-day agriculture, where cropping systems and crop management prac- tices are radically different from the condi- tions under which they evolved or were do- mesticated. Thus. it is imponant from the cmp impmvement perspective to identify the specific physiological attributes conuibut- ing to their adaptability to the moisture-defi-

cit patternr of their native environment. This would assist breeders to selectively combine \ome of these physiological attributes into the high-yielding cultivars required for preent-day production systems (Rosielle and Hamblin. 1981).

This section evaluates adaptation5 that gram legumes have developed durlng the course of their evolution or domestication. their relevance in different drought \trer\ enbironmentr. and the scope for thelr favor- able genetic manipulation. We follow Lebitt's (1980) clesification of trarts relevant Lo coplng wrth druught, wlth dehydration avoid- ancc and tolerance considered as compo- nents of druught re\r\tance. Howe\,er, we al\tr recognize that tratts can he described at different levelr of plant organization and thus di5cusr ~ntegrated tralt5.

A. Drought Escape

1)roughl escape I \ a particularly inipor- tant 5trategy of matchrnp phenological de- ~c lopmcnt with the period of soil morsture avsilabil~t) 10 rninimr/e the ilnpact of drought \[re\\ on crnp productli~n in enbironmenth whcre the growing seasiln i?, ahon and termi- nal drought strrsr predomlnare\ (Turner. I9Xha.h). As an example. local cultibarj uf coupes f louer progreshivel) earller along a transect from south la north through the Sudanlan and Sahelran mnes of Africa to- ward the Sahara desert (Hall el a]., 1978). Flowering coinctdcs uith the average time of cessation of the rainy geason (Bunting and Curt~s. 1970). which is an adaptive charac- teristic. Plant hreeding programs should, therefore. aim at developing high-yielding genotypes with phenological patterns to match probable heasonal soil moisture avail- ability of a given environment (Fischer et a].. 1982).

Landraces of chickpea, pigeonpea, and groundnut growing in their natural environ- ments often face terminal drought stress, as

evidenced hq a yield increase if imgation is given during the reproductive phase. This suggests that. despite their evolution and selection rn specific cn\rronrnentr. the dura- tton to maturity of the\e landraces is too long in relation to the amount of rtored \oil moisture available (Singh and Suhha Reddy. 1986). For inrtance, newly hrrd \hurt-durn- tion (early maturing) genotype\ of ground- nut are generall) more succe\sful compared uith traditional long-duration genotypes In West African region9 characterized by \liort growing season\ (Vlrmanr and Singh. 1986). Seberal shon-duration gcnotbpes 0fleguiiie\ \how htgher and more stable yields than longer durat~on types tMcBlain and Humc. 1980: Hall and Gfiintr. 19x1. Hull and Patel. 1985: Rose el 81.. 19921. For most crop \pc- cies, breeding for shorter duration I \ a major objective. not only to match phenulogy 111 season length. hut also for other reasons such as to I'll crops1genot)pe Into more tntensive crop rt~tatic~ni.

For the ICRISAT mandate legumes. con- rtdcrahle profre$\ ha\ heen made in shorten- ing crop duration without unduly pendli~ing yreld potential (Table 2). Sinirlar progrerk has been repofled in developing shcirt-dura- lion cowpea cu l t i~a r r that escape terminal droupht. but have yreld potent~als colnpa- rable %ith long-duration local cultrvars (Hull and Patel. 1985). There ir rcope for more ludiciour matching of genotype durat~on %ith n~os t probable soil moisture pattern using soil moisture balance models (Ritchie. 1985) in asstxiation u ~ r h crop-weather modeltng and GIS technology. A multilocation field testing program is costly and thu\ better preselection of genotypes to test. based on their fit to the probable soil moirture envi- ronnient, should aid overall efficiency of the process. The more predictable the environ- ment, the better the growth duration can be optimized.

However. various penaltier are a s s t ~ i - ated with reducing crop duration to better fit likely soil moisture patterns. Primarily, ear-

Itness ultimately reduces the potential yield of the crop by reducing dr) matter at anthe- sis and the nuniher of sites for po.;tanthests gram filling (Fischer. 1979). To some ex- tent, [hi.; can he overcome hy increasing the plnnt density. which is indeed a common pri~ctice where \horter duration genotypes are used. The degree of earliner\ required is generally a cotnpromtse hetween develop- ment ~I'rufficicnt hionin\\ without reducing \oil u;ltcr 10 it ]?\el that will limit reproduc- tikc gnihth (Fircher. 1979). Simulation stud- icy in snyhe;tn have shown that curly geno- types yield more than thore nlaturi~ig later if late dnlughr rcducer the yield of later t~iatur- ing cultivnr\ hy at Icn\t 40'1 (Muchow and Sincla~r. I'lX6). It i \ \nmetimeh oh\erved that c;trIy mnturrng culti\,arr have rhallow root \).\tern$ (e.g . w e Ferercs et :!I.. IYX6: Arihara et al.. 1991 ). T h ~ s renders such cul- tibars more '~u\ceptthle to intermittent dry \pell\ i f ' grcwn as J ritiny-\ca\r,n crop. tn additton 11, a reduction in yicld pr~tcntial due to reduced water u w (Fcrerer el al.. 1986). Huwcver, genotypic d~fference\ In rooting depth hale heen reported in a nuniher of' Iegumer withln a given duralton, and thur thi9 trait could he improved if necessary (see Section 1II.H.I on ruot aur~hutc\) . Also. the u\e c ~ f carly maturity a\ an ercape strategy ir limtted rn some cnvirontnents. such a for chickpea and lentil In Mediterranean envi- ronments, where too early llower~ng could expose the crop LO low temperature and frost damage.

6. Dehydration Avoidance

Crop species have evolved several mechanisms to mainvain plant water status within reasonable limits for normal meta- bolic functioning under limited water supply or when evaporative demand of the atmo- sphere become excessive. These can be broadly divided into two groups: maximiz- ing water uptake through improvrng the ca-

pacity of the roiit system to acquire water. and optimization of the uae of absorbed hater for the production of dry matter.

1. Root Attributes

Root \ize, marpholog). depth. length. demit). hqdraulic conductance. and func- tion are basic to meet the transpirotiunal demands ofthe shoot (Pas.ioura. 1982). For maximizing extraction of moisture iron1 thc so11 the requirements ore ( I I deep penetro- t10n of roots: ( ? I adequate root densit) through the \oil profile; and (31 adequ'ue longirudinai inductance in main roots (F~rchcr et al.. 19X21. The hater uptake pat- tern acre.\ the depth of rooting zone 1s nut untti~rnl. In general. nsarlq 10'3 of the total ~ a t e r uptahe occurs 11\er the flrst one i'ourth of the rciot rone. 30% o \e r thc second. 20'1 w e r the third, and I(lrk o\er the last fourth of the total rooting depth (Dtrorenhor and Pru~tt. 1975: Nngesuara Rar~ and Wright. 19911 Crop p lan t often tnaintaln higher root length densities than are rcquired In the sur- face layer: ( I I to facilitate rapid uptake of recent r a ~ n before 11 c\ilporate\. I ? ) to pro- \ ~ d e reserte capacit) in case of disea<e (ir pest damage: (31 to extract relatively imni(~- hile nutrients l ~ h e P. and (3) to compete ui th ueeds or other ne~ghhoring plilntr for both uater and nutrient\ (Pa\ ioura, 1983). De- pending on the target environment. certain root trait\ may be more mportant than 11th- ers and targeting of genetlc ~niprovement depends on the type of trait\ required rela- tlve to thme present in current cultlvar\.

a. Rooting Depth

In the rain-fed environments, the depth of rooting and the ability to hustain an unin- tempted supply of water are imponant fac- tors (Gregory, 1988). Even though terminal drought stress is common for many postrainy

\enson legumes. crops are not necessarily l ~ m ~ t e d by a deficiency of stored sc1;1 mois- ture, hut hy an inability o f the crop elther to fillly extract water stored deep in the profile tir extract It fast enough 101 yicld formation (Jordan et al.. 1'4831. Thus. inclusion ofdeep- rooted lilies in a breeding program for area\ uhere \uhatantial amount\ of uatcr arc' left In the subsoil at maturity seems Ju\tilird (Gupta. 1992).

The g r o ~ t h of rcl~ts Inlti deeper soil lay- er. ill :I funct1011 of hoth getlotype and envi- ronment: the interaction between the two often rn;thes It difficult to di\t~nguish geno- thplc difference\ in root growth (Gulman and Tornel-. 197Xl. .41so. the growth dura- tion cifthe genotype (long vs. vhmt duratitrnl affects nxrt length, density. and rooting he- havior In general. For exalnple, in pipeonpea. \hart-duration gencitypcs often extend roots only to about 5 0 cm depth compared wlth long-duration genotypes whose roots can extend tcr nt least? m depth (Chauhan. 19931. Genc~typic vanation in rootlng depth has been reported in st.tSel-:ll legume\ t Kaspar el a].. 1984: White and Castillo. I Y X X I .

Deep rooting wa. po\itively cc~rrelated w ~ t h wed y~e ld , crop grr~h'th, cooler canopy temperdturr, and so11 hater extraction in hean (Spr~nchiado el al.. 10891 Drought-tolerant bean genotypes ctruld extend their root. 111

1 .2 m depth in drought enviriinment\. whereas thc wnj~ t ive genotype\ c(~uld not extend the~r roots beyond 0.8 m, and these difference\ In rooting depths were rellected In overall shoot growth and yield (White and Castillo. 1988). In groundnut. sub\tantiai genotyptc variat~on in rooting depth, root volume. and water extraction pattern at dif- ferent deptha has been reported (Ketring. 1984; Mathews el al.. IYXRa; Wright et al., 1991; Chapman et al.. 1993a). In soybean, differences in drought resistance were asso- c~ated with rooting depth (Cortes and Sinclair, 1986). Wild relatives in many legumes po\- sess deep rooting capability that could be

tramferred to cult~vated legumes. A number of Phaseo1u.s species, such as P. ucurifoliu.\. P. rete1isi.5. and P. cocclnrus. have deep and tuberous primary root attribute\ (Singh and Wh~te , IYXX).

b. Root Length Density

Ho~it lcngth dens~t) (Lb. in cm c W ' J usually dec r rae r cxpiinent~ally w ~ t h depth (Wiehc. IYXO). In many crop\. Lv I \ mtirc than sulficient to extract all ava~lahle water In the rurlace laqcrr it'arr~oura, IYX.7). How- ever. at deeper i~cyerr, hclow 0.3 m. Lv may not he rulTicient to deplete availahie r(111 ntolsture. A plant that achieve\ L\ = 0.5 rhould he ahle 111 rapidly eutrdct water w~th- out any difliculty (Pd\r~oura. 198.1). Although root\ in thc rurlace roil may extract water hclow [he Iowcr limit of ava~lahility of 1.5 MPn, root\ In the deeper ( 1 1 1 often tail 10

extract w\.;lter 111 [hi\ limit tHurd. 1974; Jor. den arid Millcr. IYXO). Thus. 11 cdn he argurd that it ' the plant could di\trihute its roots r110rc unifornllq throughciut the root ~ r ine . it could hettel- Illeel itr unter needr at \arn)us stager ol'growth without inbert~ng addit~onal dry matter 111 r(lotr.

In Inan) gram Icgunlcr. l.\ ir liruer (0.13 to 0.70 cm cm '1 th:~n in cereal. idround 2.4 cnl cni '1 (Gregory. I Y X X ) . Genotypic dif- ferences In L\ h i ~ w hcen reported in many legumes. includ~ng Ihha bean (Lcn)Ler. 19781. chickpea (Brown et al.. IYXY), groundnut (Wr~gh t el 31.. 1YY41. pea ( B h a r a d ~ a j et al.. 197 1 ). cowpea (Bahltlola. IYXOI. and lentil (IC'ARDA, lYX5). In 111arly legumes. Lb con- tinues to increase even after anthesis (Kospar et al.. 1Y7X). whlch Ir in contrast to cereals where Lv stabilizes or decline.; after anthesib (Mengel and Barber. 1974). However. not all legume species continue active root growth during podfilling. Abilnilate label- ing studies and water extraction pattern5 suggest that root acti\'ity in boyhean often

decl ines during podfilling (Hume and Criswell. 1973). Nevenheless. in perennial legume5 such as pplgeonpea where a range of phenological plasticity i i.e.. degree of inde- terminateness) exists among vaneties and genutypes, roots could be continuousl) ac- tive in genotypes that are more indetermi- nate in nature (Sheldrake and Narayanan. 19791. The difference< between legumes and cereals in their root growth pattern offer at least partial complementarity in exploiting $011 resources in intercropping rystem\ (Reddy and Willey. 1981 ).

The type of root distribution required for a crop species depends on the target environ- ment. In environment\ where the crop is grown on stored soil moisture. high Lv in the rurface layer\ ir not required. In this case. efl'on\ rhould be directed toward increasing Lv at depth. On the other hand. if the crop is targeted for an enbtronment where rainfall occurs in \hart spells during the growing season, thcn high L\ In the surface layers l<0.5 m depth) is nd\,;~ntageous,

c, Root Hydraulic Conductivity

A decrcaie in root hydraulic cr~nductiv- itq could help in conser\ing qoil moisture early In the season. \r> that it is available for grain filling (Passioura. 1972). The induc- tlon of a Iargc hydraulic resi~tancc within the plant (Passioura. 1983) should be benefi- clal In enbironmcnts where grain yield de- pends on the amount of available water left In the coil at the onset ol'flowering (Passioura, 1972: Turk and Hall. I9ROa.b). This should improve and stabiltze yields where crops are raised with stored sail moisture by increas- ing the proportion of water used after the onset of flowering. By selecting for smaller metaxylem vessel diameters in the seminal roots, Richards and Passioura (1981a.b) de- veloped wheat genotypes that could use water more slowly in early growth stages. How-

ever. thls screening method may not he ap- plicable for legumes where secondary growth in the root increases the xylem tissue (Gupta. 1992).

d. Scope for Genetic Improvement

Deep rootlng and increased root length density involve a substantial investment in carbon and maintenance cost\. In sorghunl. an increase of Lv from I to 2 cm cm '. require5 partitioning an additional X(I0 kg ha-' dry matter to the root cyqtem (Jordan and Miller. 1980). Passlourd 11983) argued that the dry matter galn associated with the increased water supply will i i f fet the dry matter investment into new roots. Simuln- tion \ tud~es In sorghum indicated that ylelds of deeper rooted genotypes were at least 20L?r more than ciintrol genotype\ in 1 out iii' cver!, 3 to 5 years across Iiication\ (Jordan et al.. 1983).

Screening and selection for rooting depth on a large scale is expenrive and laborious (Blum. 1988). Thuq, this type of evaluation is normally restricted to a few prinnislng \elected germplasm lines or cultibars and in chooring potential parents in a hreeding pro- gram (Flrcher el al.. 1982). One method that permits ebaluation of a reasonable number of germplasm liner wa\ developed by Robertson et al. (1985). A selective herbi- cide is introduced at an appropriate depth and, as soon as the roots of an entry reach the herbicide, the plant develops toxiclty symp- toms. Thur, its use would be limited to the screening of lines or hybrids that are geneti- cally reproducible, Identification of nonde- structive herbicides would make it possible to apply this technique to evaluate segregat- ing populations (Khalfaoui and Havard. 1993). This methodology is still being de- veloped and requires standardization for any particular legume species before it can be used routinely for screening germplasm lines

(Hall and Patel. 1985: Khalfaciui and Havard. 199.7). Root pulling resistance has been sug- gested for characterizing root growth and has been used \uccessfully in rice hreeding programs at International Rice Research In- stitute (IRRI) (O'Toole and Soeniartono. 1981: Ekanayaka el al.. IY85). Amiponicr gives a coarse estimate 01' polenli:~l rrioting depth. hut i t also is a difficult techntque. Tensiometerr have been used lo determine rooting depth (F~rcher et al.. 1982). Root effect~keners can also be quantified hy mea- wring the apparent sap velocity (ASV), and culti\ar biui:inon in ASV ha.: been reportctl In groundnut (Ketring. 1986).

Genotyp~c \ariatl(in f r ~ r root attr~hute\ has heen reported for ihba bean (Looker. 1978: ICARDA. 1984). chickpea (Nagara-

jarao et al.. 1980: Brown et al.. IYX~I), pea (Bharadwaj el al.. 1971 1. lentil (ICARDA. 1985), groundnut (Nageswnra Rao and Wright, 1994: Wnght el al.. 1994). and al- fall'a lM~dil.(ifio \~irii,(il (Barnes, 1983). De- \pile adequate information on genetic vari- ability, the u\e 01' rnot t r a ~ t s In crop Itnprovement programs ir only beginn~ng. In chickpea, a drought-re~istant genotype (ICC 4958) had 304 h~gher root dry weight than the standard control cultibar 'Annigeri'. which is relatively more r enh i t~ve to drought stress (Saxena et al., 1994). Effort\ are underway to cotnhinc thir root trait w ~ t h the adaptlve and h~gh-yielding trait\ of "Annigeri". Currently, a number of lines wlth ICC 4958 root phenotype and "Annigeri" shoot phent~type are being tested and some are uhowing promise of higher yleld~ng abil- ity in drought environmentr (Legumes Pro- gram. 1992). In durum wheat iTririr.urn du- rutn), the deep and extensive root system attributes were combined with the agronomi- cally superior but poor root system traits of cultivars such as Wascana and Wakooma. which led to the development of cultivars with root systems better adapted to drought conditions coupled with desirable agronomic

attributes IHurd et al., 1972. 1973). This demonstrates that root traits are amenahle to genetic manipulat~on through normal hreed- ing methods, provided suitahle parents are idencified and appropriate envlronmentr are urcd for exprersion 111' the trait.

It ir ~rnportant 10 evaluate the su~tahility rlt the screening syrtems for germplasm cwIuat i(~n 01 root tralts a\ the so11 environ- ment plays a tniijor role in expres\ion o f r tn~ t attributes. Genetic variability in root charac- terirtlc\ 01 sorghum grown In rrllution cul- ture iJordan et nl., 1979) ls not expressed t~ the rame degree In thu field during droughl tJordan and Miller. I Y X I I I A reordering of \aflluwcr i('(rrfhi~i~lu\ I I I I ~ Ir~rrir.$) penotypcr tor 1.v occurred when the growth medium changed l'r~im sand 111 cl;ry (Harngan and Ijarrr. IYXJ) These I L . ~ cxalnple\ imply that the sui tah~l~ty of the model \y\tem to the I'ield situatic~n mu\[ he demonstrated heforc larfc-scale e\aluat~on 01 perrnplarm lor root attribute\ ir undertaken

2. Shoot Attributes

A r~unihcr of shoo^ :~!trihute< play 1111- pon~int role\ in regulaung walcr ubc of crop pltlnlx when grown under m<riaturc-lim~tlny en\ ~ rw~mcnts . Thcsc liiclude de\elopinent. structurc, and surface propertier of [lie u;~iiopy. ahilitq lo aQu\t the leafnrca accord- ing 111 mois~ure availahil~l), and functional attributes \uch as osmotic adjustment.

a. Canopy Structure

Canopy rtructure is determined h! leaf size. leaf shape. leaf surface characteristics and retlectance propertlea, leaf angle. and the geometrical arrangement of leaves in the canopy. These traits determine the light ex- tinction coefficient (k) and radiation use ef- ficiency (RUE) of the crop, and canopy struc- lure plays an imp~rtant role in controlling

water loss from the canopy. Under field con- ditions, the boundary layer over crop cano- pies causer gas exchange to be less depen- dent on stomatal conductance, and thus can influence transpiration efficiency (TE) (Jawis and McNaughton. 1986). Boundary layer resistance is a function of the thickness of the unrtirred air boundary layer adjacent to the leaf , which depends on leaf s ize (Parkhurst and Loucks. 1972) and canopy architecture. The aerodynamic resistance of the cnntipy determines the relat~ve impor- tance of rtomatal conductance to TE. If the canopy resimnce to heat and water vapor dtftusion is large, an increare in stomatal conductance ig,) would tend to cool and h u m ~ d i l j the alr In the houndar) layer, thus lower~ng the leaf-air bapor pressure deficit Ivpd) and increasing TE (Farquhar et al.. 1989: Read et al.. 1991).

With a closed canopy. roler radiation is attenuated downward wirh cumulative leaf area indcx (LA11 In accordance with Beer's law. with a I. characterist~c uf the canopy (Law11. 1989). Crop cultivar\ with more erecl dnd n a m ~ w Iea\er. and lower k values, and hence hlghet crltlcal leaf u e a index (CLAI - leal' area indcx that intercepts 95% of the tnccimlng solar radiation), generally have higher crup growth rate5 (Duncan. 1971). The advantage of narrow. vertically oriented leaves with a resultant higher CLAI and RUE has heen well demonstrated in groundnut using mutants with variable leaf size and shape (Nageswara Rao. 1992). The narrow leaf mutant. TMV 2-NLM. has CLAI values around 5 to 6 compared with its parental line TMV 2. with CLAI values around 2 to 3. although in horh cases LA1 reaches 5 at the onset of flowering. The RUE of the mutant is consistently higher than that of the parent (Napesward Rao. 1992). Also. narrow leaves are considered to be an adaptive trait to stress conditions (Blum. 1980). In addition to this, because of aerodynamic implications, nar- row leaves are usually stressed less (Gates, 1968).

Many legumes produce leafarea heyond CLAI. which results in inefficient use of water and radiation for dry matter produc- tion and yield. For example. in gniundnut. the LA1 often reaches >6. although the CLAI IS only about 2 to 3 (Wil l~ams et a].. 1986. Nagesuara Hao. 1992) Leai lo\ses of up to 50% can be tolerated hy many legunirs, in- cluding pigeonpea and groundnut. wlthout any major effect on jleld tfroni our ohrerka- tir~nsl. For pigeonpea genotypes growing vegetatively durlng the rainy heason. large \,egetatl\e growth \\as fiiund to he Ic\s hen- eflcial on a \oil (if low miil\ture \torags cap;icity tsheldrake and Narnyanan. ly7Y1 Thus. genotypes 01 legumes that are conser- vatike In .leaf arc'l de\elopment h q o n d CLAI. and also have namih Ica\cs that per- mit hettcr ut1li7atlnn (if r ; ~ d ~ a t ~ o n and water. should he ad\antagcous in water-li~nitcd eni'lronmenth.

b. Leaf Movements

Once leal area development I con~p lc~ i , an ~rnportant mechanim hy which Iegutne~ can adapt to drought a t r e \ I \ thriiugh chdngcs in leal' angle (Begg, IYKOJ. Leatlctr oricnt perpendicular to Incident light in the ah- hence of a hater deficit but parallel to it dunng water deficits (Squire. 1990). Thl\ can efttcuvcly reduce the radiation load on waler-\tre\sed leavec when less water IS

available to dissipate the energy as latent heat, thus minimlzlng heat damage ( L u d l o ~ and Bjorkman. 1984: Forseth and Teramuru. 1986). Advantages in ability to change leaf angle or orientation parah heliotropism^ are reversibility, rapidity of recovery on the re- lief of water deficit, and a minimum reduc- tion in yield during water deficit (Shackel and Hall, 1979). The leaf orientation will allow maximum radiation interception when evaporative demand is low and WUE is high (Turner. 1982; Muchow, 1985b). Almost all legumes show a paraheliotropism response

to radiation and water deficit. howeier. the degree of movement can vary among le- gume\ or among genotypes of a given le- gume species (Lawn. I982a: Ludlow and Bjorkman. 19x4: Muchow. I9XShI. Geno- typic differences in p;~raheliotropism exist In rome legumes. Including grr iundnu~ [Lawn. IYXZo: L u d l ~ w and B,jorkman. 1984: Mathews cl nl . 108Kh) and hean (Wicn and Wallace. 1973, Sato and Gotoh. 197'11. How- ekes. undcr \cverc molsrure deficlt enclron- rnent\, ths contrihut~rin (11 paraheliotropism to genotyprc pcrl im~imce would he limited ~hI , i theh\ el ;!I . IYXXh).

c. Leaf Surface Characteristics

A rn~ooth le;~f IS I~hcly to Io\u morc hater than a crinkled leal, whlch tends to crcatc m a l l pochets oi still arr (Hosenhur~. I C ) i X I . Thc increased pubescence and wax+ nc\r (ihser\ed under stre\+ In home ipecie\, ~ncluding Icgume\, increa\e+ Icilfrellectance and rcducei water lor\ (Ehleringer. 1980). Siryhean 11nck with den\e puhcrcence habe higher TE (Baldoch~ et ;]I . 198.5). Leal pu- hercence may have an adaptive value in wilter-limited en\lronnient\ as the hairs ill-

lo\* thc leaf to fix morc carhon, to avold potentially lethal high leal temperature\, and Iorc lew water daily, whlch allows the plant to extend growth fiir a longer period into the drought (Ehleringcr. 1980).

d. Stomata1 and Cuticular Characteristics

By adjusting their apertures. stomata achieve the best comprcimise between the requirement for COZ and the need to con- s e n t 'water (Berninger and Hari. 1993). Sto- mata play a major role in regulating water loss ro as to match the evapotranspirat~onal demand to the water-supplying capacity of the roots, but this comes into operation when

the fract~on of transp~rable water falls to 0.3 or below (Sinclair and Ludlow, 1986). This is essential in mainlaining Internal plant water status above a critical threshold level, thus contributing to dehydration avo~dance 5trat- egies (Turner et al.. 1984). Genetic variabil- ity for stc~matal character~stics. such as sto- matiil dens~ty, aperture size, sensiuvity to changes in internal and external water sta- tus, h a heen \h(iwn ( C ~ h a and Brun, 1975: Tanzarella et al.. 1984: Markhan. 198.5). The heritability f i r stomatal character~stics I S

hlgh. ~nd ica t~ng the feasibility ofgenet~cally ~~ lan~pu la t ing thl\ trait (Jones. 1979: butte^ eta] . . 19'43). The role ol abwisic a c ~ d (ABA) In regulating stomatal function and improv- ing TE has been highlighted recently (Man\ficld and Da\ie\. 1983; Hartung and Dalies. 19'4 1 ) . Genetic variation in the ca- paclt) t~ accumulale ABA exists In many legume specles ( b e el al.. 19x3; Same1 el a].. lYX41, lnduclble traits such ;I\ osmotic ad,justmcnt (see Section 111.8.3 for further d ~ s c u \ s ~ o n ) could Icnd to stomata1 adjuat- tnent Ip;~nial opening) (Turner and Jones. I~XOI. Legumes thal underg(1 liltle osmotic ndjust~nent during water defic~t. such ;is cow- pea and siratru (Mor rr~l~rrlrrrr~i orrol~rrr. p ~ ~ r o u r n i , effectively cltiw their stomata to avoid dehydration (Shackel and Hall. IYK3: Ludlow el al.. 1'485: Muchow. 1985c). In confr;1sr, crops such as pigeonpen and my- hean. whcre osn~otic adjustment occurs. per- mit stomatal adjustment unlll a critlcal inter- inal water slatus is reached (Lawn. 1982a: Flower and Ludlow. I9Xhl. Thur, differences In stomatal aperture reflect the inherent dif- ferences In metabolic strntepieb adopted by crop plants to regulate water loss. Therefore. stomatal size may not he directly amenable to genetic manipulation. rather it could be so indirectly through changing the efficienc~es in metabolic strategies. Also. stomatal size in general is functionally related to TE; thus. it may he possible to select for optimum stomatal aperture size by selecting for higher TE (see Section III.D.4 for details).

Morphological features, such as a th~ck cuticle or wax deposits on the leaf surface can reduce evaporat~onal water losses from the leaf surface and thus minimize residual transp~ration rate (Jefferson et al.. 1989). Genotypes with lower residual transpiration rates usually have s functional advantage durlng moiaure-limiting environments as this leads to efficient water use (Walker and Miller. 1986: Paje et al.. 1988). Genotypic variation In residual transpiration has been reponed in soybean (Paje et al.. 1988) and cowpea (Walker and M~ller. 19861.

3. Osmotic Adjustment

Osmat~c adjustment (OAI can he defined as the actlve accumulation of solutes wirhln the plant tlsvue (either in roots or shoot\) in response to a lowering of soil water potential ( W P ~ I (Morgan. 19X3). This could lead to lowering of osmotic potential tOP). w h ~ c h provides the dnvlng force for extracting water from i11u W P ~ . Osmotic adjustment can play a mqor role in determining the drought re- s~stance of n given genotype by: i I ) main- taining turgor ober lluctuating ioil water potentlalv. ( 2 ) mainlaining slomatal conduc- tance and thus photosynthesis. (3) maintain- ing groarh. (4 ) increasing dehydration toler- ance, and ( 5 ) increasing the extraction 01' soil water (Turner and Jones. 1980; Wright el at.. 1983: Flower and Ludlow. 1986: Ludloa. 1987).

A wide variety of organic solutes accu- mulate in plant tissues during water and salt stress and contribute to OA (Gorham et al.. 198.5). The chemical nature of compatible solutes varies from one taxonomic group to another, but most are primarily organic con- stituents. particularly amino acids. organic acids. sugars. and derivatives of polyols or nitrogen dipoles (Meyer and Boyer, 1981). Also, inorganic ions accumulated from the soil can contribute significantly to the OA: K'. and to a lesser extent NO: and C1-, can

accumulate t o osmotical ly signiftcant amounts (Morgan. 1992). The relative con- trihution of organic and tnorganic solutes to OA varies among crop species. For example. in cotton. sorghum. and soybean. organlc solutes play a major role in OA, whereas In runtlower. inorganic ions contribute a major \hare to OA (Jones. 1980). Reduction in d u t e potenttal tSP) can alwi occur through changes in the turgtd weighudry weight ra- tio (TWIDW), reducing the o\rnotic \olume without accumulattng addtt~onal riilute\ iLudlow. IYXOa).

OA a l l ~ ~ w s the plant tn nia~ntain gradi- ents for water tlow and to extract water from the s o ~ l at lower WP,. while si~nultaneou\ly Inatntatnlng turgor IMorgan. l992i. Also. OA I \ a\sociated w ~ t h \ t i~nulat~on of root growth in pea (Grexcen and Oh. 1972) and cereal\. For example, wheat genotype\ w ~ t h low OA can not u\e water hclow a depth ril'

0.77 m, wherea\ those u ~ t h high OA extract water toadepth (it 1.5 ni. ind~cating a greater roiit~ng depth and more root growth due 1 0

OA (Morgan and Condon. IYXb) Bec;~ure OA allows more water tci he extracted e ~ t l i e ~ hy improvlng water extract~on efficiency or improving the mot growth hy p r o v ~ d ~ n g ad- ditional carhon (Morgan and Condon. 1086). it helps in maintaining atomatal crinductance and photo\ynthe\is (Boyer. 1976). OA per- mits stomata1 adjustment and crintinued tran- splratlon and photorynthe\is under low wa- ter potential\ in wheat and sorghum (Ludlow, 1980a) This may lead to continued fixation of carbon and thus plant growth over a longer peritxf compared to genotypes that do not adjust osmotically (Ludlow. 19XOal. How- ever. in some cases. 0.4 is m,t associated with tmproved root growth. If the additional water available to the crop by lowering the leaf WP ( W P , ) a few bars is not large enough to sustain transpiration for more than a few days, the impact on drought resistance will be limited (Jordan and Miller. 1980). Also. the Impact of this additional carbon in deter- mining yield depends on the growth stage at

which the crop experience\ the water deficit. If water deficit occur5 during the heading or gratn-filling stage. the addttiiinal carhon avatlahle due to OA may play a crucial role in preventing .;p~helet hterility, and in in- creasing grain sel and secd-f~lling. ~ h u s im- proi'tng HI (Bingham. 1966. Morgan. 1980: Pierce and Raschke. 1980).

Osmotic adjusrtnent IS postt~vely corre- lated with y~e ld under drought environments In u heat (Morgan ct al.. 1986). barley (Blum, I989), and sorghum (Morgan, 1084). In- creil\es In grain ytcld (it as much ;I\ 50 to 60% habe heen attrihutcd to OA in wheat (Morgan. 1083) In some c;i\cs. even a dou- hling iti yield ha\ heen repnrted (Boyer. l0X?). H o w c ~ c r , other report\ tndicate no rel:~t~r)nrhip hctween OA and growth or yield under Geld conditions (Shackel and Hall. 10x3: Munns. I Y X X : Blum ct al.. I9X0). Al\o. in \ome case\. tmproved OA rcrulted In smaller cell size (Ackerson. 1981). and thus \mall organs and \mall plant\. Thu smaller m e (if plant organr. ar either source or .;ink. can re\ult tn lower pritential yteld (Hlum. 19881. A negarlve correlation hetwecn OA and yield also has heen reported (Grumet et a]., 1987).

The increase in siiluter that occurs with a reduction in W P , eventually redchts a limit, and this wries among crop species (Turner and Jones. 1980). The ecological hahitat [if the genetic mater~als, the gniwth stage, and growth conditions can influence the degree of OA (Morgan. 198.1; Blum and Sull~van. 1986: Gtrma and Krieg, 1992). Significant genotypic var~ation has hcen reported in a number of legume crop\ (Table 3). Al\o. related wild spectes in legumes may posress high OA, but this has not been evaluated systematically. For example, the related spe- cies P. ucur~fi,liu:, had a higher range of OA compared with cultivated bean (Singh and White, 1988). More studies are required with germplasm lines and related species collected across wide peographtcal areas to examine the range of OA under moisture deficit.

TABLE 3 Values and Ranges of Osmotic Adjustment (OA) in Grain Legumes

Legume specles OA (MPa) Ref.

OA in shools Soybeans -0 3 to -1 Muchow. 1985c Cones and Slnclair. 1986. 1987.

Oosterhu~s and Wullschleger. 1988 Pigeonpea -0 12 lo -1.28 Muchow, 1985c: Flower. 1985; Flower and Ludlow.

Groundnut I 987: Lopez et a1 . 1987

-0.3 to -1 58 Bennett e l al., 1981. 1984. Erickson and Ketring. 1985. Black et al.. 1985. St~rlina et al.. 1989. Ketring, 1986

Greengram -0.2 to 0.44 Zhao et al , 1985: Muchow, 1985c (V radrara)

Black gram -0 05 to 0 5 Muchow 1'985c Sinclair and Ludlow 1986 Ashraf IV unau~culata) and Karlm 1991

cowpea 0 to -040 Shackel and Hall 1983 Muchow 1985c Sinclalr (V unguculata) and Ludlow 1986 Lopez el al 1987

Lablab beans -0 2 Muchow 1985c (Lablab purpureus)

Lupin -0 14 lo -0 48 Turner et al 1987

OA In roots Pea -0 3 to -0 8 Greacen and Oh 1972

C. Dehydration Tolerance

Thc nhll i ty 01 cell\ to cont~nue neta ah^. I i n n g ;I[ I(IW U'P, is t c r ~ i ~ e d dehydration tolerance. The last line 01' defen\c that J

plant has againht wa1crde1'1c11 1s a drhydra- tion-tolerant protopla5m (Turner and Jane\. 10x0). D e h y d r a t ~ ~ ~ n rc\nltr in i r re~ers ib le d~sruption o l cellular organ17ation and me- tahol~sm. although phot<~synthetic rcautions can occur at ret'ere hater def'~cit\ in \ome lower plan1 lorms (Santar~ur. 1967). Most crop plants hcl (~ng tu the dehydrat~on-intol- erant category: In general. plant5 %ith poorly developed drought-avoiding n~echanistn\ have the greatest dehydration tolerance (Beuleq. 1979). The relevance of dehydra- tion tolerance i n determining productivity under moisture-limited environments is de- hntahle as, agriculturally. severe desicca-

tion repre\cnt\ a \m;tll proportion (11 thc total In\tance\ (11 drought (Boqer arid M ~ P h e r s ~ ~ n . 1975). Furthernlore. i e l d re- duciion duc to uater deI'1cil heco~iie\ 1111- portant hefore \ c ~ s r c de\lccation occur\. H ~ u e \ r . r , enhancement o f dehydrat~on 1111-

trance. which reaulth in continued leal grouth and decrea\ed senescence during ml ld or mt~derntt. drought. cc~uld habe a positne et't'eet on agricultural production. Slnclair and Ludlou ( I9861 considered crit i- cal r e l a t ~ \ r water content as a most mean- ~ n g f u l indeh for ldent i fy~ng legumes k i t h contrasting differences i n dehydration tol- erancc. I n enLironnients where hater defi- cits can occur at any stage of growth. dehy- dration tolerance may have some role I n survival of the crop unti l soil moisture lev- els improbe wi th succeeding rains (Turner. 1979).

1q2noip q)!m alolauo:, IOU saop sss!p joal uo osails ~ a l e m pasnpu!-9gd uo paspq aoueia[ol uo!loiphqap leql ~u!s ! J&~~s IOU Sl 11 '\nqJ. ws!p ,lea[ uo ssaJls pannpu!-ggd 8u!inp snsso IOU Lsu (catu,Czua puo Taunlqtuatu 8u!1sa1osd u! wloi so@u Lold q q n ) t!s -!,lap aJnl\!olu a1 paloa[qn\ uaqm 1aAal iueld aloqm aqi lo Innso lnql \a8unq? an!ldepe iaqla puo leuotusrlq pun 'spunodtuon saqlo \no!lrn '.;aln[os :,!uz8ig .\ss!p ,leal U I \\ails pesnpul-(I)3d) [os,ild aua[Lq~a,Clod ~ U ~ I J I

luaiau!p LJJA S! suolllpuon pla1.1 Japun usail\ uo!ln~pLqaa ' ( ~ ~ 6 1 ' n q p n 7 puwaq7 \1g , n d w ~ ' i l - 01 5")- \ h n 11 S . J I / I ~ I ~ S O ~ ~ I S U l pUV (SX6[ 'JaMOI.4) l?dW i 8- 111 6'9- tul)J,l p28urJ rsduoad!d U l a3UEJalOl Ulllll?Jp -Lqop (I! U ~ I I I ? I J ~ . \ aql 'Salpni\ luauluollhua pa[loilunls uo pa\rg '1$8(1[ ' J a n q S ) \awn8 -21 ix110 01 ~ ~ . I P ~ L L I O I anunialo~ ~ ~ ( ~ ~ ~ c i p i q a p 111 \la,\" qritq a\1?q dd\ s.?I/IIIu\o/,\'~s au1n8 -a1 aimvt:d ~"xdr~. l i aql pur raduoari!d

'(iXhl ' I J L I I ~ I 111~;7al -u! .Ina[snu 1~a11iid lrql \ L U \ I U ~ : ~ ~ ~ U U I apl\ -as it:ul 11" a111 111 r n u r q o ~ u o ~ ~ n i l d q a p jo \I\rq ~ I [ O ~ I ! I J ~ I a111 ' \ n q l .uo!tr?ipiqsp >.rat -3s J u ~ i n p pailiilrap .lo paririuap \ I \na[>nu 241 3iaq.n ~ lon ld do.ls iuoru 111 \ I \ r i lu ( r rlqi :lsrlu! \urntuai a a q ~ 1 0 \nq>nu aqi 'uo!lrspAqap i l l pa~.r~l 'qn\ a . 1 ~ rlurld araq1 u a q , ~ , ( i ~ h l ' i a i ~ g ) (att:l\ . ~ p I!I: s q ~ 01

dn ) uo!~~:.~pdi[ap a s ~ \ a \ (11 I L I ~ . I ~ O I , i [ a i u a i ~ ~ a a.tn i ~ \ ~ ! a [ awq,n P , ~ U . I V (11 avmu \a!xd\ aulc,\ ql ln ( I ~ ~ U I ~ U I ~ U O J (is>\ u.noqs uaaq srq S I ~ . L ' ( i s61 '.I&<oH) vnq2nu 3111 SPI\

-In11 \a[[.wxR.lt) \I.>llJl~U(l.>.2J illfl~ll?SJll!d~l 11" 31 11!q1 s>ilnhai uolll?~piqap a .~a \a \ 111 ["I \.In\ Illy1 B U I , ~ [ ~ U I I '.73111?J171111 1111111?~p~q~~~ ioj ll:!~uas\a v! ,<1!sB2111! i o a l ~ ~ i u 10 a ~ u r u a i -u!qq ' ( y ~ h [ ' L I I ? ~ puvqqrt\qsg) 1uosa101 U O I I

- ~ : i p q a p I O U s i r I I ? ~ I csraad.; qt ln pasrdluos <OIII?J uo! .>!~11!8iou!/ir8ns a[qnlo.; ~ a q r i ~ q a\rq ia4nuirp Buua~ln.: 1noq11.n 1alr.n itaw 10 h ~ h 01 dn a\ol urn sa!.>adv asaql) anucia[ol uo!mipXqap l o s(a\al aulailua ql!.n c a v ~ e ~ d ~ L L I O S ' ( y ~ h l ' n o l p n l puo .tamold) yo 01 palnlai . i ~ i m t ~ ! ~ s! a,x~rialol uo!lrip.<qap

'eaduoa%!d ul '(9861 'iaqaH put qemq:,S : ~ 8 6 "["a o e ! w j a9etuep pasnpu!-uoge~p -.iqap l u o ~ j satuizua pue saueiqmalu aql Isa, - o ~ d au!alaq pue au!~L[3 slt q5ns spunodiuo~ tun!uomme h u a l e n h pun 'au![o~d so q2ns sp!x ou!ue 'siadnt so qsnS valnlos afqiled - m o 3 '(0x6 1 ' g e g ) su!alold jo uo!~e~nlouap luanaid pue sausiqwaru [[as aql jo .C~!.~dalu! urltluletu LII sllas aql jo Llrllqn aql uo spuad -ap .il~uaiedds uo!~eipLqap 01 anueia1o~

' ImNinr 1~31 U I \aau$iall~p 411% pal" -!nossn a.m iualuos iaien [rJ!ilss u! sawn8 -q Iuowr \asuasaljlp inql ~.;aridn\ (11 rlep paiua\aid i y ~ h l ~ o l p n l p u w ! o l ~ u ~ ~ '(0661 "1" la I!ipUl?~Jl!UlaJd :')Xh[ ' 8~1 i l ax :PXh[ "It? I2 lUl?Klt:U~\lJ~ 'tX6[ ' l l R ~ ~ P d q > s pun ru lnpnoa) \slun;ia[ 8ulpnlau1 .\io\rlln.? put: \alaads dois 8uoutr pauoda~ uaaq crq m u r -JJ[(II u0111!ip,(q;lp U I UO!IBUP\ 2pln r ' i 30 -1opoqi~u1 riuluaa~n\ JO adi l \ ! q ~ uo pa\na .I [ x(, 1 "["a p [ o d o a ~ ' I 861 'uoxaqg pur utnlg) salLl0.11n~la j11 a8nyral i q ps+\s.;vr aq LII?J a 8 r ~ u r p a ~ l r i q t ~ ~ a ~ , i l ~ l ~ q n Ga.\03a~ J ! J ~ I so1 palo>\ u ~ q l put: waur lrsq pur ainl\loul 10 q a \ a l aia.\a\ (11 pal>al'qn\ air d u l l p ~ a i q a a q n alqrlm\o a n v p l , q l s ~

'ln\t \in.; luald i l a r ~ u ~ l l n pul? uollsunJ irltqla2 01 1wuau1~1ap \ ! Sin1 ->nil\ auniqlua~o 01 adrturp i u r .sari! \ I I JP

n~l( iqcls~u il:lnlla,> 111 I R S I U ~ am \aucJquram asnoaag . ( , V , X ~ [ 'LLIIIIH) ~ r " t u q m x \ i q d i[afi1t:1 a i r tnqt radunq:, [ n i n l ~ n l t r .lo uo~lep!xos~d prd11 01 anp innm inlu aiaplo -c!p aursqwalu .YS!Z~ICJI\ a.'uepIo\r U O I I ~ J P

- iqap qfinosq~ %[Ian aqi u! latern jo ["a[ \lql u l a ~ u l r ~ u iouun,l \ ~ u r l d j[ , ( ~ ~ 6 1 'wn[a ) aim -snits auriqiuaul JCI .il!ia'a~u! louo!lJunj pue asurualulrlu aql fill <[[a3 luold U I pairnha~ v! salon lo ?,J~g 01 8 ; lo mnm!u!lu y

resistance based on either lield or contn)lled environment evaluation of plant5 (Hall eta].. 1984).

Little ~nformatlrln I \ available regard~np genotypic variahility for dchydration toler- ance in legume6 Also, a \ dehydratl~in toler- ance 1s a \urvlval trait. the cxpre+ritm o f t h ~ \ tratt and ~ t \ cclntrihutliin to drought rcslr- lance o f a g ~ v e n crop \pecles depends (in the levcl (~ f s t r e r s that the cnlp experlencer dur- ing the growing rea\on. Furthermore, it\ usel'ulners can he real~lcd only i l 11 I \ placed in a gcnetlc hackgniund that ha$ other mecha- nlrrns related to maimcnancc 01 prrxluctlon under moisture-deficit cnvlrunments. H ~ g h dehydratron tolerance I \ usuil1I.v a \ ( c i a t c d w ~ t h \low rater 01' growth and development. and t111ist plants w ~ t h good dehydration tol- erance lack dehydration-avotd:ince strategies ( L u d l ~ ~ w . IYXOh). Mechani\m\ that allow pliints to avoid dehqdratilln could (~verrlde thc henelit5 01 dehydrat~on tolcrancc, thus ~ n a h ~ n g it dtfl'icult to quantify the contrihu- lion 01' this trait to drought re\lrtance.

Ncvcrtheles\. 11 I \ con\idered that mod- erate dehydrat~in tolerance is derirahlc in crops of semi-arld en\wronments (Steponhur et a].. 1982). Dehydrat~on ~olerance coupled with stomalal adju.;tmenl c~ntr ihuter ; to greater dn~ugh t reslst:ince of rorghutn In em^-arid arcas (Ludluu. I9XOh). Also. de- hydration tolerance ir not energy consuming (Bcryer. 1992). is rtnhle from generation to generation, and I \ related to f ~ e l d perfor- mance during dniufht, at least in some cases (Wright and Jordan. 1970. O'Toole et a].. 1978).

D. Integrated Traits Assisting Crop Performance

Integrated traits, such as seedling estah- lishment, early growth vigor, and leaf area maintenance. are delermined by a number of functionally integrated mechanisms that may

contribute toboth dehydration avoidance and tolerance aspects. Such traits are easler tc~ quantify In a hreeding program where many Ilne\ must he e~a lua ted .

1. Seedling Establishment

In arld and semi-and envin~nmentr. zu~ l molrture in the seed-bed can he suhopt~mum. wh~ch causec reduced germination and emer- gence. and results In low yields (Saxena. 1987) Even for portrainy sea\on crops that are ra~sed on stored 31111 moisture. germina- tlon and \eedling ertahli\hment ma) he 81'-

lected by suhoptlmal \urlace so11 moisrure lcvel at rowing. In aouth Asia. h ~ g h tempera- turer and e\aporative demand hetween the end of the monsoon ram.; and the time of jowlng result In a rapld lo>\ of \ 0 1 1 molsturc in the rufiace la)er\ Consequently. rnois- lure at reeding depth IS ofien in$uflic~ent t i r gernllnatlon. emergence, and e$tahlishment. Poor and irregular plant \lands are a major cause (Iir the large lield gap hetween farrn- ers ' field$ and experiment \tatlons, f o r chickpea at least (Saxena. 1987).

Rapid rool d e ~ e l c ~ p m e n t and gr~iwth would factlitate succesrful establishment of seedlinfr when soil ~ntlisture i.; suhoptrmal after sowing (Asay and Johnson. 1983). Suuccs\ful e\tahl~shtnent of seedlings dur- ing periodic dr) spells requires a primary rool capahle of rapid downward elongation hecause of frequent and severe d ry~ng of the \eed-hed. which may restrict development of lateral rootr (Jordan and Miller. 1980). Also. the lateral roots may be required to prow through relatively dry layers of soil to reach motst regions deeper in the profile. Because mechanical resistance to root growth increases dramatically as soil moisture level decreases (Taylor and Carson. 1974). these roots must be capable of exerting consider- ablegrowth pressure. possibly through alrer- ations in mot diameter or OA in roots (Hsiao

et al.. 1976). For crops planted at the hegin- nlng of a rainy season. the capacity to quickl) develop a deep root system may not be re- quired ~n~rial ly. as (only the surface \oil would be wetted initiall). However. \ ~ g o n ~ u s root development and growth are apparently irn- plrtant for seedllng survibal under condl- tlons where the \ o ~ l surface driek rapidly. hut sufliclent soil moisture ir d\ailahle in deeper zones. This i \ prohahly one of the reasons k ~ r the linding that \lgorous seedllngr sur- vlbe better than those le\a \~gorour (Blum et al.. 1977). Also. dehydrat~on tolerance of the leaf tissue and inenstems ning play an inl- ponant r d e in protecting the rcedling. and thur rna~ntainrl!g the plant rtand.

Gentrtyplc variation in the ah~llty to ger- mlnate and estohlish seedlings under ruhop- timal moirture levels har heen reported In legume\ (Wright. 1971: Bou\larr~a and Schayaugh. 1083. Kang et al.. 1985. Sdxen:~. 1987. Seong et al.. I9XX: S o p ct al . I'IRXI. Generally. crop, requlrc a cr~tical reed m01r- ture let el tr1 gernl~nate and thir varier unlong c r t ~ p specie< For example. for chickpea. a 20% soil nlolrture level is critical in a Vertlhol (32% \oil mrii\ture ir ciore to f ~ e l d capac~ty In t h ~ s Veni\ol). hel(1w w hlch none of the tested genotypes could germinate (Saxena. 1987). Genotypic dlfterences were reported In chickpea for emereence at the 21 to 2 2 1 \ o ~ l moirture level, reberal genotype\, such as G- 130. Rabat, and Annigeri, were cc~nsid- ered tolerant compared with genotypes L-550 and K-4-1 (Saxena. 1987).

2. Early Growth Vigor

Early development of the crop canopy is necessary to optimally utilize the production environment (i.e., light, water, and condu- cive temperatures). This is particularly so for short-duration varieties in order to match the crop duration to the length of the grow- ing season. In Mediterranean environments.

early bigor would lend to irnprove~llent in WUE hecause water use earl) In the season. when bapor pressure deficits (VPD) arc \rn;lller. would Impruve TE. con~pared w ~ t h water use at the end (it' the se;iron when increasing ~e~nperaturec and h~gher VPD reduce TE. Rapid early growth and canopy dcbeloprnent will reduce \urfhce soil evapo- ration. thus ~ncrcasinp the moisture abail- able f i~ r transpir~itlon. Howeber. if the crop i raised entlrrly on \tored soil moisture.

then earl) grc1~1h vigor need, to be halal~ccd with ratc of mo~sture use 111 cn\urc that cnough moisture 15 left 1111- the gra~n-f~l l ing period. Simulat~on modeling ct~uld a s r~r t in crtlmdtlng the degree of grouth vlgor re- quircd I?lr ;I glvcn production envlrtinnlent. taking into account the rtorcd soil water sup- ply i u h ~ c h ir inllucnccd hy ro11 type and depth) and iletnand (which I \ inl'luenced hy VPD illid LAI). Genct~r. var~ation has heet~ reported In early t igc~r in several gram le- g u ~ i ~ r on in^. 19x3: Silim el 21.. 1YV3). In- dicilt~ng the lea\ihility 111 manipulating t h ~ r t~a i t or requircd lor s p e c ~ l ~ c cnvlronmentr

3. Leaf Area Maintenance

Leaf area expansloll I \ more sensltivc to drought than photosqnthes~r and transpira- tltln (Turner and Hegg. 1977: Muchr~w. IYXSb; Horlgenho(1m et al., 1987). Signifi- cant genotypic differences in leaf area ex- panalon have been reponed In legumes such as royhean (Muchow. I9X5h). When rnolr- ture del ic~t develops slowly, crops tend to adjust their tranrpiring sutiace area thrtlugh reducing leaf growth and enhancing senes- cence of older leaves to balance transpira- tional demand against reduced water uptake (Hliao, 1982). This is a mechanism for re- duclng water loss as, below an LA1 of about 3, crop transpiration is reduced linearly with leaf' area under dry soil sutiace conditions (Ritchie. 1985). This occurs for production

Several attempt\ have heen niade In cc- real and legume crops to a\ \e \ \ and estahllsh genotypic variation in TE uslng A a\ a tnol. Genotypic variation in A has heen reponrd in cunflouer (Vlrgona et al.. IYYOI. ground- nut tHuhlck et al.. 1986: Wrlght et a].. IYYJi. cnwpea(Hallet al.. IYW). 1992). hean (White el al.. IYYO1. and uheat IFarqi~hiir and Richards. 1984: Read et al.. 1991 1. An In- crea\e of I',;r In A c(irrespclnd\ to a decline of I3"r In TE of Alwi wild rye tEl\ntrc.\ rl~rltrrr qp.1. 26% in crerted whcatgras\ ( A R ~ O ~ J Y ~ ~ I I I d r ~ ~ r t r ~ r i i t r ~ ~ tJohn\r~n el ul.. 1990). 19% in uhcat (Farquhnr and Klchardr. 1984). and 17% in groundnut (Huhich el al . 19Xh1. The \drliltlcin In T t among st.vcr;il crop3 has heen quantified. \ i r , jlJ% in wheat (Farquhar and Richard\. I'YX41, 6.5% in groundnut IHuhlcA et a].. IYXh). and 67'4 In cowpea IKirchhofl ct a].. I ~ X V I .

Ikzplte considerahlc generic and e11v1- ronmental Inutntlon. Itght inten\it). water \iatu\! et'fccts on the indlvldual components of A and g (\tornatal conductance) wpa- riltcly. Farquhar et 81. (I9871 \ugge\ted that varlatlon in the ratlo ot Alg. hence PIP,. and 3 i \ llkely to he small because of coordina- tlon hetween A and g Thi\ is rellected in coeffictrnts of \miition for A typically <4.SrZ (Johnson et al.. IYYO). making selecti~in lor A highly attractive to plant hreederr. Thls coordination hetueen A and g could lead to predictable genotypic differences In P , P , and A (Huhick et al.. 1988)

In groundnut. genotype x envirtlnment (G x E ) interaction for A 1s not slgnificanl, suggesting that A is primarily under genetlc control (Nageswara Rao and Wright. 1994). Genotypic ranking was maintained at differ- ent sites and between wheat genotypes grown in pots and in the field (Condon et al., 1987). The broad sense heritabilities for A ranged between 60 and 90% for wheat (Ehdaie et al.. 1991) and for groundnut (Hubick et al., 1988). In groundnut and cowpea. heritabili- ties for A were similar under dry and wet

conditi<inz (Huhlck cr al.. IYXX: Hall el nl . IYYO). Slmil;lr finding\ were reported thr other crops ruch a \ wheat t E h h l e el ,I\.. IV9l I and \unilower (Virgona el dl.. IY901.

5. Developmental Plasticity

Dcvel(ipmcntal plasticity can he defined a? the ilhilny 10 adprt thc duration of differ- cnt growth pha\es to \ u ~ t ni(iisturc auailahll- ity dur~ng the growing \e;l\on. This includes the ahility ol'a plant lo recover fnim a perlod ot drought .;tre\\ Inc(~rporat~(in of a wmider range 01 dcr clop~licntal plasticity 11110 crop5 groun under rn(~isture-liliiiting cnbironmenls i h an important \trategy to consider in debel- oping cu l t~ \a r \ with stable perform;ince in drought prone arcas ( M u n g o m c ~ ct al.. 1974: Saced and franc^\. 19x3: Wci~ver el al.. IOX3l. For In\tance, landraces of'heun grown In elidctnic drought area\ of the Mexican highlands po\sei\ a high degree tif develop- mental pla\ticlty, !hit\ giving assured hut Iou ylclds under drought conditions ( S ~ n g h and White. IYXXI. Cowpea is more drought toler;lnt than {oyhean mainly duc lo its greater de\.elopmental plasticity (Lawn. IYX?h,c). However, developmental p1;lstlcity ic prima- rily a conqervative trait, and there is nor- mally an lnevltahle trade-otf with yield po- tentlal under optimum envininment\.

1)evelopniental plasticity include\ an ahiliry lo adjust canopy development pattern according to so11 moi$ture availahility. Thlr would involve leaf area adjustment, leaf movement\, stomatal control, and ability to produce new leave\ on relief of molsture defic~t. Developmental plasticity also de- pends on plasticity o f the root ryrtem. Flex- ibility of reproductive development patterns i q also required, with ability to adjust matu- rity according to soil moisture availability, and ability to produce new flushes of flow- ers and pods when the moisture situation improves (e.g., as in pigeonpea [Chauhan el

a].. 19871). Scrrne genotypes cil'chickpea can initiate pod set and f i l l ~ n g at lower nodes at an early growth stage to ensure f i l l ing rif at l e a t some pvds In a receding \nil moisture situation (Saxcna ct al.. 1993). In groundnut. peg in~tiation and elongation ceases when soil moll molsture 1s depleted to 80% o f the cxtractahle limit (Chapman. 19x9 I. However. those pegs ln~tiated prior 10 ~ r r durtng the drnught per~od had the capacity lor renewed elongauon afer rewatenng. Thi\ pla\ticity in pod development apparently play\ an Impcrrtant role in thc adaptatccin 01 gn~und- nut ro Intermiltent drought s~tuations lHarri\ el a].. 1988).

Phcnolog~cal plasticity 1s a function 01'

degree o f \ensit~vity to pholothermal rime. I-lowertng in legulne i r generally renrltive to iarl i i t lon in hoth temperature and photo- period tSumn~crfield and Wlcn. IYX(I: Hell and Hi~rch. IYY I I . Re\ponre to photothermal ctrndition\ 1s ;I m40r component o f C x I: ~~ncractions (Sun~merl'~cld ct a1 . IYX.51. In hcan drought adaptation trlalh. cultivar\ pcis- \eshlng day neutral rc\pon\e pni \ed adban- tiigeous at hlghcr latitudes. uhcreah photo- period-\en\lti\e ~naterlal\ were better ~ u i t e d ;it Iciu latitude\ (White. I OXX). Phenologtcal plastlctty deterniines the ~ p t i m u r n reprciduc- l ive growth duration. rate of pod lilling. and the patlern o f pod de\elopnienl and thu\ could play sn Imponant role In detcrm~ning the adaptation o f ;I genolype to a given target production mvir~ inment.

Genotypes wl th reproductiw develop- nlent spread over an extended period, and whlch can he delayed hy drought. are more suited to envlronmenth w ~ t h himodul ram- fa l l d ~ s t r ~ h u t i o n or where drought can occur at any time. Genotypes wi th repro- ductive development compressed or delayed l i t t le hy drought would he Inore successful when grown on early concentrated rainfall or under stored and receding nloisture con- ditions (Harris et al.. 1988). Thus. the de- gree o f phenological plasticity required i n a

gcven cultlvar depends on the production environment. Indeterminacy, plasliclty i n hranch~np. potent~ally long growth dura- tion (e,g., perennlality), phenological plas- t ~ c l t y (i.e.. ontogenlc t lexrhil i ty) (Lawn. 1982~: Muchow. I9XSa). profligate produc- 11017 ol' slnh capaclty (Ojehmon. 1970). and ahillty to produce sequentla1 flushes o f t l ou - e r ~ n g and poddlng provide scope for detel- crpmental plasticity in many legumes and play major role\ in adaptatlrrn to marginal env~ronments (M~nguez et al.. 1993). In pigeonpea, the flowering and early ptddlng \[age I\ particularly kusceptihle to (oil mot\- lure def ic~t (F.B. Lopez et al. ICRISAT. unpuhlishcd data). Thu,. indeterminate types (or determinate type\ with Iesi \ynchronou\ I lower~ng) u,ould perm11 greater chance o l pod \ct under intermittent drought condi- tlon\. Funhemiore, ~ndetcrminatc plant t y p e are hetter ahle to produce neu lea\es 1111

ruliel (11' Jnrughl \Ire\\. and thui cont~nue the \egetatlve phase to produce mure hiom- as\. Howeuer. Jetermlnate pigeonpea also show* penotyplc \ar~ahlltty In ahl l~ty 111 re- grow. or ratoon. after harbest or damage 10

an eclrl~er I lu \h tJohdn\en et al.. 1991). De- terminate and indeterminate type\ habe hoth mdun l ad\antages and disadvantages de- pending on the target cropping \y\tenl and product~on en\irtinment.

Generally. hlghcr levels ofdevelopmen- tal plast~city have contributed to vegetatively \ , i go~ ius hut poor-glelding phenotypes and contrihute to n lack o f adaptation to inputs o f fertilizer and ~rrigation. mechanical hanest- ing. and other management Inputs. Soybean has been considerably mtdi f ied by man to better adapt to higher levels o f management and thus shows less plasticity than legumes such as pigeonpea, groundnut. chickpea. cowpea, and mungbean. However, signifi- cant genetic variation can be found within many legume crops i n various components o f developmental plasticity. Genotypic varia- tion in rooting pattern has been reported for

several crop species, including the adjust- ment of rooting depth and root length distri- buuon pattern to changing morsture avail- abi l~ty (Caner el al.. 1982). Similarly, for other components such as canopy itructure and rate of phenological de~elopment , geno- typic variatron has been reported (Ftscher and Turner. 1978). Plant breeders have been quite successful In rnct)rporating Larlous drgrees of phenological plasticity (i.e.. de- gree of ~ndeterminacy) in rtiany legumes. rnclud~ng prgeonpra. chickpea, and ground- nut (ICRISAT. 1990. 1991 ). This ha\ Icd to the de\elopnient ol 'culti~ar\ that can he41 fit panrcular prtduction e n v ~ r ~ n m e n t \ (Hall et al.. 1978: Turner. 1979).

6. Mobilization of Preanthesis Stored Reserves

Remohilization ol rtarch resenes htiired In \[em\ contnhute\ rignificantlq to grain y~e ld of legume\ (Ctinrtahle and Hzarn. 1978). These carhohqdrate reserves act a \ a buffer against arailahility of current photo- rynrhates. particularly dur~ng grain filling ISchnyder. 1901). Becauce translocatron i \ more tolerant than photusynthe\i\ and respi- ration to mol\ture deficit (Boyer. 1976). the ability to store and mobllize large quantiticf of carbohydrate, for OA. or I i r grain filling under terminal drought. +hould improve the ability o f a cultlvar to perforni under drought conditions (Bidrnger et al.. 1977: Blum et al.. 1983a.h). Also. ABA ha\ a major role in rnducing mobilization after anthesis (Tettz et al.. 1981 ). as at t h ~ s stage grain growth enters its exponential phase (Blum et al.. 1983a.b). A significant positive relationship exists between the rate of stem dry matter loss after anthesis and grain production ca- pacity under conditions of moisture deficit across a range of genetic materials (Rawson et al.. 1977). Also. this could play a crucial role in determining the sink strength by pre-

venrrng mega-gamete sterility, thus protect- ing reproducti\,e developinent. if stress oc- curs at the tlowenng stnge (Boyer. 1992).

Grain growth in leeunies is partially \up- ported h> translocated plant reserbes (Meckcl ct al.. 1'184: Westgate et al., 1989: Wright el al . I991 1. Thehe reserves are rna~n ly nonstructural carhoh)drates, which can be r n o h ~ l t ~ e d for various plitnt ncedr under nioi\ture defic~t, which is particularly neceh- sary it' current a\s i rn~l;~t~on cannot meet plant requirements (Ludlow and Much~iw. 1990). The relative contrihuti~in of there rehcrves 111

total g r a n yield is, l i~~wcver , dependent (in the growth duration of the cultivar, crop specie\. and the enrrr~inmental conditions during pod filling. In crops ~ u c h ;l h h a bean, nearly 4.5% of the total ctem weight cotnprirc\ remohilirahle stiirch rewrver. conipared w ~ t h only l X I A in joyhean (Hanwity and Weher, 1971 ) &early YO'? ol the \ecd nitrogen and 42% of the \red dry matter in riiunghcan canic t'rr~m remobil- i~a t ion (it' re\crveh \tored dur~ng prcanthcris (Rushby and Lawn. IYOZ). In soybei~n. 25V' ofthe grain wcrght was produced from stern rescner under ram-led conditions (Constable and Heorn. 197X).

Remoh~lrrat~on (if skired reserve9 can influence the perfiirmance of s genotype in hoth ~ntermitrent and terminal moisture-defi- cit envrronmentr. In intermittent mtiisture- deficit \~tuat~on$. stored carbohydrate\ de- termlne the ahilrty of a genotype to recr~vcr from stress (Sheldrake and Narayanan. 1979). Under cond~trons ol' termrnal morsture defi- cit, ner phritoynthesis decrease9 (Berry. 1975). thus the proportion 01 translocatron of stored soluhle carbohydrates for grain fill- ing becomes larger, although it is esrentially the same in absolute terms (Austin et al.. 1977: Bidinger et al.. 1977; Fischer, 1979). In chickpea. nearly 15 to 20%, of the total grain weight is derived from remobilization of stored carbohydrates (Saxena, 1984; Singh, 199 I). For legumes. which are inde-

terminate in nature, the que\tlon of reserves contrihuting to grain yield i\ complicated hy the long time interval over whlch flowering extend\. Nevenheless, i t IS possible that re- serve\ that accumulate hefore the onset of l l ( ~wc r~ng can huflcr yield agaln\t stre\s during seed lilling, as ha\ heen wggested for dyland ~oyhean tCon<tahle and Heam. 1978: We\tgate et al.. 19891, munphean (Bushhy and Lawn. 1992). and chlckpea tSaxena, 19x4. Singh. 1991 ). Thu\. for dry Mediter- ranean environments. Richard\ and Thurllng ( 197X) sugpe\ted relectlun for greater growth helorc anthehi\, when watel is unlikely to he Ilmiting. hence placing greater cmphas~+ on reed filling reliant on mohillrahle reserves lhnned durlng early growth.

Method# arc nu\*. ava~lahle to quantify the c~intrihuticln olr~ored preanthew reGene\ lor gram ylcld under terni~nal drought rnbl- ronmcntr. This in\olvcs removing the green leal area hy \praying chernlcalc such as rnapnes~um or \odluni chlorate (49, actlvc ingrcdicn!) 14 d altrr Ilower~ng. thus I)rclng plant# 111 re14 entirely tin \tern reserves tiir groln filling (H lu~n c! al.. I9X3a.h). Thir has heen attempted in cereal\, ~nc lud~ng wheat. harley. sorghum. and millet\ ~ h r n i ~ ~ ~ r r n and Pt~nnivrr~tnr \pp.) (Rluni et al.. IYX3a.h). In reveral crops, includ~ng groundnut, geno- typic varlutlon har heen reported in the ahil- ity to \!are and 1nohil17e carhohydrates for seed filling during tern~~nal moisture svess (Chapman. 1989: Wright et al.. 1991). Seb- era1 tall landraces of wheat and harley pr\- sess substantial ahility to store and remohi- lize carbohydrates for grain filling compared with modern wheat and harley cultivars (Blum et al.. 1989). Similar variation has been reported in mungbean, whlch suggests the possih~lity of genetically manipulating this trait (Bushhy and Lawn. 1992).

Although huild-up of higher levels of nonstructural carbohydrate reserves during the prellowering stage is advantageous i n stabilizing grain yield across environments

with fluctuat~ng $011 moisture levels. this trait also may have a number of neeative effects. Flrsr, constitut~ve adaptation of accumulat- Ing large reserves before anthesis lo guaran- tee wed filling involbes a penalty to yield potential under optimal conditlona. Al\o. dunnf the early reproductive stage. repro- ductive \ink\ will have to compete with the re\erve mobilization. whlch could lead to sink limitation. I f the crop face\ optrmum moi~ture le\els at the \eed-filling \[age, this would mean yteld would he limited hy sink rii.e..and also the rcsenes that were hu111 up would remain unulilired. Second, rapld rnoh~ l~~at ion ol largc qunntltles ol preanthe\~+ carbohydrate\ during seed filling w(>uld also enhance the danger ol'pred~\posing the \tern to fungal ~nlcctlons or Indg~ng could occur due to weakenlng of the stem. Th~q ha\ been rhown in sorghum. in uhlch genotype\ that rnohil ix li~rge quantitle\ 01' carbohydrate\ \tored in the \tern durtng seed filling were nlorr icnsltl\e lo charcoal rot tMo(.ro- phonrr~~rr ~hi i ,~ro/ i r roJ under condltl(lns of terni~nal drought stre\\ (Roaenow st al . IYX3).

7. symbiotic Nitrogen Fixation

The interaction helween drought stre&\ and sqmhiotlc nltrogen tixatlon In grain le- gumes must he understcxd. not on14 to es- tablish how hest to meet the nitrogen needs of the legume itself. but also to optlmize add~tion of fixed nitrogen to the cropping

rystem in drought-prone environments. Sym- biotic nitrogen fixation is particularly sensi- tive to envimnmental stresses: conditions that may not he stressful for elther of the pannen alone could be suboptimal for the symbiosis (Chapman and Muchow, 1985: Sinclair. 1986). The delicate balance between the host plant and the symbiont demands an opti- mum soil moisture environment for maxi- mumefticiency of N2 fixation (Swaraj, 1987).

Mort tropical and temperate legume\ require soil monture level^ of at lea.;[ 70 10 80% of field capaclty for optlmum funct~on of the legume-Rl~i:~>hrr~rr~ symbiosis (Johnron ct ;I].. IYXI: Swaraj. 19871.

Because of the high sensitl\it) of the legume-Rhrrohr~mi syrnhios~\ to water drti- C I ~ . nitrogen abailahil~t) i\ often a l in i i~ lng tkctnr for growth and p n ~ d u c t ~ \ ~ t ) of many legume\ under mo~sturc-limiting i.11viran- men[\ (Sinclairer al.. IYXX: S:ill and Sincliiir. 1941: Kuhad et al . lYY2) Man) l icld and contn)lled en\lnlnment \tud~cr on tropical and temperate legume\ have reported an adver\e effect of mc~irturc deficit on rhc le- gumc-Rhi:r~/~r~o~r +ymhinsi\ (c.g . see Engin and Sprent. 1473: Chapman and Muchnw. IYXS. Sinclair el a].. 19x8: Davie\ el a].. 1980: mckoun and Planchon. IYY l i The degree of rrn\nt \~t ! may bar). among gr;iln legu~nr \pecie\ (Smith et al.. I O X X I or even among genotypes of a given gwin legume tWilliam\ and Siciirdl-de-h4allorca. 1483. Sall and Sincl i~~r. IYY I ) .

The legume-Rhi:oh~rr,,~ sqmh~o\i\ In- ~ n l \ e \ a ct~mplex interilction hetween the host root. the rhizohial strain. and thc envi- ronment. Moi\rure defic~t may affect any phare of the legume-Rirr:ohiitr?~ \ymh~os~s. v ~ r . Rhi:ohirirl~ surbival and growth In hulk \oil or the rhirosphere. rhlzohial inl 'ecti~~n of the host root ti\rue. in~tialion. devcinpment. and functlon ofthe nodule and growth (11 the host legume. ~ncluding its ah~llty to malntain a supply of phottrrynthates, nutrients, and water to the root nodules.

a. Rhizobial Infection and Nodule Initiation

Rhizobial survival. multiplication, and migration in the rhizosphere of the host root is important for the successful establishment of symbiosis. Rhizobia can survive in soils of low water potential, are quite resistant to

\oil dr)ing. iind <;in sur\i\e In water lilnis \urround~ng WII particles (Williams anil S~cardi-de-Mallorca. IYXl i . Rhi:~~hiitnr \train7 dift'er in their ahility to \ u r~ i ve under ~no~\ture-deficit conditions. In gcner:ll. the f. ,i\t-g"~w~ng rh~roh~a l \train\ arc nl{~re .;en-

,iri\e to roll deh!draunn Ihiin \low-gnlw~ng \train\ (Sprcnt. 107 I ) . Hoac\c r . nln,t rhi7ohial .;tr;iiri\ can \ur\ l \e ot nioi.;turc Icv- el\ much ht'low those cr~t ic.~I I'or pl;int gr11wth. Thur, rhirohinl sur\l\al nl;ly nut he ;I I~n~ i t i ng fiictor for the \uccer.;lbl e\t;~hlish- men! 01 rylnhio\~s ~ ~ n d c r moir t~~rc-dcf~ci t condition\ (U'orrall and Roughle\. 1976: Swsrq. 19x7). Hc~wcver. low roil 111,11\ture content rl i lw* or pre\clit\ Illovenlent 01 rhilo- h ~ a to the root iurl icc (Hamd~. 1970). as the wcitrr-lilicd pores In Ihc so11 hecimc dircon- tlnuour. thu\ contr~huting 111 pilor n(~dulntinn in Icgurne\ gr<lv+ing in roils w ~ t h water lev- el\ helow Ileld capaclty at e;irly growth stager.

The rhi/oht:~l III~CCIIO~ prtjcerr i r Inore rt.n\rtl\e to mol\turi' dcfic~t tlien rhii.uhial \uiv~vii l and multipliei~l~on in the rhii.~~spherc. In many case\. plant\ fail 11) ncidulate under mnl\ture def~c~t. even with rutficlent rh1711- h ~ a in therhir(~\phere (Worrall and R(~ughley. 1976) Moisture deficit may rerult i r ~ greater adhes~on hetween root halrs and S~III par- ticles (Sprent. 1975). thu\ affecting the p h p - cal rel:ltionship hctween hacter~a and rant

ha~rs Production of extracellular material by one or both panners may af'fect the estah- liahment of a symbiotic system under moly- lure deficit. Also, di\tinct wetting and drying cycles, cr~mmon in semi-arid environ- ments. enhance n~trification and can pro- duce \c~il nitrate levels sufficienlly high tu inhibit nodulation (Br t~ku,e l l and Whaller. 1970). Thus. rhiwhia capable of success- fully establishing a \ymhloais under mors- ture-deficit environments may also require tolerance to high nitrate levels.

The higher levels of endogenous ABA i n roots of many legumes during water defi-

Sheaffer. 1983). but thore of pigeonpea (Sheoran et al.. 1981) recovered only up to 85% after expenencing a mild water det ic~t of -0 6 MPa. In legumes such as cowpea. mungbean. and black gram. nodule recoverj Ir incomplete (Sprent. 1971). Thu\. legume species vary in the ahihty ol'rheir N, fixation to recober from water deficit ( Apariclo-Tejo et al.. 19801.

Nitrogen-fixing activity decl~nes In many 1egume.i as the crop approaches mattint) I Lawn and Brun. 1974) Nearly 60 to 7 0 q of the leaf nltrogen i \ d~vened to the pod-filling process and t h i ~ leads to early cessation of photoqynthetic ability. termed a\ ;I "self-de- struction" prck-ess tSincla1r and dewit. 107.5). M~nllnal nitrogen iixatlon occurs durlng reed filling In legumes like cowpea. sokhean, end black gram, further aggravating the \eIt'.de. structlon process (Cure et al.. 19851 Terrni- nal molsture def'lcit. common in long-dura- tion legumes \own during a rainy ieason and on receding sot1 moi\ture during a postrainy seavrn. could accelerate [hi\ de\ t ruct~on (Curs et al.. 1985). Thsrel'lrre, the ahility to extend nitnrgen-fixation activity into the re- productive growth phare under-moi\ture Ilm- lted envtronmenls could play a major role in ~rnproving grain legu~ne prt~ductlv~ty in these environments.

IV. GENETIC IMPROVEMENT STRATEGIES

A. Screening and Selection

Characterization of the drought environ- ment of the target production reglon is a first and crucial step in undenaking a genetic improvement program aimed at improving yield and yield stability in drought-prone environments (Campbell and Diaz, 1988: Robertson, 1988). Production environments can he grouped into subsets of environments using canonical variate analysis (cluster analysis) (Seif et a]., 1979; Shorter and

Hammons. 198.5: Malhotra and Singh. I W I). hq con\ldering .:uch factor.: ;I\ i o ~ l water balance, temperature reglmrs, the potentla1 e\apotranipiration of the growing en\.iron- rnent (using long-lemi cllniatlc dat:~). or hehcd on G x E interacrlons (Seif el a].. 19791 Thuz. mc:ln and var~ance of moi.:ture deft- clts l i h e l to occur during the growing sen- \on 2nd length of the growing \citron can he ~.alculated. T h ~ s ~ I I I i~s\ i \ t in e\tlniatlnp the intensit) and dura t r~n of \Ire\\ the crop IS

I~hel) to experience. and ;it which crop growth \tape. thus guiding development of relckant screening methodolog) lor cviilu;~t- ing gernipla\m I~ne\ . Thl\ \t!.ategy also w1l1 help in ~dcnt~f'ylng the envirotln~entr with sinillar drought pnltcrn\. to gutde nlu1t1- Iocational tc9ting of genolypes developed fur specific drought patterns. Recent devel- opment 01' G1S techniqoes allour 11r corn- puterl~ed niopplng, and thu\ ready visual- ization. ol such i\o-drought envtronrnent\.

In mo\t cn\e\, screening Inr dnjught re- \l\tance ~nvo l \ e \ ekaluatlng gerrnpla\ni l ~ n c \ under field clrnditlt~ns wlth and without ~rr i - p;iti<in. Yield. its cornponenls. and t(rtal dry matter arc normally estiniatcd Line sourcc rrrigallon I \ also u\cd lo ekaluate germplarm liner undc~ a gradient of water deficit (Hank\ el al.. 1976). Motsture rejponse L.urve\ itre generated by regreusing the yleld of ~nd l - bidual entrles against water applied or envl- ronmental y~eld. to calculate the slahility of a genotype across a range c~f rniusture avaiiab~l~ties using the stability analysis pro- cedures of Flnlay and Wilkinson (1961) and Eberhart and Russell (1966). Line source tmgation studtes with gr~rundnut showed that genotype\ with stability and high mean yield could be identified in early and mid-season drought patterns, but not in those drought pattern9 where genotypic sensitivity ic nega- tively correlated with yield potential (Nageswara Rao et al.. 1989; Singh el al., 1991).

Yield stability analysis has been sug- gested to define drought resistance in terms

(11 y ~ e l d (Ftnlay and W ~ l k ~ n s o n . 1963; Eherhan and Russell, 1966). provided that the maior c o ~ n p i ~ n e n ~ o f vanation In the en- \ininmental ~ndex could he attrihuLed to the water regme. Stability ot yield acre\\ envi- ronment\ could he represented by the slope irt cultivar yield regres\ed on the unviron- ~nental Index (mean site yield). Houever. t h ~ s appniach doe\ not con\ldcr the con- Iiiunding effect\ of phenology on yield or thc clfects (11 yleld potential on the \lope 01 thc regrcsrlon and hence on the mtcrcept and drouglit rerl\lance F~schcr and Maurcr ( I9781 prtipo\ed a dronght susceptih~l~t) In- dex, hascd on rs lat~\e ytcld. 10 account 111r the ~ t~n l i i und ing c l l c c t i i I ylcld potentla1 of gcn<ityper. To ;~ccount I r i r t~n ie 111 f l i iwcr~ng. a\ri~ci;~tcd dniuglit e\cape, and )iclcl putrn- t1;iI. H~di l igcr c l ol l lqX7;l.h) p r ~ i p o ~ i l a drouglit rcrl\!ancc 111dcx t1)KI). In w h ~ ~ . I i I)KI 15 harcd ,In the r ~ \ ~ d u : ~ l \ 3 a r ~ a t ~ ~ i n In gr.1111 yield nailji~rled lii~ cxpc11111cn1;il crrilr. 'The ~ndex I\ po\~l lvc ly correlated n l t h ylcld undcr dtriught. ;lnd ~ndcpcndcnt o f yield p i ~ t c ~ i l ~ ~ ~ l LIIILI tlnic 10 l ~ l o n c r ~ ~ i g .

B. Difficulties Associated with Yield- Based Selection

Crop y e l d 15 lhc t~utciilne ill miin) prtj- cesscr o c c ~ ~ r r i ~ i g at dil'lr'rctn tinir. \c;lIe\ and level\ i i f~t i tcgrat~on (I 'ar\~~iur;~. IYX3. IYY?) Alro. a ch;iractcr ruch ar drough~ rc\l\tilncr is crimplcx 11 lnca\urrd nr crop ylcld be- C~IUIC 01' the many possihle phys~ologlcnl, ~ncirpholog~i.aI. and h~<whcniical f;ictorr re- ldted to drought resirtancc. Plant characters rli;~t influcncc pcrfrirnmance hale differing r~pponunities tor exprehsliin 111 dlt'furent years (Ceccarclli et ill.. 199 I ) . Select~on k i r drought res1ttanc.c hased on yleld alone ma) not hrlng the required genetic shifl I n rpec~f ic physl- ological attribute5 as different mechnnisnis would havc dif l trent oppnrtunitir\ for ex- pression under difl'eren~ conditions. Genetic gains made In one season may he lost in

\uhsequent seasons due to the variability i n the tlrne, duration, and intensity oi'mo~sture deficit acroa years. Thus, screening and \election hased ent~rely on yleld or yield derived indices may he o f limited value for genetlc improvement In drought envlron- rnents (although this 15 the cnterlon gener- ally uacd), a+ thc genotypic variance compo- nent I\ low compared w ~ t h cnvironmen~al and G x E karlancc under these condltlons IUor~el le and Hanihl~n. 19x1. J~ihnron and Cieadelmnnn. 19x9)

Adaptation to n~indniught lactor\ 1e.f photi)period, temperature. pests. dlseases. etc.i'ma) havc an ovrrrlding effect on yield. thus nldrhlnp potentially huperlor drought re\lstancc attr~hutes 01 germplarni I ~ n e \ IU'IIII~. IYXXI. k ~ c n thclugh landracer niay pii\ser\ a h~gher degree o f drought re\is- lance due to their hcltc!-udaptatinri to drought en\ lronmcnts compared u ~ t h ~mpraved cul- t~ \a r \ . !hi\ w i~u ld not nece\sar~ly transli~te ltit i , h~gher drought re\lstancr. ratings when cvalui~tionr arc hased on qlcld or yicld-de- r i \ed ~ n d ~ c t l \ nlotli, due 10 their po\r~hle poor ddaptauirn 10 the Ir~catlon hhere they here e\iiludlcd or the~r lnherent low harks5t 1nd1- ces tEhdale and Waine\. IYXX: Ehdale et al.. IYXX. Blum et al.. 1992). Yleld-hahcd indl- ccr may result In the development o f genetic niatcr~al\ that ha\e ;tdaptattc)n to the e lec - tion rite hut ha\e a l ~ m ~ t e d role in devdop- Ing genetlc stochs or vanertes ru~tahle for other ~su-drought liication\ The probability ot 'succes~~ In tranri'err~ng a rnultigene adap- tatton. such a5 drought resistance, by hreed- Ing could. houe\er. he inipro\ed by select- i n g l o r Iniportant components o f the rehistance r n e ~ l ~ a n ~ s ~ n s involved rather than selecting at a funct~onal phenotypic level (Blum. 1979: O'Toole and Chang. 1979: Jordan and Miller. 1980).

The approach that is put forward for ctrnsiderat~on is conceptually different from the "ideotype" approach proposed by Donald (1968). uherein an ~deotype IS defined by a

\et of morphophqsiolog~cal attr~butes that are thought to improve the genetlc yield pitential lor a given crop specie\ acres+ a numher of production envlronmrnrh. Th~s could he termed a? the "\tatic idrot!,peV ,~p- proach t L a ~ n and Imr~e. 109 1 I. One of the mosl serious crit icism\ of this "static ideotype" approach i, that it doe\ not take Into account the G x E interaction. nnd this denies the hreeder thc c~pp~inunitq to exploit spccitic adaptatton. 111 I ~ I < caw the moih- tu1.e-a\a~lahil~t) paltcrn\ dur~ng !lie grouing \cdvon (Lawn and Imrir. 1991 1 . The l im~ta- trims ashoclatcd ui th thir appmach are thor- ~ i ugh l> d i w u i e d hq Marsh;ilI I190 I ) . Sedg1t.y ( 1991 ).and Laun and In~r ie I I99 I 1. thus they are not repeated here.

G ~ \ e n the range and p.ltterrl\ ~I 'n io~sture \[re\< that can occur in drougl~t-prone c n \ ~ - ronment\ 01 an) gi\ en crop productl~in rc- glon. d u ~ d e 5pectra of hililogical modclr or deiired phy\i<~logical ~dcot)pc\ need 111 he dcvel~iped 111 ,uil the specif~c rcquircn~cnt\ of \arious target prtiduction cn\ll-i~nment\. Thl\ could bc ternird as "dynainic crop pl,lnl ~dcotypes". uhercln II I\ ~mplicit l) recog- n i x d thar an optitiial ctimhlnati(1n (11 mor- pholr~gical, phenolog~cal, and physiological trait\ md) diffi-r froni onc product~on c n ~ i - ronment to another (Lawn and Imric. IYY I : Sedglry. 199 l l and u r~u ld he cc~nt~nu~~u\ ly subjected to calidat~on during the course 01 a hreed~ng program. Alro. I l r ph)<iolog~cal traits that are of a quantlrative nature. such as TE or OA, in which the degree 01 cxprc#- hion is on a continuou\ scale. i t i\ to he recognized that the degree ofdeslrcd lecel of expression of these mait< will barq accrirding to the needs of the target en\'ironmentr. For in\tance, in dururn wheat dr~iught evaluation trials. i t was noticed that most of the high- yielding dururn genotypes under droughl stress had an Intermediate range of residual transpiration rate (excised leaf water 1055

rate), whereas genotypes at either extreme did not yield well, although there i \ gener-

ally a negatnc corrclauon hetween grain yield undcr drought and res~dual transpiration r;ue (Clarke et al.. 1989). This underlines he need for choosing the tiptiinurn le\.el of ex- pressioil o fa desired physloltrgical tralt. and [hi\ optimum expre\sion \rill vary according I0 the ncc~ls ofthe tarpel environment. Mrisr idrot) pe hrceding has heen preoccuplcd with extreme mo~.phological cIi;lrnclers. such a\ the i ~ r ~ ~ c u l m ch;lcter~stic In wheat 1ir barley, xlthour rcalir~ng the lim~tationr in v;lrlriu\ productio~~ cnciionrncllt\ IScdglcy. IO'JI I. Alro, c~~npo~ ien t idc~itypes of n crcip \pccics In a gi\cn largcr pr~~duction environment need to he con\~dcrcd to cniurc ;I h;~l:~~~ccd \ct af ~h . lcc t i~e \ in a hrecding progm~ii and al\o the possih~lit) 01 intcr;icricin\ helwccn ~l i l lcr ing ideolype ch:iractcr\ \ctc.h as t i i i~tr as\ciciatcd u ~ t h niorpholog). phenology. c;lnupy rtructure. and di\casc resirlance (Scdgley. 149 1 I.

Thw. hrecdcri are expected 10 ta i l~~r crop plam\ lor adaptation to specif~c environrncnt\ uhcrc moi\~ure def'ic~t\ arc uc l l char;ictct.- 1lt.d. Cc~nccptually. hi\ I\ ;11i Important dc- \iation froin thr tl-.lditlonal vicu 01' pla111 breeding, which is ni~rtnally 111 develop Lari- etim adapted tcr a widc range (11' cnbiron- men!\ Thi\ could Icod t i 1 dcvclopn~enr 111

genotype\ivarictics th,il could ~~p t im i r c cx- plcritation ot'spec~iic adaptati~~n in given tar- get production env~ronmcnt\.

There iirc a number of advantages (11 addrer\ing this prohlem through ;I phyri- t!log,cal gcnetic appr~~ilch, ~ncluding

I . Developmenr 111 conceptual hiolr~gical model< (dynamic ~de~type<), which are defined in terms of combination\ of trait< that suit the requirements tit' a target production \ystem.

? Selection focused on specific physi- ologi~al attributes that wil l result in the identification or development of genetic stocks possessing various lerels of ex- pression of a given attribute.

3. By defining the biological model at the heginning of a hreedlng program, the team of scientistt lnvolved wlll have a heuer ctimprehens~on of what they are speclflcallv Itrok~ng for. rather than treating adaptatlrin to moisture-deficit environments ar one 51nple compinent. This also helpr in evaluating the ex]\(- tng focally adapted vanetie\ Ibr traits (using hlolrigtcal model\ as a has]\) that they may he lacking: thus, the genetlc impro\,ement <if thew exl\tlng adapted varletlcr cr~uld he focused on certain physiolog~cal attribute\ conridered worth inlprovlng. The hreeding meth- ~idolcigy and \clcctn~n cnterta criuld then he well defined. so a \ to improve the chances of genetic Fan\ .

4. The variou9 genetic n~a te r~a l \ that are ~dent~fled ar havlng ureful phyriolog~. cal attrlhutcr. and the lcvelr ofexpre\- sion of a given attr~hutc In varl~iur ge- neuc material\. can he entered into a datahasc. T t~c re genetic \tochr would then he availahlc for other hrcedlng prcigr21[11\ in other agro-ccolog~cal ~cincs.

Hq lollowing the a h m e approach, the plant hreeder then can more s)stematicallq translcr genes related tu particular relevant traits and accumulate these In a step-wi\e manner instead ot'awalt~ng their colncidence. Such an analytical approach, however, re- quires cons~derahle knowledge of'the physl- olog~cal and gene ti^. ha515 liir dnrught resis- tance. The subsequent sections are focused toward tmplsn~entlng this approach.

C. Trait Identification and Evaluation

The word "trait" invokes several consid- erations: one is the level of integration or hierarchy of plant organization that should he addressed. Generally, plant breeders do not face the problem of choosing between

levels of inteprat~on, hut a physioloptst con- tlnut~usly faces this choice (Acevedo and Ceccarelli. 1989). Because ~ncreased expres- rlon of mort drought-resistance mechanisms reduces the potential for max~mum yield. an optimum lebel of expres\ion is requlred for each situatron (Jones. 1980). The major d ~ f - liculty in determin~ng thls optimum is the unpred~ctahility of the weather. although the capacity of the plant to tense and respond to changes in the env~rtinment and water sup- ply I S also imponant. Gi\,en the derailed knowledge of the environment and water availahillty during the course of a \eason, it is p<;;sihle. at least In principle. to calculate retrcbpectively the kind ~ifphys~ological and morphological attrihuter required la maxl- n11i.e kield.

The steps 111 fc~llow before a particular tralt can he recommended for use in a hreed- ing program a ~ m e d at improvlng drought reststance are

I . D e ~ e l o p a hypo the\^$ and \alidate the potential contnhur~on ot'a panlcular trait

2 . Search for genotype\ with high l e ~ e l s of expresslon of the der~rahle tralt

3. Detemiine the mode of inhcntancc of the trait

4, .De,elop rapld and effic~ent rcreenlng methods for segregating populations

5 . Incorporate the trait Into agronomically superlor genotypes

The greatest obstacle to a physiological approach is production uT convincing evi- dence linking high levels of expression of a trait to improved crop performance under drought. Conipensatory effects and inlerac- tions with the environment make it very dif- ficult to assess the value of particular physi- cilogical attrihutea in improving drought resistance (Jordan et al.. 1983; Clarke et a]., 1984; Ceccarelli et al., 199 1 :Marshall, 1991 ). Also, little information is available on how different physiological attributes (mecha- nisms) interact in determining a given level

of drought resistance (Jones. 19XO). The ef- fectiveness of a particular trait wil l depend on the nature of the drought stres.; occurring

in a particular area and prtwing seaxrn tGamty et a]., 1982: Crccarelli et a].. 1991 ). Each physiological pathua) to niax~miza- tlon of yield will be rl'fer.rive only when the genotype. environment. and ihe consequent ph?\ioliiglcal pathway arc ciirrccil) matched.

4 common nppmach tr1r ar\e\slng ihr value of tra~tk is hy mean\ of i\ogenic I~ne\ or populations (genotypes with \iniilar ge- neuc hachgrc~und\ hut ctintra\tinp in the cxprer\ion of a pnrticul;lr trait) iGnray and W~ lhe lm . 198.3. Jiihn\on rt al.. IqX.3; U~chardr et al.. 1YXh. Grurilct et dl.. IYX7). Such an appruach ha\ o tinit \i.nIc c1f )cars. ar i t deprndr on finding r i gn i f ~c~n t ht'ritahle varlauirn for the rtre\\ rc\pnn\c trait. and thcli a crn\\lng (prehretd~ng) prclgram to inccirporatc d~fferent Ie\r.l* 0 1 ira~t cxprer- >ion into a uniform genetlc hachground. Another Itmrtauon ass(rclatcd w ~ t h thi\ ap- proach I\ that trillis niay he cxpre\scd d11ft.r- ently In d~ffercnt genetic hachgroundr. I f \<I.

the information froni isogenic lines ma) he ~ i l ' l~nuted value as plant hrecderr would l ~he to he ahle io ure the irair in different gcnctic hockprounds depcnd~ng on the target envi- ronrnent (Ceccarclli et al.. 1991: Walldce ct al.. 19931. Al\ir, pleiotropic effectr could oh\cure the cxprerslon 01' the phyr~iiloglcal trait In different genetic hachgr~~und>.

Different combination\ ol'a giben re[ of traits may rerult in a similar le\el ofdrought recistance tAce\edo and Ccccarelli, 19x9: Ceccarelli et al.. 1991). One wa) to o\er- come these problems while asse?sing the value of a trait i? through divergent selection for different traits related to drought re\]\- tance (Acevedo and Ceccarelli, 1989). T h ~ r has a number of advantages over the sog genic line approach: ( I ) it offers the possibility of assessing the role of individual traits as well as a combination of traits in randomly ar- sorted genetic backgrounds; (2) i t generates information on realized heritability of traits

under contrasting environment\: and (3) i t

allows ;t coniparis~in of the \eleci~on cffl- r+nc) hctuet'n a ) ~cld-habed I..\. ;I tra~t-h;~hed approach

Sininlation modeling i s another pos\ihle approach 10 as.;r\r the value of a trait (Meyer. 1985. MLIC~OW el dl.. 1991: Shorter et dl.. 199 1: Hunt. 1'19.3: Muc l i~ \ r ;~nd Carhemy. 199.7). Simulations are performed with the trail present or ah\ent to \arytng degrcc's. w ~ t h :III other locti~r, h ~ l d cotirtant (J(1nc.i and Zur. 19x4: Hunt. 1093). Alvi, rimtil,~- tion m(~dcl\ plu) an 1nlpori;int role in \up- gesting Iiypothere\ on which traits arc u'orth puruing and \;i l~dat~ng tCec.cal.cll~ el ;I]..

I99 I1 Growth ~II I~II~~I~I~II tilodclr \uch A

BEANGRO (Huogcnho~~nl ct al.. 1988). PkAN1:TGUO tBoiite ci al.. IYX5). and SOYGRO IJonc\ and Zur. IYX4) nre hcing u\ed to prcdiut thc ~ntcgr;ited ct'l'ects ti1 dil- Iercnt mcchani\~ns in the comcxt ~~ l ' vur~ah le climat~c conditi~nr and agronom~c practicer. However, the ucrfulnesr of s~mulation mod- eling depends on the avarlahilit of \ull '~- c~ently rohuri niodels Icir ihe particular crop arid suff~c~cnt undcrrtanding 111' the traii and 11s mode ~iloperation. To our knowledge, in the care {il plgeonpea. ch~ckpea. and munghean at Icaat, such models arc no1 in such a rufficientl) advanced state to u\e I i r this purpo\e.

D. Physiological Mechanisms - The Underly ing Phi losophy

Plants are highl) intepratcd organisms. and when \trei\ disturbs proce\ses in the rystem, a variety of control mechani\m\ are brought intu play to adjust olher proceqses so ar to maintain functional balance and thus cope with adversity (Hsiao and Acevedo, 1973). It i\ important. however, to differen- tiate the physiological mechanisms that as- sist the plant to survive and grow under moisture-limited environments from mere physiological consequences o f reduced growth and injury. Most of the physiological

information available only describer the con- tequences rather than the cau\es or mecha- nl\ms that could contribute to ma~ntained productivity under mt~isturc defic~t (Jones and Corlett. 1992). Al\o. when genotype\ are evaluated for their metahol~c effic~ency under moisture-defic~t environments, care \hould he taken to \eparate other factors, \uch as dillercnces in plant water status. that could mask metabolic difference\ (Singh et a1 . 1973).

Plant\ arc honicl~static sy\tcm\ with rnuch coupl~ng and hull'ering hetween pro- cesses. and w ~ t h a set-point for the \un,ival 01' tllc genclme IHardwlck. I9XXl. In engi- neering term\. "the "critical trail" in a ho- meo\tatic syslem i \ not Ihc rille of any one procc\\, rather i t ir the dclcrnl~nallon o f the %el-point" (Cram. l9X.31. Thus, it may not he easy to define which of thc phy\icrlog~cal mechanism\ I \ the most cr~tical in detcrmtn- ing droughl re\ist:rnce in a g ~ v e n crop Many report\ are a\ 'a~lahlc on nonl~near itrringl? huffered plant responses. such as \t;irch in the pod w;~lls 01 \oyhean (Fader and Kc~ller, 198.5) and stems of Viqrlrr rrrdil~rrr and V , nzutrqo (Muchow and Charle\-Edward\. 1982). Physiological sclecuon criteria involve two dnma~ns 01' drought \tress: the domain when \tressed plants rnainla~n a p ~ s i t ~ \ e carho~i halance, and the domain when plant\ enter a situation of negative carhon halance ;ind are merely huniving. Seleclion for re- \istatice in the first doma~n in\olves tactors that \uslatn ~ranspirntion. efficir.nt moisture ex~raction. higll WCIE, and h ~ g h HI (Blurn. 1988). Selection for resistance in the second domain involves the conservation of \iahle meristems and the capability of plants to ruhrequently recover and prtduce a reason- ahle yield.

E. Conceptual Framework for a Physiological Genetic Approach

The underlying theme of any crop irn- provement program is to develop cultivars

capable of utilizing the target production environment to the optimum extent geneti- cally feasible. A clear conceptual framework is necessary for integrating the analytical approach of crop physiology into the prap- matic approach of breeding a crop for mols- ture-limiting environments. We helieve that the conceptual model\ that could he used for the rainy season and postrainy seawn grow- ing environments are different. In the first case. the crop growth model proposed hy Duncan el al, 1978) would \eem more ap- propriate and, in the \econd case. Pars~oura's (1977) model could he used. Although horh models can explain and predict yield accu- rately. the choice (if the model for a given cnvirclnrnent I \ determined hy the ea\e with w h ~ c h components ol'the miidel can be mea- \ured. Although the\e framework\ are quite \ i~nplc and brnad. they nevenhelesr help focus on a w ~ d e range of physiological. rnorphnlr~g~c;~l. and developmental attribute\ of pos\ihlc \~gn~ficance to drought rerirtance and. turthernmorc they help aspess the im- portance of these attribute\ IPassrc)ura. 1977. 19921

Duncan's (Duncan el al.. 1978) crop growth model de\crihe\ yield a\

where. Y = yield of pcd or seeds (kg ha I ) :

C = mean crop growth rate (kg ha day-'): D, = duration of reproductive growth (days): p = mean fract~on of crop growth rate pani- tinned to yleld (Y) . This can he derived by dividing the mean rate of yield accumulation (YID,) by mean crop growth rate ( C ) .

Passioura (1977) cons~ders that. under moisture-limited environments, grain yield is determined h) the relationship:

where. Y = yield (kg ha-'); T = amount of water transpired (mm ha-'); TE = transpira- tion efticiency (kg ha-' rnm-'1: HI = harvest index.

Each subc'ompnnent in hoth mciiel\ i\ an integrated function of a numher of morpho- logical and ph)siological altnhures [Hard- wick. 1988 I. The relative importance of these attnhutes and their expression in the optl- mum exp l~ i ta t~or~ of a target product~on en- vironment Ithu\ in determin~ng yield) bar- ie\ across environments. Thus. one could u\e this analytical framework to tdent~f) and a\\ers the limrr~ng factor\ and the phyrl- ological attnhute.. nccc\her) to opt~rmii~e the \arious components ( ~ t the model in order to best exploit a givcn production en\.ir~inment. No\el approache\ are n\a~lablc to quont~fy the \hool growth raw\ and part~ri~ining on a

thernmal nme ha\^\ (in ;I li~rge \tale. using nonde\tructi\e n~elhodtilngie\ (William* and Siixena. 199 1 ). T ra~ l \ ( m ~ r p h ~ i l o g ~ ~ ~ a l and phy\iological) should he e\alunted in term> (>I thc~ r funct~cinal relationsh~p and the \mrenfth of thc~r c~~rrelaticlnr to onc of thew comp~inenrs iKranmrr. 1980: Ludloa and Much~iw. 1990) having rnaxlmunm as\ocia- tlon u ~ t h yield In the growth niodel.

We heliebe that ;I more dircc~ed ph ) \~ - ological approach I\ relc\ant lo genetic im- probernent con\~deratirins. The two main approaches that we \ec for achlevlng t h ~ \ are the "black box" and "phy\~i~logical ideotype" approache\. For the \ake (11 L.on\enience, we dr\cu\s these two approaches separntely, hut bo~h c ~ u l d tomi componenla 01 an ~nteprated approach of a crop Impro\ement program. The "black box" appniach leads to the iden- t~fication ofpotential trait\ and genetic st~)cks that would form the huilding block\ for de- veloping a physiolog~cal tdeotypc, which acts as a conceptual framework for the genetic improvement program.

1. Black Box Approach

The black box approach proceeds from e\tablished phenotypic differences, that ia. from differences in drought resistance, to the underlying morphological, physiologi- cal. and biochemical mechanisms (Fischer.

IYX I ) . Genotypes are ev:~luated in the target environment to eslahlich genetic difference\ in drought resist:~nce hased on hieid or) icld- denved \election indices. Mult~\:ir~ate sto- tist~c\ can he e~nployed 10 ~dent~fy poten- t~nll! uh~t'ul c ~ ~ i i h i n i ~ ~ ~ o n \ t>f triiilh thal correlate a ~ t h qicld or y~eld-derived indice\ (White. I O X X I Once a rource of drought re\lst:ince I\ ~dcnt i f~cd in the cultivnled \pe- CIC\ or it\ wild r~I :~t i \e\ . the ncxt \tep is 11)

undcr\t:lnd thc ~ncchani\nm\ i ~ndc r l y~ng drought re\ist;incc. Simple and cfl'ccti\c me;lns 01 rcrecnlng refrcgat~iig popul:iti~~nr for \pecific p h > \ ~ c ~ l ( > g ~ c ~ ~ l or ~iiorph~ilngic;~l altr~hute\ mu\t he dc\clc~ped tor a auccc\sful hreed~ne program. P:irtlti(~n~img dilf'crcnces arnung groundnut genotype\ in drought cn- \ ironniemr hate been reporled con\~stentlq and Are attributed to dit'fercnccs in drought rev\tancc tGreenherg ct :iI.. 1992). How- ever. 11 i, Important I(> undeisland different \tratcgie\ that genotypes adopt in thc~r re- productive hchavior. which rcllectr theirdif- Ierenccs In part~tioning and thuh drought rc\l<tnncc: once thi\ is under\tood, i t then \\III he much easier t ~ c h o ~ n e strateglcr that \ U I ~ tlie target cnvinin~ncm. For cxernplc. griiundnut genotype\ dllfer in thulr pod dcvel- opmenl attr~hutes: genotypes such as TMV 2 and Kohut 33-1 can produce and fi l l pod\ ;it a moderate. hut con\tant, role irrespective of the ch;~ngc\ in the intens~ty of the drought (Ha r r~s ct al.. IYXX: Chapman et al.. 1993a.h.c). Thl\ adaptation is hest \uited for Ierrnlnal drnught \Ire\\ environments where the growlng season 1s more predictable. Thus. the ahil~ty to maintain reproductive devel~ip- rnent in dry \oils may play an important role in determining their adaptation to these en- vironment\. In contrast, genotypes such as Kadiri-3 continue peg initiation during drought period\, but pod development \tops until moisture deficit is relieved iHarris et al., 1988). Therefore, this genotype is more suited to environments where the moisture deficit is intermittent and unpredictable dur- ing the growing season.

D~fferent adapted landraces or cultivars have evolved a variety of mechani\ms that contrihute to yielding ahility under a glven pattern of moisture avai lah~l~ty. Genotypes attain a level (I! drought rcwtance through thew own combination\ of various physi- ological attribute+. F11r Instance. one geno- type may attain a glven level of drought resirlance through itr deep root sy\tem. whereas another may attaln the same level of drought re\istance through it\ higher TE. However. \ome genotqpcs may have hoth artr~huter. This wa\ the case In groundnut, wherc hlgher TE wa\ a\rociated with In- creased root grrlwth (Wright et al.. 19941. However, In groundnut. TE i > ncgat~vely currclated with HI (Huhlck et al.. I Y X X : Wright et al.. IYXX. N a p w a r a Rao el al . 1993). Thur, the rame level ot drought re>!\- tence expre\\cd In lerlns 01' ye ld can he attalned through c~thcr h ~ g h pan~tioning :ihll- 11). or higher TE ILcgu~ne\ Program. 19921. Soyhran genotypcr with deep rooting uhar- actcr~sticr ha\c rclali\el! p m ~ r OA capah~l- ~ t y and the rever\c i \ true liir pcnotyper with shi~llcrw n)oting cliar;ictcr!\t~cs tCorte\ and Sinclair. 1986)

The underlying ph~lo \ophy 15 that. 31-

t l i o ~ ~ g h d~fferent punotype\ rnay ahou the ramc level (11' drought resi\t:lncz, they can attain this through d~ft'erent phy\iolog~cal mechan i \~n \ or nttrihute\ (Ace \edo and Ceccarelli. 1989; Cuccarcl l~ ct al.. IYY I 1 The\e can he iden~ified using the hlach hox approach within the conceptual Irame- work descrihrd earlier. T h ~ s infiir~nntion will ;isrist in ident~fying potential parents having co~nple~iientary physiological at- tributes and also will help to direct selec- tion in the segregating materials toward these specific phy'ilological attributes. Thi'i approach may lead to development of ge- netic materials segregating for a higher level of drought resistance and also Im- prove the chances of selecting for improved levels of drought resistance from these populations.

2 Physiological fdeotype and Pyramiding Approach

An ideotype i < defined as "a hypotheti- cal plant dewihed in terms of traits that are thought to enhance genetic yield potential" (Donald. 1968: Rasmu\son. 1'1") 1 . A physl- ological ideotype for drought resi\tance could he defined in terms of spec~fic physiological traits expected to contribute functionally to optlmlte yield product~tm and jtability un- der moi\ture-deficit en\ironments. Howeber. 11 15 Imperatc\e that the climat~c regime for u 'h~ch the plant ir being dertgned he thor- oughly under\ttrod brcau\e a glnple univer- sal idrotypc will not he adapted to all mu]\- lure-11mitt.d enulronments. Rather, many biological model\ will he required because thc mo~sture-dclicit environments are d~vcrse ( W ~ l r o n . I ' ) X I . Lawn and I m r ~ e . 1991. Sedgle). 199 I 1

Conccptu~lly. the phy\~olog~cal apprr~ach tor Iniprovi!ig drought re\l\tance in crop plant\ \hould hc to hring together varlou\ physi(lloglca1 traltr that complement each other In a p)ram~rlic (building block) man- ner by relecti\e ~ncorpura t l~n of these tralts into a single cul t~\ar , that is. accumulation <if seberal. prohiihl) ~ndepcndenc, phyaiologi- cal i c c h a n ~ r m s into a cingle cultivar. An dnal~jg) ciln he draun from disea\e resl\- lance breed~ng: horizontal re\lstance (defined as rerl\tance to a number of physiological race> of a disease) can he achieved by pyramiding different genes carrying resis- tance to indlvldual physiological races into one cultivar (Prcrlevliel and Zadoka, 1977). This contrihuter to stability of the cultivar ~ I C ~ O L years in disease-prone environments. The same concept also can be applied to drought resistance, wherehy pyramiding of genes that regulate different specific physi- ological traits into a single cultivar could provide that cultivar with the necessary ge- netic means to respond to the d~fferent types of moisture deficit that it is likely to experi- ence at different locations. sites. and years

within a specific reglon. The various steps invol\,ed in t h ~ s kind of approach are

I . Define the var~ous physiological tralta that habe functional significance for product~bity in a given crop under un- ter limited environments using the con- ceptual models menuonrd In Section W E , realiling that the t ra~t require- ments will vary depending on the targct enbironment.

2 . Establish the genetic variahillty and locate sources of high level, of cffl- ciency I'or each physiological trait in the germplasm or itr related wild spe- cies. that 15, selec,tion sh(iu1d hedirected touard the individual components of drought rerirtance on s trait-hj-tra~t haris, irre\pective ol the yleldlng nhil- ity 111 the genotypes carrylng these traits. This can he done effectivelq through establirhing physiological nurserle, for screening rpecific traits s ~ r n ~ l a r lo dl\- care-\creening nu~serie\ .

3. Ehtahl~sh the genetic hahi\ lor cach physiological trait under consideration by studying it# inheritance, and ertl- mating heritability to determine the fea- \ihility of using that particular trait in a breed~ng program.

1. Develop genetic markers. huch as ran- dom amplified polymorphic DNA (RAPD) and restriction fregment length polymorphisms (RFLP) markerr, !feu\- i14 identifiable morphological. p h y i - ological. or olher markers are not readily available for each physiological trait. as this can increase the efficiency of selection from segregating materials.

5 . Identify genotypes for each phy\iolopical bait that have good combimng ability.

6. Incorporate relevant traits into agro- nomically acceptable hackgrounds.

Information generated through this exer- cise could be stored in a database and made available to plant breeders interested in in-

corporating drought resistance component\ into their hreeding programs. T h ~ s is s im~lar to the datohases a\,ailahle f'or ~norphological traits from the ger~nplasm evalu;~tion pro- granis at the Consnltatibe Group 11r Intcrna- tlonal Agricultural Research (CGIAR) Cen- ters.

3. Development of Genetic Markers for Physiological Traits

Most phys~olug~cal traitr are controlled hy onc tc10 many genes located throughout the chromosome complrmtnt Each 0 1 the ttidt~'idual genei o f ' u ~ . h a p(ilygcntc. \ystct,, contrihutc\ ;I snlall poritibe elfict to the trait of Interest. Clear dominance I \ not exhih- ~ ted , and the phenotype ha\ a large conipo- nent of en\~rt~nmcntal ~a r i ance . All I I I L . ~ ~ characteri\tic\ conspire to ~nahc phy\nilogi- ciil trait\ wry difficult (11 ani~lyle Thus. convent~onal Mundcl~an ~ n c t h ~ d s 111' annly- sir. u h ~ c l l ;ire ruitahle lor traltr controlled hy a \ inf l t or a l'ew gene\, cannol he appl~etl ti) ;inalyh~r oi'there ph)\icilogical tralts. Thi\ ir one or the rea\cin\ why physiohiplcal trait\ have not heen urcd extensively In drought resistance hreed~ng programs. ;iltIiough rcv- era1 phyriolog~cal t ram w ~ t h tunctional rig- nificance tn product~on under nioirture-lim- ited enblronmcnts have heen idenufled hy crop physiologists (Ludlow and Muchciu, lYL)O) and some breeders have advocated their judicious incorporation into hrecdrng programr (Rasmurson. 199 1 ).

Deuelopment of genetic markers. such as RAPD and RFLP, for each of the physl- ological traits contributing and insorporat- Ing drought resistance should increase the efficiency of \elect~ng these physiological traits and ~ncorporating them into an im- proved culuvar (Paterson, 199 1 ). It is pos- sible to analyze complex polygen~c charilc- tern such as physiological t rair a\ ensembles of 5ingle Mendelian factors using genetic markers (Tanksley et a]., 1989). Because

KAPD and KFLP marker\ can he used to +imultaneou\ly follow the segregation 11f all chromosome segments In a cnisstng program, the has~c Idea I\ 111 detect corre- latlon3 betwern the phy\iologlcal tralt and \pec~fic chromoatime segment\ marked hy KAPD or KFLP I f corrclat~ons ch~st. then the inlercnce I \ that the ~hr irmi i \ome \ep- ment must he involved In the quantitat~bc trait. Thc dlff'lcult p:irt in thl\ procedure IS

e\tahli\h~ng c~r rc la t i<~ns hetwecn the tralt and \pec111c chroriiii\omc scgrncnt\. The KAPI) and KFLP marker\ can he ea\lIy \crircd, hut the ph>s i~ i l~ ig~ca l trait mu\[ he charucteri/cd In the con\entional fashion tTanksluy el al.. IOXY: Rafalsk~ et a l . I YY I J . Oncc t h ~ \ mo\t difficult proce\\ i \ complctcd, and \pecific clironios~me seg- ~nent\ arc ~mpl~catcd in the tralt. plant, carrylnp KAPI) and KF1.P markerr u ~ t h ;I

p o \ ~ t ~ v c ctlcct on a quantlt.lll\~i. tr:tll can hi. \i.lrctctl Irorn :I \cprepating populal~lin Thi, 1s pos\~hl' hccauw \inflc plant\ can he c:~\lly scored tor \c\r.l-al KAY[) (lr KFI-P rn;trkcr\ \~multan~~c~u\l!.. lrcc ironi en1.i- ro111ii~ntal I~IILICIIC~ Or gene ~nteracrion. C;~rhon ~\olopc I"(') ~ I i \ c r~m in :~~ ion uur rati\t';~ctorily prrdlctcd I lom three KFI.P\ In tonlatil tMar t~n et al . IYX'),. Sitii~larl!. osrnr~tic ail,ju\trncnt In \orghulii I\ ~ep<irted to hc linhed h i l h two KAPD marAcr\ (I.udlow el al.. 1993) Thur, the an;~ly\ir of qnanutatlvc tra~t\ . \uch a\ physlologl- cal ch;iracter\, can he carried out hy con- \en t~ t~na l Mendellan analbria. Howe\er. this approach may he I~mited to ;I fcu he- lective traltr ct~nr idcr~ng the practical d ~ l - ficultics in\ol \ed III de\elnping these pr - netlc niarlr.r\ ior a given physiological trait and also the linl~tauona assoelated with generating large numhers of early segre- gating materials to get a desired le\el of recomh~nation i i onc has to handle several traits simultaneously in a hrecding pro- gram (Marshall's "tyranny o f numhers") (Marshall. 1991 ).

4. Limitations Associated with ldeotype Concept and Concluding Remarks on Physiological Genetic Approach

The concept of "dynamic physiological ~deotypes" that is intrtduced here could oter- come some 01 the conceptual problems a s w ulated with the 3tatic identype in guiding the crop lmpnibrlnent programs 10 exploit \pc- cif'ic adapr;ltion 01 genzuc mechanisms to well-defined, drought-prone target produc- tion \).\tern\. Ncverthelc\\. wrne of the In- hercnt conceptudl and practical diit'~cuIt~e\ a\\oclated with the atatic ~deritype approach will still pcr\l$t. Some o i the most Important 011e\ are

I The challenge\ a\\ociatcd w ~ t h charac- terilation l r l thc target drought en\l- ronment, ;I\ [hi\ would determine the appropr1;iti. h~olop~cal model(\) required lor :I crop \arict)

2 . I l~ l ' l icul t~e\ ar\oclated ~21th dcterrnin- I" the approprlatc hiolt~gicill model lor a target cn\wronmsnt, glben the con- ceptual d~tflcultics \uch as the unllheli- hood of the\? he~np a sinele optlmurn h~ological model tor a given target en- \ironn~ent: a turnher of h~ological model\ may ha\? the \ame level of efficietlcy. T ~ I \ opens up more optitin\ 10 the breeder\ to choose the right com- hlnatlons of waits. taking into cons~der- ation the practical dit'ticulties of incor- porating cendin physiological t r am

3. The dlfficultieh associated with finding appropriate genetic bariability for a gi!en phy\iological trait, and the limi- tations assoc~ated with developing the appropriate screening methodology: the amount of f~nancial. personnel, and technological resources required to operate such a breeding program for incorporating specific traits could com- pound these problems.

4. Interrelationships arilong individual traits and compensation among traits to maintain the internal merahol~c honteo- stasis may prevent the breeder\ lion1 realizing the desired Impact ot' an im- proved metabolic efficiency on pheno- typic expres\ian.

5. Pleiotropq I I e .one gene ait'ect~ng n i (m than one character) ie g.. the negative relatton hetwern TE and HI In ground- nuti could oh\cure e\pres\lon ol' an imponartt trait and in rcalirat~,in of the expected ~n ip ro~ement r ~ f ett'lclrnc) (11 the phen~it) pe.

6. Genetic hnclgri~und efkct \ . ruch 4,

when dehirahle gene\ erpre\\ only In inferioi parent\

Cenainl! the phy\iol<~g~c;~l-genttic ap- priiach IS not an ea\y \olut~on to :11l o f t h e prohlrms associated \rith genetic inipn)\e- mrnt pnlgrnrnr aimed nt d r ~ ~ u p h t - p n ~ n e cn- vironnient+. Howc\er. the dynam~c phg\i- ologlcal ideotype\ approach could prokidc ,I \ i~und conceptual frameworh I l r hetlcr de- fining hreedlng ohject~\cr . and thu\ w<~illd as\!\t In dirccting genetic impro\etnent pro- g'amp. This can guide breeders in direct~ng their eftiinh more \pecifically tuward \elec- tive trait+ or combinations of trait+, thus pro- viding more oplions to okercomc \ome 01 the practical and conceptual limitat~trn\ a\- s tuated with the trait-based appri~uch Hreed- er\ mny henefit from the trait-based appn~ach by specifying the desired goal of phenotype for each uait. thus goal \etting and generat- ing of hypotheses wlth~n breeding program+ would induce the necessary \cient~fic tem- per (Rasmusson, 1991). which we hetieve would certainly be heneficlal to the program. However, the physiological-genetic approach that is advocated here should not he qeen as an alternative or substitution to the empirical approach in which selection is based on yield or yield-derived indices; rather, it should be seen as complementary to the empirical ap-

proach. with the a\r:lreness ot' the pr:li'tlcal ad\antagr\ ;~ssoc~ated with thc latter op- pn~ach. There are d~lt 'icul~ies ;ls\~ciated with the t r a i t - h a d appn~ach and practical limi- tati~in\ in implemenl~ng thi\ approach. Thew hale hcrn cotnprehen.iively h~ghlightrd hy Laun and lrnrle (IYY 1 1 . Mar\liall i 199 11. Ka\ni i~\w~n i IYY I I , and Sedglcy i 1991 1 .

Houever. ~den t i f~cn t~on and ch;iractcr- i~ot ion o fgcne t~c \ t (~cks that h:ne key 11ict;l- hollc tfllciency trait\ (wch a\ O A . TE. and the dh~llty lo r e m ~ h i l i ~ e prcantlicsis-stc1rr.d non\~ructi~raI ~i~rhol i! dr;lte rc\cr\er f1irgr;11n till~ngi. 'ind hy ~n t roduc~ng \uch tr:llt\ into \~iit;ihlt~ ad;ipted ~ C I ~ C I I C hi lcI \gr(~~~nd\ . c(111ld hn\e n 1113jc11. i~iipi~ct 011 crop prt~dui'tnin. and lhu\ cl.op ndapt;llirin to di~~ught-protic cnvi- rcrnrnents. One 01' the nio+t succcrt 'ul c x - ;~~ i ip le r 01' ,I tr'~ll-h;~\ed appro;lcIi I\ hou "Norm- 10'. dwnrling gcncs rcv~lutioni/cd the genetic yield potenlial <rl'whe;il all over the world iR:~\mus\on. IUVl). 'rhu+. with the potential liv uidc\pread I I \ ~ o l '~ntpr~\ .ci l genruc stoch\ in rerc:~rch and hrecd~ng pro- :rilm\, and h! sh;lrlng the uork 01'incorpo- ratln! \pccilic phyri(11oglcal tr;llh into rupe- rlor gent t lc h; iclgr t~und\ . i t \hould hc po\\ihle to develop "phy \ i~~ log~ca l trait col- ICCL~(I I IS" . ju\t a\ thest are ccrllectlons of genetic stock\ dnd ctrllecl~ons 01 gcnrs lur d~ \ease re+i\tancr iKa\lnus\on. 199 1 ). Thih would he definllcl) ;~d\antageous f ~ r long- term crirp iinpro\cmem need\ and, keeping i l l mind thtlt thc cnvinrnrncnral ctinditions. pan~cularly rainfall pattern+ ol'a plven agro- ecl~logical production zone, are continuously changing uver rime. thus necr+sitatlng the dekelopment of new biological mcidel\ to cope up u ~ t h changing physical envlron- ments.

V. FUTURE OUTLOOK

It is difficult to quantify the impact of' moisture defic~ts on crup yield losses on a

glrihal \tale, considering the transient nature of the problem in the many differing agroecol<iglcal producticin environments. Perhaps the complexity could be minimized by better defining the target production jys- ten)\ In term5 of geographical houndaries and \oil, climatic, and cropping +y\temc character~+tics herein. Then, hetter directed genetic ~ m p r ~ v c m e n t may he pri\\ihle elther hy an cmpirical approach, or hy Incorporat- ing jpecific p h y \ ~ ~ i l ~ i g ~ c a l rralts that would enhance drought resistance. Genetlc strate- gies can he ciptimlzed only to help nilnlmize the impact on legume pniduct~vity nt moder- ate level\ of moi\ture deficit that are com- mon in rain-fed production sy\teni\. The level (if improvement rrqulred and the fra\ih~lity cii realiling rhc h i r e d ~niprob~enient will vary Inlm cnip tci crop. Thi\ \r 111 depend on the inherent ahility of the crop to t~ilcrate dry envir~~nmeiit\. the amount 0 1 genetic tari- ahility in physiological component\ (if

drought re+istance. ;lnd the ;ivailahlllty 01' rcientific and cctinoniic resource\ lor de- plciyment to crop impro~emcnt . In thc more developed ctiuntrleq. ~mprtiving drought rc- slstance laberage pn~ductivit)) over a tinre span (e.g., 10 10 20 yearr) may he realistic and acceptable. Howcber. In lew-developed c~iuntrie\. survival of thc \uhristence farm- er\ and the general economy depend\ en- tirely (in ~niniiniiing the prohahilily (if crop failure. This can 111 some extent he addressed hy adopting short-temi drategieh (if incor- porating various physiologtcal d r f e n c mechanisms into crop cultivars to allow a cenain level of realtied yield in a more reli- able manner. Howe\er, this is likely to be accomplished only at the expense of yield potential. Despite the difficulties, the ultl- mate test of any drought irnpro\.ement en- deavor is the demonstration of lmprnved yield in drought situations in farmers' fields. Quan- tification of this impact needs to he factored into the overall genetic improvement pro- gram to allow assessment of the efficiency of that program, and hence guide future such efforts.

The previous 2 decades of research on drought resistance in legumes have led lo a vastly improved understanding of the vari- ous physiological mechanisms and strate- gies crop plants have evolved dunng the course of their adaptation to motsture-deficit environments Thts has led to a better conceptuallzat~cin of the biological models that are appropriate to various drought-prone environment\ Alsri. our ahllity todefine and characterize the target envin~nment ha\ Im- proved recently uith the availability of \oil- water balance model\ and techniques \uch a\ CIS. We heliebe that. if properly Inte- grated. ttieje recent development< of phqii- cally charactcrlzing the envlnlnment and appropriate biological models ct~uld fomi the ha\i\ f i r t,i~laring gt'notgpe\ wing phyii- ol(1glcal. genetic. breeding. and management \ t r a t e g ~ s f o r better utilization (if drought- prone target production en\lrtinmenr.

ACKNOWLEDGMENTS

The author\ thank Dr\ D. J. Flower. F. B Lopez. J H Williams. and F. R Bldinger for their helpful comn1ents. discus- \ions, and 5uggesrion\ during the prepara- tion 01 thii manu\cript.

REFERENCES

Acevedo. E. and S. Ceccarelli: Role of phyrlolo- g~st-breeder In a breedlng program for drought rrslrtancc. In: Droughr Rcriirnn~ e in Crrrul,. (pp. 117-1 39). Baker. F. W G.. Ed.. CAB International. U.K. 11989).

Ackenon. R. C.: Osmoregulation in conon In response to u,ater stress. Plnjrr Phy~lol. 67: 189493 (1981).

Aparicio-Tejo, P. M.. M. F. Sanchez-Diaz. and J. I . Pena: Nluogen fixation. stomata1 re- sponse and transpiration in Medrra~o sarir,a. Tnfolium repens, and T, suhrerraneun~ un- der water stress and recovery Physiol. Plant. 48: 1 4 (1980).

Pl i i t~r Brerdirig i ~ r rhu lYY(is. (pp I 8 1-200). Stalker, H. T and J. P. Murphy. Ed,.. C-\B lnternat~onal. U.K. I 1992).

Br~gg'. L . J. .lnd H. 1.. Shant~: The water re- quirement\ o f plant* II A review of the ltterature. i!Si)A Biir. /'/tir~i 1110 8lfIl Xo. 285: 1-96 1 l Y i 3 1

Broun. S. C.. P. J Gregoi.!. P J \I Col,per. 2nd J D. H Kr'!tingc: Koot ;III~ <hirot p r o u t l ~ illid w.lter 11,e i l t c l i l ch~~c ;~ I ( i ~~ , ' r o , , ~ , l ~~ i i i ~~ , i ) grown In dryl;)nd c o n d ~ t i o ~ ~ \ clIzct\ 01 \ou - ~ n g dateatid genot)pe..l A q ~ i i \ < I , 1C~itrt1, i 113: 41-49 110RY1

Buii t~ng. A H .(lid D L. ('i111i\ Li>c.il ;t(Iapl,~- tliin o f \(~rfhuln bdrietle. t ~ i ~ ~ o n h c r n N~ge - ria. AIittt(ic11f 1<1,//0 1 IIII. Sci~iiair~f R,', I j i i l / 30. 106 I 1')701

Ruihhy. H. V A, and R. J L'tun. Acci~ini~l,~t ion of parlltlilnlllg ill nlttogen nnci dry l na t t r~ h! contrarrllig gmnoi)pc\ 01 rnunfhean r \'rqrt<i ruciraio L , WiI~~:ek,, ,41i\i. .I A ~ r t < . H?\. 43: 1609-lh?X tlYY2I.

Buttery. B R . C' S. Tan. K. I Bur / r l l . J. D Gaynur. and D C XlaoTd\~\h. Stomatill number, 01 \oyhean and re\pon\r to uatcr +tres*. Pior~r ,%,ti 149: 283-288 ( lY(1i l

Campbell. G. S, and R. D I~L : S~mp l~ f l ed ~ 1 1 1 -

water balancr model\ to predict crop tran- \plratron In: Droughr R~\eurr. i i Pr ior i r r r~ for rhe Dr!l(trtd Tropic.\. (pp IS-26). Bidtnger. F. R, and C. Johansen. Edi.. ICRISAT, Patlincheru, India. ( IYBXJ.

Caner, P. R.. C. C. Sheaffer, and W. B. Vmrheer: Root growth, herbage yield. plant water \la- tu\ of alfalfa culttvar\. Crop Sci. 22: 425- 427 (1982).

Caner. P R, and C. C. Sheali'er: Al ial f ,~ re\ponre ti) v~ i l water d r f~c i t \ 111. N i>dula t i~n 11nd N, Fi\.~tiot~. (',vrl, Xi r . 23: 985-9YO ( 19x31.

C:i \ to~ig~a), > diid A H Markhart: Saturated r.uc\ n f photo\?nthe\~, In ui l tcr \trei\cd It.;t\ei ot comnioti hc;m ;ltid teptiry hcan ('ri~/r Si,i 31: IhOZ-lhl l I 19'll I

('eccarrll~. S.. F Acc\cdo. .III~ 5 . (;r.tnd(~ Hrred- ill; tor ) icld \tdhilit) in ilnplcd1ct;lhlc en\ I- ronlllclit\ Stilplc Iralt\. Intcr;lctt<rtl hctwt'cil ~I,III\, -IIILI . i ~ c h ~ t c c t t ~ r e ( 1 1 s e ~ ~ o t y p c \ b.ii/~l~\rit (I 56: 169- 18.5 ! I W I ).

C~I:I~~III.III. A I. , ~ ~ t i l R C ' v l ~ i i l i ~~u . NII~II~CII : i ~ ~ ~ ~ i ~ t i i i ~ l . ~ t e ~ l ,111cl p . ~ r t ~ t ~ ~ ~ r i ~ ~ c l ,I[ !lt,iturlty h) gr,lltl I cgu i~~c \ ~ I O U I I 1111clr.r i l~l ferent w.ltcr replilic* in ,I ie i~ i t -d l td II.O~IC:LI e t i v i r ~n~ i i e t~ t 1 i<,/tI C'IO/~\ Kc,, 11: (>~1-70 (19851

CIi:~pnid~i. S (1 'l'he EtIcet ill IJrui~ghl 11~r11 ig Relxuducl~\e Dcvcli,p~~rent 011 the Yield ~ l f

Ct~I t~ \ : i r \ 01 G r < t u ~ ~ d n ~ ~ t !,111i(Iii$ IIV/>,J~<I~,~I L . I. Ph D tI ie\~\. I l n~ \ ' c r \ ~ f ) (11 Quecnil:tnd. B~l \h ,~nc. Au\tr i t I~,~ ( l4X01

Chapman. S C.. M M . Ludlow. I.. 1'. C' Bldlitcy. illid K . S. FIIC~C~. f:tfccI (11 dr<>upht durttip edrl) repr~~duuti\.e dcvc l< ip i?~e~~l on gruwth i>fci1111\.1r\ ~~ fg ro~ tndnu t lAr<i,111\ I ~ L / J ~ ~ C O ~ ~ ~ I l I I t l I i l ~ / , ~ t ~ i ~ n 01 i . id~i i t~on c ~ ~ l c l m'itlcr ~ U I .

ing dir~ught, l i< , l i l ('rijpi Hr,. 32: 197-210 (lvcj3aj

('hepm.tn. b ('.. hl M. 1.udlow.I.. P. ('. Bldnicy. and K S F~\chcr t l t c c l (11 drriupht durtng c:lrly repri~ducti\c dc\clirpnicnt [in gn~wth (11 cult t \sr\ of gr~~undnut lArc~< h,\ h !po~o r i i L.). I1 B l i~ma+* production. pod dcvelop- mcnt and ) ~ c l d Ftcld C r i ~ p < Re\. 32: 21 I-- 225 (1Y93b~.

Chapman. S. C.. M. M Ludlow. F P. C. Blame). and K. S . Fhchcr: Et l tc t \ <rfdrought during pod f~ l l i np on u t i l iw t~on o f water and on growth of cultivar\ of groundnut (Ar~i1,hr hvpoqueu L I . F i u i dCrop~ Re\. 32: 241-255 11993~l .

Chauhan. Y. S.. N. Venkatratnam, and A. R. Sheldrake: Facu~rs affecting growth and yield

ol jhon-duratton pigeonpea and it \ potentla1 lc~r multiple harve\t\. I. Ajirrc. Sri. (Cumh 1

10% 519-529 (19x7)

Chauhan. Y S . Ptgeonpea. In: Rooring Porrcrn n/ Troprcul ('ropi (pp. 79-9 1 J. Salam. A and W. A Wahtd. Ed,. McGrau, Hill. l nd~a (1993).

Ciha, A. J, and W. A. Brun Stornatal \17e and lrcqucncy in \o)heaii\ Crop S r i . 15: 31% 713 (19751,

Cl;trke. J. M. . I' F. T~mnley-Sm~th. 7' N McCirig. and I) ti Green (jrrlwth analy\t\ 01 *prtng whcat iul l t \ar\ (11 \;lrytng dri!ught rcrt\tance. (',rip Sl I. 24: 537-511 11')811

r l a rk~ . . J. M . S. R~rmapo\;t. S Jana, J. P Sriva\ta\~. and l' N, hlcCaig: Krlation\h~p i)l cxciscd leaf u;rter l i~\\ rille and )tcld of duruni uhritt In diverke environment\. C'NII. .I. Plutil Sri 69: 1075I! lXI I IYXV)

ondo dot^. A. G . K A Rtchard\. and C. D F:,rrcl~~har Carhim i\otopc d t ~ c ~ i l i i ~ t ) i i t t ~ ~ n I\

p<~\ i t~vely cilrrelaieii wtth grain y~s ld dnd dry tnarrer ~ ~ ~ ~ U C I I I I I I 111 f ~c ld grourt whrdl (.rap S~I . 27: OYf+l!X)I 1 IYX71.

C'on\lahli.. G. A attd A. B. Hedrn. A~~OIIOI~IIC and phy\tolog~cJ r r i p o n w t11 \o)he.tri and \orghum cmph to udter deficit\. 1. Ctou'th. dcvrlopnient a~id yteld. 41111 .I Pliirli PIII \to/. 5 : 154-1 h7 I 197K)

C(~nes. P. M. aild I' K. Slnclair. Water relati(~n* (11' field growl1 \oyhran under d n q h t Crop S u 26: 993-YYX ( IYXh).

Cones. P. M, and T R Sincl,~ir: O*niottc potetl- tial and \t;irch accuinulati~n in leave* o f fteld-pruun \oyhean. Cr.rrp Scr 27: RO-84 (1987).

Cram. W. 1.: Chlorlde accumulation as a homco- static system: set polnts w d penurhations. I. E ~ I . Bor. 34: 14R4-1502 (1983).

Cure. J. D.. C. D. Raper, J. R. P. Pattmon, and W. P. Robarpe: Dinitmgen fixation in soy- bean in response to leaf water stress and seed gmwfh rate. Crop S<.i. 25: 52-58 (19x5).

Da\le\. J D.. 1. M. Bennett, S . L. Albrecht, and K . J. Boote: Water relations. nitmgenase and r t ~ l t development o f three r a i n legumes in re5p1n\e to \oil uater deficit\ Firld C~,~IJ Rp!. 21: 215-??h 119891.

Ujekoun. A and C. Planchon. Water \tatui et. lect on dinitn)gen fixation and photos>n- the\t\ in \oyhean, Agro~i J. 83: 316-122 (1991)

Dcmald. C. M.: The breeding of crop ideot)pc\. Etrph\rico. 17: 385403 IIYhH)

D(~lrenb(li. J and W 0 Pru~tt: Cutdelinc\ lor predicting crup water requtrcnient*. FA0 I,-. r.~gulrotr [III~ I)~(JIII(I,~C /luI>c,r 110 24, FAO, Rome. Ital) I 1975).

Ilutican. W C;: I.eafangle, leaf' area. and canopy photo\ynthe\t\ Crr~p Srr. I I: 18248.5 (1971).

Ehrrhan. S A and W . A Ru\\ell. Stahtlit) pa- ratnclrr\ for compmtng \artetir\. C ro j~ S I t. 6: 3 M O ! 19661

Ehdair. B, and J. G. Wane\: Yteld potenttal and \trc\\ su\ccptihtlir> of dumm landrace* In non\trc\\ and \trr\s cn\ ironment\ In: Ptor.. 7rh /nr U'hrui Grrti.ri( SJII~I.. (pp. 8 11- Xl5). Miller. T E and K. M D Koehner. Ed,.. Cambridge. I'.K ! 1988)

Ehdate. B.. J. G. Watne\. dnd A E. Hall: Dtffer- ent~al respon\es (IF landrace and improved spring wheat genotypes to <tress environ- ments. C~CJIJ Srr. 28: R3R-842 ! 19RR).

Ehdate. B.. A. E Hdl. G. D. Farquhar. H. T. Nguyen. and J. G. Wines: Water-use effi- ciency and carhon isotope discrimination in wheat. Crop Scr. 31: 1282-1288 (1991).

Ehleringer, J.: Leaf morphology and reflectance in relation to water and temperature stress. In: Adaptorion of Pluttrs to Water and High Te~nperarrrre Srress. (pp. 295-308). Turner,

K';. C. and P. J . Kramer. Eds.. W~ le ) . N r u York t19XOl

EkanayAa. I . 1.. J , C O'Toole. D . P. Gllrnr!. and T. M Masalo. Inheritance o f root chm- acter~atics and their relatioos to Jrought re- *!stance In rlce. C ry , S(i . 25: 927-931 IIYXSI.

Engin. M and J . I. Sprcnt: Ettect\ ol'uatcr \ l r r \ \ nn grouth and nitr~igen i'irtng LCII\II! <>I Triiolirr~n re,l,t,rl\. ,Vm f ' l i i r ,~l 72: 1 17-12h 119731

EricLwn. P. I, and D L . Ketr~ng, E\.~luatioi~ ot peiinul genotype\ I'or re\l\t.ince l o *(.illi'r \Ire\\ i n \rrir. Crop SS' i 25: X70-X70 I 19851

E7r. J hl 0.. E B. Dumhn~lt'. dnd J . E. Th t~ i l ~p . w n : ECtects 11f tcnlperarure and mc~i*turr \Ire\\ (111 the :i~.curnul;t~~i,n ot iih\ci\ir. iiciil inhe'ln I'111~roi P~~IIII 58: 171)-IXiiIYX31.

t.adci. G ht and H K. Ko l l r r Seed p o u t h rate and cdrh<rh)drate pc~ol \ i /e\ ol the \oyhe;ln 1ru11 Plot11 P/II\Io/. 79: (763-660 ( lL18Sl

F A 0 I Fuod and Agr~cultural Organl/ation ( 11 the L'nrted Nation\) I A 0 Pr.odu~~irr,t~ k>,orl>ooi. I Y Y I . Vvl. 45, P.40, Romr. lt;il) ILjY2i

Farquhar. G. D . M . H O'Lcilr!. i~ i td J A Bcrry On the rcldtion\hip hc l u r rn carhon ir~)lupc d~scrtm~natii)n '~nd the intercrllul;ir carhon d~ox idr concernratLon In lei~vc\ t11r.rr .I. Plunr P h ~ \ r n l . 8: 121-137 i IYX2I

Farquhar. G. D, and R. 4. Richatd\: Isulup~c composition of plant cdrbon ci~n.cldte\ u i t h uater use eff~cienc) of wheat genotype5 Ausi. J Plutri Phi..irnl. 11: 5.19-552 (1984).

Farquhar. G. D , K . T . Hubich. I . Tere\hrtna. A. G. Condon. and R. A. R~chwds. Genet~c xanat~on In the relat~on\hip hetween pht~tcr- \ynthet~c CO, aa\imilation and %tornatal con- ductance to water \Ire\$. In: P m g r r s 111 Phoros>nthesi~ lV. ipp. 209-2121. B~gg~ns . I., Ed.. Maninus N~ jho f f , Dordrecht. The Netherland5 ( 1987).

Farquhar. G. D.. J. R. Ehler~nger, and K. T. Hubick: Carbon Isotope discnminat~on and

Ferrre,. E.. C. ( i ~ t i ~ e n r ~ , and J hl . F~~II; I I I~~J: G e r ~ r l ~ c \ariahilit) in r~ i n t l uwe~ cu l r i \ ~o \ un- der dr<iughr. I Yield relarion\h~p\ Airur. .I A~r i , . X(,s. 37: 573-582 (I9Xhl.

Finl.!). K . W. ,inti (i N. M'IILII~~I~. The ;inal)si\ t~ f .~d. ip t .~r~o~~ in ;I pldril hiced~ng pn>granllne. :\ii,r .I ,4,qrrc X c , 14: 742-754 (lYh.3)

Fi\clirr. K A : Opt i in~/ ing the u\c 01 u.iter .ind n i l r i ~ g ~ r ~ ~ h ~ i ~ u g h hrrciling <)I cmp* l'l~rrrr \,,!I SX: 240-2711 ( 14X l )

f:i\chcr. K S . t C J i ~ h i i \ ~ ~ n . .~nd (i 0 Cd~iie,lde\. l i r c cd i~ i g ,ind r c l r c r~ r l n 1111

dn~uyht reuuancc ~n Ircrplcal rna~/c In: Drr,irgiir KC$I\I~II( c 111 Cr<yl\ ~ r h I;r:iphasr\ ,III KI(.P. ipp. 177-.1L191. IKKI. Philippine\ 1 19x2)

F~,chcr. 'vl J dnd h.1 M, 1,udIow Adaptaliw to watcr d e l ~ c ~ t \ ~n Sr,lu~irnrlrir. In: Thc Ilroi- ogt ut~dAgrononri o / J l ~ l o ~ ~ ~ i l h r , ~ . (pp. 163- 1781, Stace. H. M and 1.. A. Edyr, Lid*.. Acadcmic Pre\\. Sydney ( 19114).

Flower. D. J: The Rule of Osmutic Adju\tmenl In the Watrr Strc\\ed Leave\ o f P~gzonpea (Cujunua ~ u j u t l L. Mill5p.l. Ph.D. theaia. Unl- venity o f Queensland. Qurendand (19851.

Flower, D. J. and M . M . Ludlow: Contribution o f omo t l c adju\tmenl to the dehydration tolerance o f water rtreaaed pigeonpea (Cujunu~ c.ujun L. Millsp.) leave\. Plunt Cell Envimn. 9: 3 3 4 0 (19861.

Flower, D . J and M . M . Ludlow. Var ia t t~~n\ amony scce\\lon\ o f plgeonpea lCujo,iu$ ,UJUI~ I. MiI1.1p.i in o\motlc adju\tment and dehydration tolerance (~ f Icave\ I.ipld Crc,p\ H P I 17: 229-213 t 1987).

F i~rwth. I . N, and A. H . Teramura: Leaf energy hudgel and ial iulated tran\plratlon lhe tn- Ilurnce o f leaflrl o r~cn ta t t~~n , t r o l o n i 67: 501-571 (IYRh),

(iafl'. 1) F f 'n ) t i~p la \ r r i~~ trllcrdncc 01 extremr water \ t r r \ \ In: Adupioriort (11 Plortlr rr, Wolrr irrtd t i r r h l ,~r~p<, rc i r~r r r .Sir~,\r (pp 207-23Oi. Turner. N C ;tnd P J Krarnrr. tii*.. Wtlcy. Ncw Yurk tl')Xlll.

(i;trr~ty. I). P.. C. Y Sull~\;rn. ;~nd W, hl K~ I \ \ Altrrtidfr\'c appr~~schrr 11) tniprrnlng gr'iln urrghuln pro~Iuct~\ t t ) ondcr drcrught rtrc\\ In: l~ t~1rq111 RC~~>I~III(C 111 ( ' I , , / u ib'1r11 t.m. plt,~.!~! OII K! , c' ~ q ) . 330-350). IRKI, Ph111p pItlt5 I 19n2i.

Gtroia. F. S and I). R. Krtef. Omiotlc .~&u\t- tnctlr In \orghun~ I hlcchan~rrn\ c ~ t diur11a1 ~~smotrc potcnual chatlgc\. /'la111 Ph! ,rol. YY: 577-582 (IYY?l.

Gorham. J . . R. G . W y n J i m \ . and E. M . McDonnrl l : Sotnc rnechatil\nl\ o f salt toler- ance in crop plant\. Pl i i~rr Soil. 89: 15-40 (19x5)

Graham. P. H.: Stre\\ lolcrance In Rlri:obr~i!tt snd Brud\rtrr:(~hrutt~. and nodularion under adverse soil condtttonr. C~rn. J . Micrnb~ol . 38: 475-481 (19921.

Greaccn. E. L , and J. S . Oh: Physrcs o f root gmuqh. N~irure New Biol. U S : 14-35 (1972).

Greenberg. D. C.. J . H. Williams. and B. J. Ndunguru.: Differmces i n yield determin-

ing proce\ \e* of gruundnut (A ra rh i x ~YIIURNP(I L i fmut)pe\ in \aried drought cnvtronment\. A ~ r n Ap/11. Bin/. 120: 557- 5hh l lYY2I

Gregor). P J: Root g rou th o f chtckpca. fahahean. lent~l. and pea and effects o f water and \alt +[re\\ In: Wr,rldCn,p~: Cnol St,u.ron FOOIILL~~I~IIIPI (pp 857-8071. Summerfield. K J.. E d . Kluwer Academtc Publt\hrr*. London. U .K t lY8X)

Grumct. K.. K. S. Alhrecht\en. and A. D. Hdn\t)n Gn,uth and )tclil 01 hale) ~*~rpopuIa!~on\ dtf ter~nf in xrlutc potential. Crop S i i 27: 991-9115 119X7l.

(;ulrnan. 5 L and lu (' Turner. Dtlfurencr. 111

root dnd ihoi l t de\elopmenl o f ttlmato (1.y~ o/~(,r!it (,!I c'\ci!1~~1trr1111 L i \ anetie\ dcro,, conlra\ting r111l rrn~ronments. Pliitr! Soil 49: 127-136 t147Xl

Guptn. (1 S ('rr,11 / l l l / ~ ~ ~ ~ ' I I ~ f l l 1'01 I . P/ri.\r- r ~ l r r i . r ~ i r l A!rrrhitr<,\ Oxtord & IBH P u h l ~ \ l ~ - Ing CII P\t 11~1 . N r u Drlht. l nd~a (1Y021.

Hall. h E . K W. Fo\trr. and J. G. Waine\ Crop ad:ipldtion 10 icmt-x td m\ i ronmmts. In: .4gr!c 1111trrc in .St,~i,r-,-lr~i/Eit~ trotr!itt,n!s (pp 118-17Yi.Canncll.G H.. 4. E Hall. and H C ~ u t o n . Ed, . Spr~ngrr-\'erlag. N r u Yurk (19781

Hall. A. E and D A Granrt. Druught reurtance of cuupea irnpro\cd h) \electtnp for earl) appe;tratice o f mature pods. Crop Kc!. 21: 161 -4M I IVXI I .

Hall. A . E and P. N. Patel. Breeding for reah- tancc to drought and heat In: Cowpea Re- ~c r r r~~ l r . Prr,drrcrro~t. nrtd L'ri1i:nriort. tpp. 137-151). Singh. S R. and K. 0 Rachie. WI.. Wiley. U.K (1985)

Hall. A. E.. R. G. Mutten. K. T. Hubick. and G. D Fuquhar. Genotypic dtfferences incar- hon isotope discriminarton by cowpea under wet and do field condittons Crop Sci. 30: 3W.705 ( 199(1).

Hall. .4. E.. R. G. Mutters. and G. D . Farquhar: Genotypic and drought induced differences

in carbon ~\olops dlrcr~m~natlon and go\ ex- change of c o u p e Cnrp Scr. 32: I4 (100?)

Hall. .A. J . . C. Ch~mcnu. N. trap an^. F. V11cIIii. and R. Cohsn de Hunau. Ylcld In rater- *tre*\ed rnalre pcno!ypc\: A \ \~ r l ; l t ~on u lth trail\ mea\ored In \eedl~np\ and In flouer- Ingplantr .F~cld( '~o/~( Kt,.\. Y:4I-57 1IY841.

Humdi. Y . A . Soil udtcr tcn\len and the mo\c- men1 of shlfohln 70ilH1rd BIOC 11<.nr 3: I ? I- I26 (19701.

Hank\. J. K . . 1. Kcller. V P R.~\nill\\t~n. illid G D. Wll\on Lltic riiurcc \prinhlcr kir con- !innou\ \driahlc 1rng.ltlon crop prudilctloo \tud~e\ So11 . S c , 5 0 , .,IIII. PI,JC 40: A?(+ 429 lIY7hl.

Hd~i\r:l!. J. J and C R U'chrr N. P . I I ~ K percenlagc, ~n \o?hrd~i i(;l~<!rt(, nro< 1. ,Wt,rri/l. I plarlt part\, .~CI,III. .I. 63: 28&2YO OY7I i

Hardu~ch. R. C Crrt i~.~I pIi)\~ol<,pl~al tiall\ 111

pulw crop, In: Hiir ld (',ry?$. ( ooi Srir,or! Foo~ l l .~~ r r i~ r r (~ \ tpp 885-XYhl. Summerfield. R J . Ed . K lu rc r Ac;~denilc Puhli\her\. Dordrccht. The Ncthcrldnd\ i I')XXI.

Harrigdn. E S dnd H D. R,lrrr. ('c>,rrr,r ]or / ,n-

q(iri,,r~ Keicor.< 11. 1983-84 rrporr (pp. 11.5- 118). CSIRO. GrlSl'ith. USW. Au\tr;ill;l I 19x4).

Hur l \ . D . R. R hldtther\. K . C. Nage\rara Ran. and J. H Ki l l iam\ The ph!\lr~ioglcaI ba\!s for yield diflrrencc\ k tueen four gcnu- t)pes of groundnut iAru<111.\ h>pr,qul,o) In reipon\e to drought I l l . Developmental pro- ceases. Elp. Ayrlc. 24: 215-226 11Y8X).

Hanung. W. and W. J. Da\le\ Droupht-induced changes In ph!<iolog) and ARA. J Erp. Ror. 42: 9 11991).

Hoogenboom. G.. C. M Peterwn. and M G. Huch: Shmt growth rate of wybean a\ af- fected by drought stre\%. Axrort. J . 79: 598- 607 (1987)

Hoopenboom. G., I. W. Jones, and J W. While: Use of models in studies of drought toler-

ance. In: Kc\t1on.Il olt /),?~~rplrr T~r lcro~!~.~, rrt COIIIII~OII B < s o ~ ~ . (pp 192-230). \'hit?, J . H' . G. Hoogenh<x~li~. F. Ihi~rrn. and S. P. Singh. Ed,., TIAT. C(,lomhld ! IYXX).

Ho\hlh;~ud, K. . Stgn~l~zancc t11'legunle crop, 111

IIII~~~I\II~S !he pr~rductl\lty ,ind \ lnhl l~ty of cr(~pplllp \)\tcrni. In: Plro.\/~/ror~r\ Nlrrrrrrrrrl C J / (;IOIII L,,,,~I~I,IC! ;!I r11c .S~,r?r~.Ari~/ 7'rq1i< !. ipp. 173-1 X l I. Ji~hdl~*e~t. C.. K K . 1.e~. illld K I.. S.~hrau;lt. Ed\ . In!c~-i i ; l t~~~~ial Clop\ Rcsc,~scl~ III\~I~IIIC for tlic Se~n~-Ar ld Tt l~p- li,. P;lf;incIicru. Ind1.1 llYi)l 1

H\l;ln. T C The io l l pl;lnl attno\pIiclr coil. tlnu~rni In relutron to dr<x~ght alld crop pro- d u c t ~ < ~ ~ ~ In: 111011,~lrr R<,II!I,~II<,, 111 ('1,1/1\ I 1 1 1 i I I 1 9 -52 ) . IRKI. Phillpplne\ (1482).

Hl i i l i~ , 7 . C . J. C O'To(~lc. E B. Yamhao. .mtl N C' Turncr. Inllucncc o l osmotic adjust- Inlent irn Ir.il rtllllng and tl\iuc dcnth ~n rice 1Ur>:u \urihu !,.I. Piur~r P/ i \ \ ro l 75: 338- 341 tlYX41

Huhlck. K . T.. G. D. Fnrquhar. and R. Shoner: Carrelatlon hetwcen uater-u\e efficiency and cdrhiui r\otope dl\crlrn~natlon in dlverre pea- nut iAru(hrsi germplaim. AII.<I. ./. Plunr Ph!\roi. 13: 803-8 I h ( 1986).

Hubick. K. T.. R. Shoner, and G. D. Farquhar: Herlrability and genotype Y envlrt>nment In- teraction\ 01' carbon isotope discr~mination and tran\p~ration efficiency in peanut (Aru- chi. h~poguru L.). Aulr. J . Piiinr Ph\.rroi. 15: 799-813 (1988).

Hume, D. J , and 1. G. Criswell: Distr~bution and utilization o f '"C-labelled a\iimilate\ in soy- beans. Crop Sci. 13: 519-524 (1973).

Hunt, L . A . De\igninp lmproved plant types: a breeder3\ viewp~int. In: S~rrernl A p p r f ~ a c h ~ for Su~/uiriuhlc A,qrrculrural I)n,~,lr,pp,nt.r~r. ipp 3-17,. Pennlng de Vrir\. F. W. T . P Tang. and K . Met\ela;lr. Ed*.. Kluwer Aca- dernlc Puhl~rher\. Dnrdrecht. The Nether- landr I 1993)

Hurd. E. A . T . F. Tounley-Smith. L. A. Paltcr\~~n. and C. H Owen. Techiiiqur\ ured in produc~np Wdudna wheat. ('riri .I Pla,~r Sci 52: 689-h9I (19721

Hurd. E. A,. I'. I: T(~wrrlcy-Sm~th. D. Mall~rugh. and L. A Pailer\r~n: Wilkiu~ma duruin uheat. (h~r J . Plunr Sf.! 53: 2hl-?h? (1071)

Hurd, t A : Phenotype and dr~rught tolerance In wheal A ~ r r i M~rc~r , r / . 14: 39-55 I lY741

IC'AKIIA Ilnlernatlonal ('ciitre lor Agrlculiural Kr*cilrch In Dry Ared51: In : Aririiiol K,,pr,ri. 1YR.I Ipp 154-155). ICAKUA. Aleppo. Syrra i IYH4).

ICAKDA. In: /II~IIII~/ Kqxvr, 1984. (pp. I9& 201 ). ICAKI)A. AIeppo. Syrld (IYXZI.

ICKISA7 ( l ~ ~ t e ~ ~ i ~ a t ~ o n a l Cri,p\ Kc\ear~h ln \ t~. tute for the S~IIII-Arid Tr<)p~c\ I Atiriri(i1 X ~ ~ / ~ ~ r r . IYXV ICRIS4T. Pat;~ncheru, liidra I Ill9llI.

ICRISAT: A~iri l i r i l K<'/~r,rr. IYYO. ICRISAT. Patancheru. Ind~;! 11991 I.

ICRISAT. M~zl iu~tr Tc,rrn Plan. \'ol I . ICRISAT. Patancheru. Ind~o ( IYY?I.

Jarvir. P G, and K . G. McN:ioghton: Stornatal control oftwn*piration: Scaling lip fmnl leaf to region Ad). Ec.01. Kc,s. 15: 1 1 9 i 108h)

Jefferwn. P. G.. D. A Johnaon. M . D. Runhaugh. and K H. A u y : Water atreu and genotypic effects cm epicuticular wax prcduction of alfalfa and crested wheat gras\ in relation to yield and exciced leaf water losr rate. Con. J. Plant Scr. 69: 481-490 (1989).

Johansen. C.. Y. S. Chauhan. S. C. Gupta. and F. M. Taylan: Assessing short-duration pigeonpea for multiple-harvest potmtial. 1111. P i ~ r o n p m Nensl. No. 13: 15-17 (1991).

lohamen. C., 9. Baldex. J. B. Brouwer. W. Errkrne. W. A. Jermyn. L. J Lang. B. A Malrk. A. A. Mlah. and S. N. Silirn: Blotrc and ablotrc \tresses con~uaining productiv- ~ t y of cool 5eason food legume\ in Aua. Africa and Oceanra In: Expanding rhe Prcr- ducnnn and Us(, of Cool Seo.\oti Food Lc- qurnes. (pp 175-1941. Sluehlhauer. F. J and W J Karxr. Ed \ . Kluuer. Acadern~c Pub- I~rhcr\. Dnrdrechl. Thr Netherland\ ( 19941.

Jnhn\on. D. A . 1 D Rumhaugh. and K H Ala) Plant irnproremeni fnr \emi-arid rangeland\, po\rihilitie\ for drought re\i\- t.lncr and nitrogen frxat~lrn Plrrrlr St i I 58: 279-3117 I 1981 1.

Jirhnum. D A . R A R~chardr. ;and N C Turner Ylcld. hater reldtlon\, &?a\ e~chanpe. and \url.lcc reflectance\ of t irar-~\oprn~c uhear line\ ditferlnp in plauciru\ne\i Crop Sc.i 23: 318-725 (198?1

Johnson. S S. and J. L Ccadelnlann lntluenic 01 u.iier \trc*r on graln yield re\pon\e ti,

recurrent ~elri.rlon In maize. ('r~l, \r i 29: 958-565 ( IYRYI.

John\on. D. A . K . H. A\\ily. I. L. Tre\rrn. J. R Ehleringer. and P. G Jrller\on. C.irbon IVI- tope d~\crrnir i~at~on Potcnual in crrenlnp cool-reawn gra\\e\ lor uater 1111uted enxi- rnnrnenl*. Crop .%.I. 30: 338-313 ( 14901.

Jane,. H. G . Stornatal hehdxlorand breeding for drought re\istancc. In: Srrt~.\\ Ph!sio/o,q! rrr Crop P l o ~ i r . (pp 408-4?7). Mua\ell. H, and R. C Staplra. Eds.. Wile). Nex York ( 1970).

Jone~. H. G : Interaction and integration of adap- tive respon\es to uater stress: The impl~ca- tions o f an unpredictable en\ ironment. In: Srrcss Phi.rolo,qr rn Crop Planu. (pp. 408- 12RI. Mus~el l . H. and R. C. Staples. Eda.. Wiley. Neu York 11980).

Jones. J. W. and B. Zur: Simulation o f possible adaptive mechanirms in crops subjected to water stress. Irr ig. Sci. 5: 25 1-264 (1981).

Jones, H. G. and J. E. Corlett: Current top~cs in drought physiology. J. Agrir. Sci. (Camb.). 119: 291-296 (1992).

Jordan. W. R.. F. R. Miller. and D E. Morns Genetic uariation In root and \hoot growth of sorghum in h)druponics. Cro/l Sci. 19: 468-472 1 IYlY!.

Jordan. H' R and F. K M i l l r r . Genet~c bar(- ahtlity In \orghum root \).rteln*: ~mpl ica- lions for drought tolerance In: Arliipr(rrto,r oj Plnnr.\ to Wurpr ortd Hlph 7r.nrpcnrr1rrc .Trr?\.j. (pp 383-39L)i, Turt~er, N C ztnd P. J Kramer. Ed\ . Wile!. N r u York I I')XO)

Jurdan. H. R.. H'. A. Dugd,. and P J Shouic: Strateglr* tor crop impro\cment for drought prone regloni Aaril. Mirr<'r M~rriirgv. 7: ?Xi- 21)') (IYX71.

Kong. C. W . G. C. Kuo. and t:. R Son Stltil~e, on driiupht tolerance rcreetilng ~ncthnd In inunghean I . Drought tolcr;lncc (11 \redl~ng \tage 111 greenhouw HI,.\. K?/I. Krtrril I)?, :lrl~rrrn. 27: IUlLlYX (1'4851

K~*par . T. C. . C D. St'lnlr). dnd H M. 7';t)lor Soyhcnn r i~ot grcrwth during the reproduL- t l \e \lapc\ of de\rli~pnient. Aq,r~!r J. 70: IlO-I I07 I 197x1

Kmpar. T C . H. M. Td!hrr. dnd R M Shiblc,. Tap rout elonpat~on rate, 01 \n)hean cultl- \dr* in the gla\thnuic dnd thclr relation 111 field ruciting depth. C n ~ p 4.1 24: Vl6-9?(1 i 19x4).

Ketring. D. L . Rixit divcr\~ty among peiltiut genotypes Crop Sci. 24: 229-232 (1984).

Ketnng. D . L : Physiological rcspnnse of grrlund- nut to temperature and water d r f ~ c i h - breeding implications. In: A~rontrfeoroiogi. lf Groundnrrr. (pp. 135-143): Sivakumu. M . V K , andS. M. Virmani. Ws.. ICRISAT. Patanche~. lndia (19RbJ.

Khalfaoui. J. L . 8 , and M. Havard: Screening peanut cultivars in the field for root growth: a test by herbicide injection in the soil. Field Cropr Rer. 32: 173-179 (1993).

Kirchhoff. W. R.. A. E. Hall. and W. W. Thomson: Gas exchange, carbon isotope

d~icr~nl lnat~nn and chloroplast ultr;~htructure 111 a chlorophyll deficient mutant of coupea (',n/> srt. 29: IW-11.5 (IUXY).

Kramer. P J . . The role ol phy\~oliigy In crop Impro\ement In: L~rtlrrr,c Kt,scnr~.lr to <'rrq~ Pnld~r<rrrrrt. lpp 5 1 4 ? ) . Staple*. R. C , and R J. Kuhr. Ed\.. Plenum Prew. New York ( I9XOl.

Kri\hn;tmani. hl R. S .J . H Yopp.and0 Myer\. 31 Leal ~ o l u t r Ic.bk~gc a\ t i droupht tolrr- ancr indtcalor 111 \oyheiin. Plr\,ror~ Ar,qr,i- ~IIII! 44: 4 3 4 9 i 10841

Kuhad, hl S . A . S N;indw;~l. I S Shet~rori. and K D. Sharma W.1tt.r rrlation\. rilrhon ex- ch;tnge unil nttrirgenzrrr in chtchpc;~ undci \oil uater (lepri\atinn :lriri, ,4r11/ ZOII<, 31: ?OY~-'l(l i I'JV?!

I.auti. R J ,rnd W 4 Brun Symhtot~c nttrogen t tuat~o~i 111 i ~~yh r i i n \ . I . Effect* (11 phor~>iy~i - thel~c \ourcc-rinL ninn~pnlationr. (‘rap S 1 1

14: 11-16 09741

I.tiwn. R J.: Respon\es { i f four grain lepurncr 111 water \Ire\\ in \outh ca\trrn Queen\land. I Phy\~ i i l i~g~c;~ l re\p,rn\e nicch;tnirrns Air\r. .I A,c!r( HPI 33: 1XI-ll)h (II)X?a)

L3,tun. K J . Rc\ponie\ ~rl'four gratn Iegunie\ to wiitcr $Ires* in wuth ea\tern Qucen*land. I1 Plmt grnwth dnd \oil water extractlnn pat- tcrnr, AN!! .I April.. Hr.! 33: 497-509 I IYX2hl.

Lawn. R J : Rcsponrri nf fi,ur grain legumes t11

water stre\\ in wuth eaitern Queeniland. Ill. Dry niatler pr~,ducttiin. ytrld and water u\e efficiency Autr .I, Agrrc. R p \ , 33: 51 1 - 521 (19x2~) .

Lawn. R. 1. Agronomic and phyhiologicul con- straints to the productiv~ty of tropical grain legumes and prospects for improvement, Ex/?. Agrrc. 25: 509-528 (1989).

Lawn, R. J. and B. C. Imrie: Crop improvement for tropical and rubtroptcal Australia: de- signing plant\ for different climates. Field Crop.\ Res. 26: 11 3-1 39 11 991 ).

Legumer Program, Anntcul Report. 1991 ipp 17-2 1 1. lnternat~onal Cropr Rerearch Inrtt- tulc for the Semi-Ar~d Tropic\. Patancheru. lnd~a ( 1992)

Lcop>ld. A. C.. M E Mu*grave. and K , h l Wtlliamr. Solute leakage rewlting from leal dc\iccauon I'irint P ~ I \iol. 68: 1222-1225 (19x11

I.cv~tt. J . Rr\po!i.\c., 01 /'/ant\ ro E ~ n ' r r o ~ l t r ~ ~ r ~ r ~ / S i rp $ 1 , ~ VO/. 11. WOIC~. XU~IIIIIO~~. .SU/I ui ld othc,r.Ytrn\r\ 2nd rd.. Acadcni~c Prcrr. N r u Ynrk I IYXO)

Lcic~kcl. C. H bludlc* uf the Griiuth and Devel- ilplncnt (11 Koot\ III \.'ri.ul frihn L . Ph D t hc \~ \ . I ! n ~ v r r \ l t y of Nott~ngham, U K I1 97x1

I.opc~, P H . T. 1.. Sruer, rnd (' R I\lcD,~vld Carhon dloxidc and l~ght ruiponre, 01 phi>- toaynthe*i\ 111 coupeo and plgr.onpca durlrig uater dellell i111d rcco\ery. Plt~rl i /'/I\ ~ i o l 85: YYOLYcJ5 I lL1X7l

I.udlow. M. M.: Ad;~pt~ve *~gntftcancc i j f \lo-

natal respcln\er 10 u;\tc~ r t r e u In: AO~il~ru- IIC,!I of P/IIII~A io H'II~CV crttd HI,~/I 7't,lt1p<~rcl- ttrlc, Srr<,.r.\. (pp. 123- 1381. Turner. N. C. ;tnd Kramer. P. J.. Ed,.. W ~ l e ) . h'eu Yorh [ IVx l l h~

Ludlnw. M. M, ando. Bj i~rkn lm: Parahelintn~p~c 1e;ll' nlovetncnt In Siratro a\ a pn>trctivr n~echanlsln apatnst drought tnduced damage to priniwy phvtosynthrtic reaction*: dani- age hy chce\*tvr l ~gh t and heal. P l r r~~ni . 161: 505-SIX (lY8.11.

Ludlow. M. M.. M. J Fischer. and J. R. W~lsim: Stomstill adjustment to water defic~ts In t h ~ e tropical pra\se\ and a tropical legume groun In controlled conditions and In thr field. Atrsr. J. Plar~r Ph?siol. 12: 131-119 (1985)

Ludlow. M . M.: Contribution of osmotlc adjust- ment to the malntenunce o f photosynthes~s

during water rtrers. In: P r o j i r n in Phoro- svnrhr\l.\ Resrrm-h. Vol. 4. ipp. lhl-1681. Bigpns. J .Ed.. Maninu\ Nljhofl. Dordrecht. The Netherlandr t 19871.

Ludl<rw. M . M, and R C Muchuu. A cr~tlcal cvaluatlon oftra~ts for imprmlng crop ytrld, In water-ltm~tcd en\lronments. Adr. Apnrti. 43: 107-153 tlY901

Ludlou'. M 51.. J. Bamajake. M Ciropcr. Y Tui~. J. Manners. R H r n ~ r l l . S Duncan. and I Gcrdw~n Oimutlc adju\trnetlt lo baler itre% In graln \irrghum lnhrrllancc and Ilnh- i gc analy\~r In: Prr,~. Mo lc~~ i r l~ t r MurJ?r< III .S~,yhlt!n iind /'et~rI M~llc,r /w 1)i.t r l c ~ p ~ r ~ g (i,irrltrlr,\, Cin~ver*~t) of Ect\I Angl~a. hor- ulch. 1J.K. iIYY1)

Mitlhotra, R. S, and K. H. Singh: C la r r~ f~cau i~n of ch~chpc;! growing en\lroniilcnt* to con- trol genirtypc h! cn\lrirnmrnt Intcractiirn l,tl/~lllllc 'I 58: 5-12 I IVY! I.

Man\ f ie id . l A, and M'. J. Da \~cs A h \ c ~ \ ~ c uc~d and uatrr strr\,. Hioi /I?I~I So(.. T r ~ i ~ i \ . I I: 557-5h0 i IYXql

hlarhhan. A. H.. Comparatt\e udtrr relation\ of / ' / ICA\~'~/IAS i t r l g ~ t ~ i ~ L .ind Phoi<,o/t(\ <l,~l l l l /~~/it l \ ql(lv. ~'1<1111 1'/11\1,,1. 77: 113-1 I7 (1Y851.

hlarshall. D. K , Alirrn.ttt\e appro;ii.he\ and per- *pectl\e\ III hrcedlng Ibr higher yield\ F l r l d Cro/uR~r\ . 26: 171-IYO OYY I ) .

Xlanln. B.. J. Neinhu~\. G. King. and A. Scharl'er: Re\trict~un fragment length pol! n~orphi\~ns a\\cwated u ~ t h uater u\e eftictenc) In to- mato. S<,I~.II<.C.. 243: 1725-1728 ( 19891.

hlatheu,. R. 8.. D. Harris. R. C. Nageswara Rao. J H. Willtnms. and K. D. R. Wadla: The phyhlologtcal basis for yield difference% heturcn four genotypes of groundnut !Aru- chlr h?j~oyuru) In rerpunse to drought. I. Dr) matter production and water use. Exp. Ajiri(., 24: 191-202 11988al.

Mothews. R. B.. D. Ham\. 1. H. Wllllamr. and R. C, Nageswara Rao: The phys~ological

h a i \ for y~eld dtfferencr, hetu een four gentr- type\ of groundnut fAror11ri h\piq<rtvir in re\piinse todrought. [I. Soldr radi;ttlnn Inter- ceptlon and leaf mo\rnirnt. Etp . .4qrrc. 21: 203-213 tl9XXhi.

McBlatn. B. 4 and D J. Hume: Ph!\tologtcaI srud~ec of h~fher !reld In nen c,irl! tn; i t l~rin~ \o>kan culti\ar\. (-off. .I Piorlr k I. 60: 1315-1326 I 19801.

!lechel. L.. D B Egll. R E. Ph~llip\. D. Redcllffl~. and J. E. Legget!: Efl'rct of liilu\turc \Ire\\ on \red growth In \~r>hton 4rvi8rl .I 76: M7-650 (1984).

!vlciigel. D B, and S. 4. B.irher. Ue\'l<~pn~ent itnd dtrtrthutlon of the corn root \!\teln un- dcr t~r' ld cond~tti,n* ,A~JOI I I 66: 341-344 tlY74l.

!vleterler K m p . H. K A,. Re\pon\r to c,~rl! planltng and trrlg;itlon 01 a I;ite rndturlng grt~undnut \arlrt) K/ii,Oc\i~r -Igrrr .I 64: 127-110 11Yh71

\Ic?cr. G t.. S~niulation i r l tiiui'tiIrc \Ire\\ cI- fcch on ,o!he,ln y r l d cr)mponent\ In Nc- hra\ka. Trcrri\. / \ n~ . Sr*. Aqrr,. ttrp. 28: I 18- 128 lIYX51.

Ile!er. R F arid J S. Boyrr O\~~iorcpulat~irn. wiute di\ ir ihi ir~r~n. and grouth in wyhean \eedhng\ habing low wdtrr p tmt ia l * Pliif~ro 151: 482489 i 1981 1.

Minpue7. M. I . B. Ruif-Nupueird, and F Sau Faba bean prnducti\tt) and optlmum canopy de5clopment under a hleditcrranean cl~niate. Field Cropr Rc.5. 33: 235447 (1'11131

Morgan. J. M. Osmocic adju\tment in the \ptke- lets and leaves of uheat. I. Exp. Bor 31: 655-665 ( 1980).

Morgan. J. M: Osmoregulat~on as a \election criterion for drought tolerance in wheat. Au\r J . Axric. Rev. 34: 607-614 (19x3).

Morgan. J. M.: Osrnoregulation and water stress i n higher plants. Annu. Rev. Plonr Ph?rioi. 35: 299-319 11984)

Irlnrg.in. 1 h l , and ,\. G. Condon. Water ow. gram !leld and orrnoregulut~<~n In uhc;~t. ,411sr. .I. Plirftr Plrv~rol. 13: 523-532 ( IqtXi).

hlorgan. 1. M.. K A. Hiire. and R. J. Fletcher: (;cncitc \ drlatlon in o\marefulntlun In hrcod and duruttt \\hc:it\ ;tnd it \ rel;ttion<htp\ to g?dii ! !dd i t t ;I r;rnpi, of l icld ?I,\ Irtlitment\. Airs!. .I. 4rrr< Kc,\ 37: 440-457 I IYXhl.

hluch(~u. R. (' Sliitiiaf;~l heha\i i~r gr.it1n It.- gumc prown under d~l t r rcn l ,011 haler re- pmc \ 111 .I icml-ar~d trop~ial cli\,lrnnnicnt. f i<,/cl ('IO[J\ KO\ , I I: 2L)l-307 (1 YXScI,

Muchow. R C and T. K Slnclatr Waicr and nltropcn I~mltatlonr in wyhcan praln pro- ducuon I1 Fteld and mudel andly\ls. F i~, ld ('rill'\ Kc,\. 15: 143-156 119Xh)

Muchow. R. C.. G L Hammer, and P. S. Carherry: Opt im~?~ng cnrp and cultlvar \c- lection in re\pon\e to climatic rt\k. In: Cli- mririi Rfsk in Crop Pror lu~~fron~ Mod~1.r und Mo~iu~rrnenffrrr Ih? Sumi-Arid Trl~pi1.a (pp. 235-262). Muchow, R. C, and J . A. Bellamy. Eds.. CAB Internat~onal. Wollingford. U.K. (19911.

Muchow. R. C. and P. S . Carbey : Design~ng improved plant types for the semiarid trap-

lc\. agronom~st\' v~ewpoint\. In: S?,\rrms Appr~~uche$ /or .lusfutrwhir AXru ulrrrru1Dt~- vul~~pmmr. lpp. 3 7 4 1 ). Penn~ng de Vne\. F. Vv 7.. I' Tang. and K . Met\elaar. Ed\.. Kluuer Academic Puhlirher\. Dordrecht. The Netherlands (IY93).

Mungomery. V . E.. R Shorter. and D E. Byth: Cienot)pe V env~rr~nmrnt interaction* and enulronntcntal adaptiltlon I. Pattern ;~naly- \I* applicat~r~n In *cryheen Atrir .I Aprtc. Hc\. 25: 59-72 (19741.

Munn,. K . Why mea\urr i~\tnotlc adlu\tmrnt" AU\I .I PIOIZI it^,,^,^ 15: 717-7x1 ( 19xn1

Nagaraliirao. Y.. S. Miillicli. and (i . C. S~ngh. Mo~+ture deplet~rrn ;ind rrlot growth ol dl i- lerent \.irlct~c* of chichpc,i undc~ rainkd c i ~ n d ~ l ~ i ~ r i r h!d ./. Axrot! 25: 289-293 1 lYK0)

Nagc\uaru Rdo. R. ( ' . S. S111ph. M V. K S~vahutiiar. K I. Srtv.~rt;~\;i. and J H. WII- I id~nr t f lec l of u ;~ tc~ del~clt 'it d~tferctit ~ I I I W ~ ~ ph,l*e* (11 peanut I. Yield re\pnn\r\. A~~U>II. .I 77: 7x2-7x6 ( lYK5i1,

Nape\uar.~ R;I~I. R C . S. S~ngh. M V K. SibaLumar. K L Sr~\,i*ta\a. .~nd J H. WII- l i o ~ n . Etlccl 01 udter def'~c~t ;It d~licrent g r o ~ t h ph;~*c* of pctinut I I . Re\pr~nre 10 druught dur~ng prellower~ng phaw ,'\,qrori. .I 80: 131--1X ( IVXhh)

N;ige\wura Rdo. K. T.. J. H Willt.tms. and M Slngh: Grnotyp~c ~ t * ~ i \ ~ f t \ t t y to drought and y~eld potential nfpranul. A,qro!t. .I . 81: 887- 893 (1989).

Nagmwi~raU~do. R. C.: Sotile crop physi~Iog~c.tI approache\ t i ~ r groundnut tmproummnt. .I. Oilsrc,ds Rrs. 9: 286-296 (1992).

Nagesuara Rao. R. C.. J. H. W ~ l l i m s . K . D. R. Wadla. K. T. Hubtck. and G . D. Farquhar: Crop growth, wnter-use efficiency and car- hon taotope dtacrimtnation i n groundnut (Arurhis h!pogacu L.) genotypes under end- of season drought cond~tions. Ann. Appi. Bi(11. 122: 357-367 (1993).

Nage\wara Rao. R C. and G. C. Wr~ght Stahil- ~ t y o l the relat~on\hip between spectfic lraf area and carbon isotope d~\crirnlnatton acrov env~ronment\ In peanut. Crop Sri. 3411): 98-103 (19514)

Nam. N. H.. Y. S Chauhan. and C. Johansen Cornpar~ron o i extra-shon-duration plgeon- pca w ~ t h shnn-season legumes under ratnfed c i~nd~t lon\ on alfiholh. Eip. A g r ~ t . 29: 307- 3lhI19931.

Nutman. P S. Ccntrnary Ircturt. on nltrogen flhatti~n In: A C'errrrrrr (I/ Nltrojien Frtarrr,~~

'H~\etrrc h . Prr\(,r~r Sliirli.~ uttd I-~iftrri, P r o ~ - /"( ! $ (pp 69-IOh). Bergcrien. F J and J R. P~~\tgatc. Eds.. The Royal Suc~ety. Lcrn- don. I:.K. (IYX71.

Olehmon. 0 0 : Eflect of continuou* rcmordl 01 ripen fli,wcr\ (in the reed y~e ld of IUO

\i tr~et~e' of coupes. Vi,yn~i rrriquii~iriola L Wdlp. .I Apti(. S i r lC(in!h 1 74: 175-3x1 I l Y 7 0 ~

Onlm. J. F M 4.\ociat111n hetuecn gralri !~e ld and drtrught re\~itance In plpeonprit In mar- ~ ~ m l r ~ t n i a l l drca, 0SKcn)a. In: Mr~r? Fooil trotrt Hrr11.r T i ~ ~ / t ~ ~ o / o y r ( p p 864-871). Holme\. J C and U. M. Tdhor. Ed\.. FAO.

. Rome. Italy (19x3)

Oo\terhu~r. D M , and S D Wull\chlrger. Drought lolerancr and ocmotic ndjuctment of \;~riou\ crop\ In re\pun\e to water \tre\\. Ark. Farr,~ Kt,,. 37: 1 (1988)

O'Timle. J. C.. R. S. Aqulno. and K . Al lur~ Seedltng stage drought re\ponse In nce. Apron. J . 70: 1101-1103 (197R).

O'Toole. J. C. and T. T. Chug : Drought resls- tance In cereals: a case study. In: S l r r j ~ P h v s i o l o ~ ~ irt Crop Piunrs. (pp. 373-405). Mussell. H. and R. C. Staplec. Eds.. Wile). Neu York (1979).

O'Toole. J . C. and Soemartono: Evaluation of a simple technique For charactenzing nce root systems i n relatton to drought resistance. Elrphyrica. 30: 283-290 (1981 1.

Pale. M . C. M.. M. M Ludloa. and R J. Lawn. Vanation among w y k a n IGli~.r~rc, rniu L Merr. I accession\ In epidermal conductance of leave* A~rsr. J. ,4grrt. Rc.!. 39: 363-377 11988)

Pmkhunt. C E, and 1. I. Sprent. Effectr o i u a t r r on [he respiratory and nrtr<igrn-fixrng

actlvrt) of * oykan root nodule* .I E~I, Hor 26: 287-3(H ( 1975).

Parlihurrt. D. F and 0. I.. LoucL\. Optimal leaf \ izr In relatron III en\,lronmen[ J Ec 01. 60: 505-537 119721

Pdrlrrliet. J. E and J C. Z,idocl\ Thc ink- grated concept of di\raie rc\l\tance. A neu \ leu' including hor i?on l~ l and \ rn ica l re\ i i - lance In plant\ Eriplr\rrt.ir 26: 5-21 i 19771

Psr\ons. L. K and T. K. Houe. Eflcct i ol water \ t r r r \ on the water rcl ;~~ion\ of Wriirt,r,Iir\ ~.ulgorr i and the dmupht rc\i\tan[ Plrirscolrr ncurr/olriir. Ptt,\ri,! Pl<rnr 60: 1'17-202 11984).

Pa\\~oura. J H.: The rffect of root gc<rmrtr) OII

the yield ot wheat prowlng 11n stored water. Ausr. .I. Ajirrc. RPJ. 23: 745-752 (19721

Pa\voura. J. B.: Gwln yield, har\c*t rndex and water uie of wheat. J. Au.11. I r ~ r i . Aprrr 5< r. 43: 117-121 (19771.

Pa\\l<rura. J. H . The role o f n ~ o r \?item chardi- terlitics In the drought re\irtancc 111' crop plantr. In: l)roir#hr Ru\r.iruncr, r t r Crop\ Wrrh Emphuslr on Rr(.r. (pp 71LK2). IRRI. Phil- Ipplnes (1982).

Passioura. J B : Ronts and drought rewtance. Agrir. Wurer Munogu. 7: 265-280 1 l98.j).

Pa$s~oura. J. B.: Overview of the procesrer I lm- ittng crop production on duplex \oils. Au\r. J. Erp. Agric. 32: 987-990 11992).

Pateraon, A. H.: Agricultural genetic5 and D N A markers. Adv. Agron. 46: 40-90 11991).

Pierce, M. and K. Raschke: Correlation between loss of turgor and accumulation of abscisic

acid In deteched Ienver. Plrrnro 148: 174- 1x2 (IYXO).

Pren~dchendra. G. S . H . Saneoha, n ~ l d S 0g:it:1, Cell rnrmhrarie \lability, 1111 ~nd~cat iun or drought tolrraocr. ;is rlilected h) applied 111- trogen In \nyhe;in ,I. A,qri< Si I ~C1111rb. I 115: 63-66 (IYVOI

Rachir. K 0 .III~ L M. Ruhert\: Grain lupuinc\ o f thc IOU-1n11d trnp~c,, A,/) d,qrotl. 26: I - 132 l19711.

Rafal\ki. J. A,. S. V. Tinpry. aiid J G. K Wrlliam\: KAPD n ~ n r h e ~ \ - a l ieu tech- nolo:). lor genetic Iiiapprng and plan1 hreed. in? A,qRir,rr~ 11 N c 14 , l r i / r~ 3: 644-648 (14911

Ki~u. A. V . and 8 . Venkale*w;~rlu. Nrtropen 11h;i- tinn as ~ntlucnced hy water rtre\r In selrc.ted crnp legume\ of thc Indtdn a r ~ d lone. Al-rtl So;! H i Kch[rh 1: 89-96 119H7)

Ra\rnm~i\~in. D C.. A plan! hrccilrr'r experirncr wrlh identypr hrredrng I,rclrl ('ro/>.! Kc,.$. 26: IYI-20OiIYYI)

Knwul. K M: Naulral hyhrrdifatlon airicrng wild. weed) and cultivated k'ryrrn unjirri,r~lrira 1. Walp. Er,/rhirrr.ii 24: 099-707 I 1975)

Kau\nn. H. M.. A K. Hagga. and P. M. Bremner. A*pec[s o f adaptatron by wheat and harlcy 10 ro i l moirturc def ic~t r . Aujr. J . Plrrrir I'i!\.r~ol. 4: 389401 i 1977).

Read. J. J . . R C. Johnson. B. F.Cnrver, and S . A. Quarne: Carhon isotope d~scr~m~nal ion, gar exchange, and yield ot hpring wheat selected for ab\cis~c acid content Crop Sr r. 31: 139- I 46 11991).

Reddy, M. S, and R. W. W~ l l ey : A rtudy of pzarl mtllel/groundnut lntercropplng w ~ l h partlcu- lar smpha.~r un the effic~encies o f leaf canopy and rnoting pattern. In: 1nrc.rnurional Work- shop on Inr r r i ropprng. Ipp. 202-2091. Willey. R. W., Ed.. International Crops Re- search Inuitute for the Semi-And Tropics. Patanchem, lndla (1 981 1.

Richard\. R A, and N Thurl~ng. Variation he- tween and s l t h ~ n \pecle\ r~frape\eed (Rroa- siirr ~ e n i p ~ ~ ~ m . ~ and /I. nupus) In re\ponse to drought \tru\\ I1 Gr i~wrh and development under natural drought rtrc\\ Auir. .I A#n< R r , . 29: 47'1190 ! lY7X)

Richard\. R. A, and J. H. Pa\*ioura. Sern~nal r ~ x ~ l rnorphl~ic~gy dlid u d e r u\c 01 uhcilt I En- birimmcntdl cffccl\ ('t,q~ ,S< I. 21: 249-252 (IYXlal.

R~chanl\. R A and J H Pi~\\tourit Scminiil root t i torph~~lirgy and water u\e 01 aheat II Ge- l ict i r \artdtliln. ( !r,,t> .St I. 21: 240-252 ! IYKlh l

KiiIiard\. I<. 1.. H M Raw\rln, atid D. A JoIiit\~m ( i I , iuc~n~~ne\ \ III whei~t I t \ debel. ~~p i i ic t i t and cl'lcct 1111 walcr u\c cl'liclcnc). g;~* rhch:~itge ilnd phi~to\!nihet~c t l \ v i r ten). lpcriiturc\. Ati\r. . I . 1'1<111r /'/I\ \rol 13: 465- 473 ! 1 ')#()J.

K~tcl i ic. J. '1' A user-i~rieiitcd model (I< tltr \otl uiiter hdlai~ce In ulie;~t. In: Hiimii (;roiirIt (IIIII M o ~ / ~ ~ / / I I ~ ~ . (pp 293- 305). [li~!. W. atid hthln. I< K . Ed,.. I'lcnu~il Pit!,,. N c u Yorh !lYK51.

Rohert\on. U. hl.. 4, t. I iel l . ,ind K W I7o\tci. A Itcld techit~qu~. lor \clcenlnf gcnot)plc ilifference, 111 nil11 &roulI i Crop S('r 25: I0X4-I0Y0 tIYX5I

Kohcn\iin. Ci H Po\*~h~l i t l c \ and Ilntitatiuli\ 01 r;tinl;~ll CI~'II)*I~ lor predicunf crop a\all- ahle u'ater In: I)r,>i,~/tr Rr.\<,iirclt Prtortrtes li,rrhc I)r?/iini/ Troplc.s (pp, 3-1.11. Ridinger. F R i ~ n d C. Johanwn. Ed\.. ICRISAT. kltanchrrti. India ( IYXXI.

Ro\r. I A.. K. S. McWhtner. and R ;\ Spul-u.iy: Identification of drought tolerance tn rarly- mntunng indetennitiatr coyhean\ ((;l\r,rnr tr~av L. Mrrr.). Aust. I. A,errc. Rrs. 43: MS- 657 (1992).

Robenberg. N. J.: Norrh Atnrric~uti D r o u ~ h r . ~ . AAAS Selected Symposium. American So- ciety o f Agronomy. Madison. W I (1978).

Kr~\enow. D T.. J E. Qui\enbeq. C. U. Wendt. and L. E. Clark. Drought tolerant sorghum and cotton germplasm Agrlc. Wafer Man- axe. 7: 207-222 ( 1983)

Ru\iclie. A. A, and J. Harnblin. Theoret~cal a\- pzct, r ~ f \election for yield tn \[re\\ and non- \Ire\* en\ironrnent\ Crop 5c.t. 21: 943-946 (19811.

Surrd. hl, and C A Francii Yleld \tahtlit! in relattl~n to mdtunty in grain \orghum Cro/i, Sc.r 23: 6x3-6x7 ! IYX3l.

Sall: K dnd T. R Sini.ld~r S<r)hean genotypcu difference\ in ir i t i l t lui t) of iymhio t~c nitro- gen l ~ x a t ~ o n tu \ o ~ l deh)dri~tn~n Plntir So11 133: 31-37!1991!

Santet. J. S.. P. hl Cone\, and T R. S~nclair. D~urna l dnd \ea*nnal changer In ahiclric actd (11 f ~ e l d pmu.11 \oyhrani 5uhjecred to driufht \[re\\, f - r ~ l d Crops K r i . 8: 49-59 !IYX41.

S.inta~~u\. K. A 4~\ im11at i~ i t i <1fC0:. N A D P . dnd PGA reductinn iind ATP \!nthe\~\ in illl.~cl leiti cellr In relation to uater coiltent P1t111ra. 73: 228-212 ! 19671.

S.itii. H, and K. Gotoh. The photntn~pic mo\e- ' mrnt and i t \ vsrietill difference In the leaf o f

corninon hrdn !Plro~t~oIrr\ 1,111qiirls L.1. .I. Ho i i a t r i o I.'IIII Edui SPI,I. 30: 45-51 !1979!.

Saxma. N. P.: Chickpea. In: Thp P l t ~ i i o / o g ~ r ~ f Trrylrc o l F i ~ i d Crop.\. (pp. 419-45?1. Golds- uonh!.. P. R. and N. M. Fiwher. Ed\.. Wtley. New Yorl, (19841.

Saxens. N . P.: Screening for adaptation to drought: Ca\e studies u t t h chlckpea and plgeonpea. In: Adnprarion r?f Chirkpea and Pr,qu~~npro ro Abinnr Srrnr. (pp. 63-76). Saxena. N. P. and C. Johansen. Eds.. ICRISAT. Palancheru, India (1987).

Saxena. N . P.. C. Johan~en. M. C. Saxena. and S. N. Sillm: Selection for drought and salin- i ty tolerance i n cool season food legumes.

In: Breedins j ~ t r Srru~\ 7~~irr i1,tr~r 111 C'orli~ Srusr~n Fo[xlLxr.ur,lu, Ipp. 245-270). S~ngh. K. B and hl C. Saxena. Ed\.. Wile!. 11 K 11993)

Sdxena. N P . L. Kr~\htid~nurth!. ,~nd C'. Ji~han\rn. Rcg~\trd~ion ol'a drought rc\lrlant ch~ckpea grrmpla\ni ('rap S, 1. 33: 142-1 119941

Schn!der. H . The rule 111 cdrhohydr.~tc i t o ra~c and red~rtrihutlon 111 thr. \ i ~u rcc - \~n l rcl:~- tlon, of wheat and hnrlr! ilurln? S~~IIII 1111. In? - .I revleu Yc i~ I ' l r~ro i 123: 213-245 1 19931

Schuah. K S. dnd l' Hehct. l'h!l.iki,ld 1iie111- hronc % ~ h i l ~ t ~ in droitpht t<~lcr,int .ind d r ~ u g h ~ \u\ceptihl~, plant, / ' / ~ r t~~ t r , 161: 37-45 11984).

Schuah. K B and U F C;;11l' Sirf.ir ;III~ ~ o i i collierit ~n leaf tt\\uc\ 01 rc\cl,ll droupht t<~lcr;int pI,inl\ i~nclcr u,lter \(re\\ ,I PIC~III Pit) \rrd 125: 257-265 I IYXhi

Sedglc). R. H . An ;lppr.it\ii ill thc l),111,1liI

rdeotjpe after21 >r,rrr F'!~li/ ( '~,i /~l NPI. 26: 93-1 I ? I IYVI I

Sell'. F.. J C. Evan,. irnd I. Y B.~li,~m A ni111- t ~ \ a r ~ d ~ e procedure lor cla\\~l>lng cn\lroli- men[\ accilrd~ng to their inlerdctliin utth penot)pei A~rrt. J ,44ri( Kr \ . 30: 1021- 1026 (1979)

Seong. R. C . H J. Chung. and E. H. Hung Varictal rerpilnres [if wyhran germlnatlon and seedllng rli~ngatlnn to temperature iind polyeth~le~i r glqcol s ~ l u u i ~ n Korerrtl J Crop Sci . 33: 31-37 ( 1988)

Shackel. K. .4, and A. E. Hall: Re\rr\ible leal' movement* in reliltton to drought adapthtliin o f cowpeas (ViR,tu u~iguicuiuru L Walp.1. Awl . J. Pion1 Phjrroi. 6 : 265-276 (1979)

Shackel, K. A. and A. E. Hall: Compari\on ot water relations and osmotrc adjustment in sorghum and coupea under field condit~onr. Ausr. J. Planr Ph?,>roi. 10: 4 2 3 4 3 5 (19831.

Shcur;in. I. S . Y. P Luthra. hl. S. Kuhad. and R. Sttigh: Effects o l uatrr rlrcs* on some cti- /!me\ of iiltrogen metahol~.;m in pipeonpe.~. P /~~ r r~< . l r< ,~ ! l i%r r~ 21): 2h75-?h77 (1981 I.

ShelJri~he. A. R :ind A I\i,u.;~ydnan Growth. dc%clopmc.nc i n d riiltrirr~r upt:rhs in pigwn- pc~l, ~ ~ ' ~ l , r t r 1 8 1 1 ~ c~rj<lrrJ. .I Agrrc Sci, (C~II!I/),) 92: 5 13-526 ( 197VI.

51l11n. S U.. M C' Stlhcn,!. and W. Ersklnr: Ad;tptutiori r>f l c ~ i t ~ i to thc Mrdltcrranc;~n en\lronmenl I. b.lcl,lr\ ;~ltect~ng ylcld ~111- <let dr<1~1gh1 ~ ~ < ~ ~ i d t t ~ o r i \ . L i p ,,lqrt,,. 29: Y- I 9 f I ')<I3 I

Sjncla~r. 'I' R ,lnd C' 'I' dcWtt. Pholos)nrhotc ;mil liltroretl reaulrentenlh fi,r iccd nnduc-

Sinc1dlr.T R . Matcr;rnd nitr~lgcn I~mttdtlirn\ In ~ ~ ~ > h c d l l gr:rtn prirduct~on I . Model debeli~p- ilicnl Pr?id ('n,,,.! HI., 15: 12.5-I41 ( ILlX61.

51nclatr. 7. K and M. M L.udli~u': lntlurncc of \oil water supply on the pldnL \cater balance of lour tri~plcul gram lepumc.. Alcvr J. Piunr Ph i t~o l . 13: 329-341 (IYXh).

Srncldlr. T. R ,A . R. Ztrnet. dnd R. C. Muchow: Change\ In soyhean nodule number and dry wclghl In re\pm\r ti1 drought, Freld Crop, Re,\. 111: 197-202 I 19XXl.

Slngh. K B.. Chtckpea breeding. In: 7ht. c'hichl~ru. ipp. 127-1 62). Saxena. M. C. and K. B. Singh. Eds.. CABI. Wallingford, U.K. (1987).

Slngh. M.. R. C. N. Rao, and J . H . Willlam\: A statistical assersment of genotypic sensrtiv-

~ t y of groundnut (Arur hi\ h)pogaeu L.1 to drought In l ~ n e \ource \prtnkler experiment\.

tuphsrilo. 57: 19-25 i 199 I ) .

S~ngh. P.: Influence ol' water d e f ~ c ~ t \ on phenol- ogy, growth and dry matter allocation In chickpee l r i r eruri~rinurn). Ficld Crop3 Re.\. 28: 1-15 (19911,

Singh. R. P, and G. Suhha Reddy. ldent~fying cnips and cropping \).\tern\ u ~ t h greater production \ t ah~ l~ ty In water delicit cn\In)n- ments, in: 1)roughr Rrrporch Priorirrn (or rhr Un' lund Trr)prp,r,\ lpp. 77-85). B~dlnger. F R, and C. Jnhdn\cn. td* . . ICRISAI'. Patancheru. l nd~a 1 IOX(>1.

Slngh. S. P, and J W White: Brred~np common hean\ Inr aduptauon to drought condltlon\ In: Xc\r,orch r,n 1)nrir~hr 7r,lc,ruri~ r 111 C'r,n- mcrn Hrorr D~icumento de Trahajo No. 41 (pp. 261-285). Wh~tc. J W.. F Hoogenh<x)m. F. Ihan-a, and S P. S~ngh. Ed \ . Bean Pro- gli::am, TIAT. Cali. Cnltimh~a (IYXXI.

S~ngh.T. N.. 1.. ti. Pdlcg. and D A\plnall Stre\\ n~e tah~ i l~ \m: I . Nitrogen met;ihr~li\n~ and pr iwth in the harley durlnf unter stre\\. Arr%r. .I ,4101. SVI 26: 45-56 ( 1973).

Sm~th. D. L.. hl. D~lah. and D. J Humc: The 'ffbct of uater del'ic~t on N: (CZH2) flxatlon hy uhite hean and soyhean. Con. .I Pl~irir SI.~. 68: 957-967 1 IYXXI

Sole. G.. A. Soje, and R. Zlrgham~: Ewly screen- ing of fahahean i V ~ ( . i ~ ~ , f i ~ h i L.1 fur drought re\~stance. FAbrlJ Nc.ila1. 22: ?&?4 (19881.

Sponchiado. B. N.. J. W. Wh~te. J. A. Castillo. and P. G. Jonec: Root growth of common bean cultlvnrs In relation to drought toler- ance in environmrnrr w ~ t h contrasting soil types. E1.p. Agric,. 25: 259-257 I 19891.

Sprent. J. I.: Effects o f water stress on nitrogen tixat~on in mot nodules. Planr Soil. Suppl.: 225-228 (197 1 ).

Sprent, J . 1.: Adherence of rand particles to soy- bean roots under water stress. New Ph?tol. 74: 461-463 (197%

Sprent. J . I.: Water dehcit5 and nitrogen fixlng root nodules. In: Warer Dtfiritr and Plunr C;r(>w,rh. Vol IV. So11 Wuter Management. Plunr R r ~ p o n s c und Bruedrrig for Droughr Rarr,rrrn~r (pp 29 1-31 31. Kor lou \k i . T . T . E d . Academ~c Pre%. New Ynrk (1976).

Squire. G R : T ~ P Ph>siolog! 111' Tropical Crr~p Produl.rron. CAB Intemat~onal. L'.K 119901

Steele. W. M.: Cowpea\. In: Evoluriun of Crop Plunrr. ipp. 183-1 85). S~mmond*. K. W.. Ed.. Longman. London (19761.

Sieponl\u\, P L., K W Shahdn, and 1. .V. Cut- ler: Osrnotrc adjustment In rice. In: f)ruirght Rtz\i\ro,i(c in Crop\ ~ i t l i Entphari\ on HI(,P. ipp 181-194). IRRI. Philipplneh lIYX2)

St~r l~ng. C. M.. C . R. Black. and C. K. Ong: Thr rc\pon*c of groundnut (-Zro~~Iir\ h!pogo<,u L.1 111 tlrnlng of ITT I~~~ IOI I 11. "C pm~t ion - ing ;lnd plant uat r r *raru\. J. Exp Ror 10: l3h7-1373 IIYXY)

Suhharao. ti. V.. (' Jnh;ln\cn. R C Napesuara Ram and G. C Wright: Tli::nnrp~ratrun cffi- clency - avenue\ tor grnellc Impro\ernent. In: Horidbool of Plilr~r ~I I I~ Crop Ph!r~ol- ogi. Peuarakli. M.. Ed.. Drldter. New York i.1991).

Summerfield. R. J and H. C. W~en. Eftects of photoperlod and air temperature on growth and yeld of economic legume* In: Adiurt(.r III Lepr~ntr S(.rc,rr<r. Vol. I (pp 17-36). Summerfield. R. J , and A. H. Bunting. Ed\.. Royal Botanic Gardens. Kew. U.K. (19801.

Summefield. R. J., S. Shanmuga\undaram, E. H. R o k n \ , and P Hadley: Soybean adaptation to photothemdl en\,lronments and ~rnpllca- tions for screening germplasm. In: Suybeun in Trupicul arid Sub-Tropical Cropping SJS- tcsm.r. (pp. 333-352). Shanmugasundaram, S. and E. W. Sulzberger. Eds.. Aslan Vegetable Research and Development Centre. Taiwan (1985).

Swaraj. K.: Environmental stress and symbiotic N: fixation i n legumes. Planr Ph?siol. Biurhrm. 14: 117-130 (1987).

Tanksley. S. D.. N. D. Young. A H. Paterson. and M. W . Bonlerhale: RFLP mnpplng In plant breeding: new tools for an old sclence. Biorerhnolog!. 7: 257-?M ( 19XY).

Tanner. C. B. and T. R. S~nclair: Efflclcnt uater ure in crop production: Rerearch or re-\rsch. In: Lirn~rarion< lo Eff;(.rrtlr U'(ir6.r I!.FP III

Crop Produrr ro~~. (pp. I-281. Taylor. H, hl.. W. R. Jordan. and T. R. Sincla~r. Ed*.. American Societ) at A~ronorn) . Madiwn. W1 (19831.

Tanzarella. 0 . A,. C, dePace. dnd A Filtppettt Stornatal frequcncq and r17e In \'I( ru tu!711 L. Crop Sci. 24: 1070-1076 llYX.11.

Taylor. H. M , and E W. Car\on. 7h<, P!u,~r Roo! atrd Irr E I Z ~ rro~in~(,ni L'ni~errtt) of Vlrgtnld Pre$h, Charlotteiv~lle. VA 1147.11.

Ttrtz. 4.. M L.udeh~g. hl D ~ n f l u h n . 2nd K Dorffing. Etlecr of ahrc i \~c dcld on the Iran\- pon of a\\lrnllate\ ITI harle). P1~11trti. 152: 557-561 t19XI)

Turk. K . J . and A E Hall. Drought ddaptdlti~ti c r t ' cowpea. 11, Influence of drought on water ure and relation\ ulth grouth and reed ).~cld. Apron J. 72: 4?8-43? ( 19KOal.

Turk. K . J , and A E. Hall. Drought adaptat~on of cowpea. 111, lnfluellce of drought on water use and relation, u ~ t h growth and reed jteld. Agr011. J . 72: 1 7 4 4 3 9 ( IYRObl.

Turner. N. C. and J E Begp: Rerponre of pas- rure plants to water deficit\. In: Plurlr Hrlii- l iolt~ in Pl rs r l r r~ (pp. SU-661. Wil\on. J . R.. Ed.. CSIRO. Melbourne. Australia (19771

Turner, N. C : Drought rristance and adaptation to water deficltr in crop plants. In: Srrrrs Ph?stolog) In Crup P l a n l ~ . (pp. 343-3721. Mussell. H. and R. C. Staples. Edr . Wiley. New York (1979).

Turner. N. C. and M. M. Jones: Turgor mainte- nance by osmotic adjustment: A revlew and evaluation. In: Adnptarions of Plan13 lo Water and High Temperalure Srresr. (pp. 87-103), Turner. N. C. and P. J. Kramer. Eds., Wiley. New York (1980).

Turner. N. C: The roleof.;hcr~t characteristtc~ In dn~ught rr.;lstance of crop plants. In: I)rorrphr Rest.\mnrr 111 Crops wrh Efrtphu.rr.\ (JII Rir?,. (pp. 1 I.(-1341. IRRI. Phil~ppine\ (IqR?).

Tumer. N . C.. E D. Schulze, and T. Gollan: The r e i p o n w of rr<rnlata and Imf pa5 exchange tr , iapor p r r w r e deflctt* and soil water content. I Specier cornpurlson* at high soil udter c ~ n t e n t r . O ~ v ~ ~ r I o , q ~ n 63: 338-342 (ILJX4)

Turner. N C Adaptation, to water dehclt\: ;I

ch.~nglng pcr\pectlbr Air!!. .I I'lrrnt PI~?\iol. 13: 175-1VO ( IYXhaI

'Turtler. N . C . ('rnp water dellc~t\ a decadr of propre\\ Ac11.. A,qrorr 39: 1-51 I ItlXhh)

Turner. N C.. W. R Stern, end P. tvanr . W;iter relatlrin< and ir*~nortc adju\tment ol le;~\e* and rcx>t* of lupins III rerpnnre lo watcr del't- cttk (".011 Sc.1 27: 977-983 (14871

Vciikate\warlu. H.. N. Saharan, 2nd M . %laherwor~. N(~dulatlon and N? hxdtlon 111

cowpea dnd groundnut dur~tlg water stre55 dnd rcclrvcry. I,rt,ld ( ' V ~ I [ J . \ Kc,\. 25: 223-232 ( lYYll1

Vlrpnna. J bl . K, T Huhlck. H. M . Rawron. (; D. Parquha~. and H. W. D o w ~ ~ e * . Genrl- typtc varldllon In tran\p~ration efficiency. carhon !\(itope dircrim~nauon and c a r h ~ ~ n allocation during early growth in umflirwer. Air.~t. .I. P I N I I ~ Ph~sia l . 17: 207-214 (1990).

Vtrmanl, S. M. and P. Slngh: Agrocl~mat~ilog~caI charactertrtics of the groundnut growing re- g u n , In the semi-and tmpicr. In: Agrr,- rnetcoro!ogy of Groirndnrrt. (pp. 35-46). Sivakurnar. M. V. K and S. M. Virmant. Edr.. ICRISAT, Patanchew, India (1986).

Walker. D. W. and J. C. Miller. Ir.: Rate of water loss from detached leaver of droughl resir- tant and 5uacept1ble genotypes of cowpea. Horric. Sri. 21: 131-132 (1986).

Wallace, D. H., J . P Baudom, J. Beaver. D. P. Coyne, D. E. Halseth, P N. Masaya. H. M. Munger. J . R. Myer\. M. Silbernagel. K. S.

Yourslone. and R. W Zobel: Improblng ef- ficiency of breedlng lor higher crop yleld Theor. Appl Gmrr 86: 2 7 4 0 (199.71.

Wea~er. D B.. D. L. Thurlow. and R. M . Patterson: Stahillty parameter* of soybean cultlvars In maturlty group\ VI. VII, and VIII. Crop .Scr 23: 569-571 (lYX31.

We\tgatc. M. E.. J. K Schu\\lcr. D C'. Rricrr\k). and M 1.. Breriner t:tfect+ of water deflcic\ on reed devclr1pmrnt in wbhcanr. II C'on- *crvation of *ecd grouth ratc I'lorir I'iri ,rol. 91: YXO-9x5 (IYXY)

White. J W Prel~m~ndr) rc<ult\ 01 the hem ~nlernationsl d r ~ ~ ~ g h t yield trial (BIDYT!. In: H~*.\can.li ori I)mii,thi 7n/cron< c i t ?

ttlort HPNII. (pp. l?h-1451. Whlte, J W . (i. H(~ogenh~r~rn. F Iharra. dnd S P. Slngh. td \ . . CIA'T. ('all. Colomh~a IIYXXI.

U'ltrtc. J W attd J A. Ca\tillo Studter i t t CIAT on mrchanl\ni\ ol d r ~ ~ ~ g h ; lolcraiicc 111 hcam In: H<,.\nrn h i ~ n Oroiichi %,Ic,rirrr< I. it, ('(,trr-

rrrorr &,ori. !pp. 146-1.51 1. Whlte. J W . (i. Hoopenhcarm. F Iharr;;. .lnd S P. Singh. h i s . . CIAT. Cult. C ~ ~ l o ~ i i h i a ( IYXXI.

Whne. J. M'.. J. A C';~\tilIo, and J Ehler~tiper: A*\ocl ln~onh hetheen producti\lt). root growth d id carhun Iwtupr d~\crinlin;i;iolt In Phosroiir\ I r r l q~ r r \ ultder wdterdcficil. Air.\! J . Plutrr Phistoi. 17: IXY-IYX !lYYOi

Wiehc. H H.: Morphol(~fical adaptation+ to watcr ! re \* . In: Allripnirtorlr o,' PINNII to M / U f c , ! crrld H ~ g h 'I;~ttq~rrurir,t~ Srru.r. (pp. 119443). Turner. N. C. and P. J. Kramer. Eda.. Wile). New Yorh (19801.

Wien. H. C, and D. H. Wollacc: Light rnduced leaflet orlentation in P h ~ i r ~ ~ o l u ~ ~.rr/guri\. Crop SI~. 18: 721-724 (19731

Willinnis. P. M . and M. Sicardi-de-Mallorca: Effect of osmotrcall) Induced leaf molsture m a s on nodulation and nitrogenahe acti\.ity of G!\rinr rtlnt. Plarrr Soil 80: 267-283 (1984).

Wl l l lam~. J. H.. R. C. N. Rao. R. Manhews, and D. H m r : Responses of groundnut geno- types to drought. In: Agromrrrorolog~ of

Gmundnur. Ipp. 99-1061. Sivakumar. M. V. K and S M Vlrmant. Ed\.. ICRISAT. Paldncheru. lndla ( 1986)

W ~ l l ~ a m \ . J. H, and N P Saxena. The u\e of nnn-dc+tructive rnea+urement of ph>\iologi- cal model\ of yleld determlnatlon to Inve\ri- gate factor\ dcterm~ntnf diftcrence\ In seed )ield bctu,een genotype\ 111 d r + ~ chickpea\. 411n.AppI.Biol 119: 10.5-ll?!1991!

Wil;on. D . Breedlng for morphological and phy\ilrlr~p~cal trdlt, In: Piorlr B r~ rd r r r ,~ 11. lpp. 233-?YOl. kre!. K . J.. Ed . Iowa State l lni\er\ i t) Pre\\. Arne\. I A (1981 I.

Winter. K: CO! ~ n d hater vapor exchange. 111dlJlr' coment dnd ' C \ r lue in Ci i c.1

crr.rcr~~?rt,rr grou~t utidsr t uo uater regime\ Z. Pfl~ir~:<,ti/~I~\ \tt,l. 101: 42 1-430 ! 1981 1.

Wonall. L: S, and R J. Roughle) The effect of niol\ture +Ire.\ on ~nfection 01 Tr~fr~lriinr \iihi~~rrutirutrr L, h! T irrfolir Dang .I Elp Uor. 27: 12.13-1241 11976)

Wright. N. L. .~nd C L. Jordan: Anificial \elec- tion tor \erJltng drought tolcrancc In buer lo \c gr:ir\ iEro,qri~si~i ~ . u r i l r I o ~ C I ~ J ~ Sir. 10: YY-102 I 19701.

Wrlght. L N Drought lnfluencr 1111 gerniination and *redling emergence. In: 1)rolrghi I t t jun ritldKrn.\rrin<r In Crops. lpp. 191.11. h i o n . K. L, and J . D East:n. Ed\.. Crop Sc~ence Society of America. Specid Publlcarlon No. 2 . Madiwn. W1 (1971 I.

Wrlght. G. C.. R. C . C. Sm~th. and J. R. McWil l~ani: Differences between two gram rorghum genotypes in adaptation to drought stress. 111. Physiologlcd rerponses. A~rsr. J. Agrrr. Res. 34: 637-651 1198.7).

Wright. G. C.. K. T. Hubick. andG. D. Farquhar: Discnminat~on in carbon isotopes o f leaves comlates with water use efficiency of field

grown peanut culrivar;. A~rsr. J. Planr soto ope d~scritnination ~n peanut under water Phrs~ol . 15: 815-82.5 119881. delicit c o n d ~ t ~ o n \ . C'ro/> Sc I . 3 4 11:9?-97

Wr1eht.G. C.. K. T. Hub~cL..andG D. Faruuhar 11944)

Physiological analyu, of peanut culti\,ir> response timing and duration of dmughl

Zhan. Y . . S. Kamliahi~. and Y . Masuda. Qui~n-

,tre5s, Ausr, J , ,4Srrc Rc.s 42: 153--170 ti1u1ibr relattonb hctwrcn oimotlc poten-

11991). tiul and epicntyl growth in V;,YIICI radrnro n\ nft'ected hy o*mot~c .;[re\\ and corble..

Wright. C. C.. R. C. N.~ge\uara Riri~, and C D. don excrslnn Phi \;,,I. Plor~r. 64: 1.3 1-437 Farquhar: Water-uhe efhclcncy and cdrhiln (1985) .