Plant Physiol. (1980) 65, 340-3450032-0889/80/65/0340/06/$00.50/0

Mechanism of Monocarpic Senescence in Rice'Received for publication February 26, 1979 and in revised form August 22, 1979

ARUN K. BISWAS2 AND MONOJIT A. CHOUDHURIPlant Physiology and Biochemistry Laboratory, Department of Botany, Burdwan University, Burdwan 713 104,West Bengal, India

ABSTRACT

Dwing grain formatin stap (90 to 110 days), the youngest flag leaf ofrice (Ouyza siva L. cv. Jaya) remaned metaboically most active (asinicated by celluhr constituents and ewyme activities) ad the third leafthe least active. At the grain developmnt stage (110 to 120 days) theabove pattern of age-related senescence of the fag leafcompletely changedand it senesced at a faster rate than the second leaf which remainedmetabolically active even up to grain maturation time (120 to 130 days),when both the flag and the third leaf partialy senesced. Removal of anyleaf temporarily arrested senescence of the remainng attached leaves, thatof flag leaf did not hasten senescence of the second leaf, while that ofeither the second or the third accelerated senescence of the flag. Removalof the inflorescence after emergence or foUar treatment of intact plant withkInetin equaly delayed senescence and produced an age-related, sequentialmode of senescence or leaves. Both translocation and retention of S2p bythe flt leaf were wman at the time of grain formation and that by thesecond leafwasmataed even up to grain maturation time. The inductionof senescence of the flag eaf was preceded by a plentiful transport of 32Pto the grains. Kinetin treatment decreased the transport of 32P, prolongedits duration, and almost equally involved all of the leaves in this process.The pattern of senescence of Isolated leaf tips was similar to that ofattached leaves. The level of endogenous abscisic acid-like substance(s)maintahied a clse inearity with the senescence behavior of the leaves ofintact and defuted plants during aing, and the rise in abscisic acid in theflag leaf was also preceded by higber nP transport to the grains.

Although a great deal of information on leaf senescence hasaccumulated in recent years, comparatively little is known aboutthe senescence process of the whole plant (12). In monocarpicannuals, the whole plant senesces abruptly and immediately afterthe completion of reproductive development. The cause of suchchanges. leading to senescence during this period remains uncer-tain (4).Long ago, Molisch (19) propounded the idea that senescence in

plants is induced as a result of diversion of nutrients from leavesor apex to the developing fruits. This has received support fromsome recent observations (24, 29). The nutrient depletion theoryof senescence proposed by Molisch (19) has, however, been ques-tioned by several workers (4, 13-15). Their experiments, conductedwith leguminous plants, support the concept that a senescence-inducing factor or a signal coming from fruits is responsible forplant senescence, rather than a depletion of metabolites fromleaves through mobilization. However, the intimate involvement

I Part of the thesis submitted for the Ph.D. degree of the University ofBurdwan.

2 Presnt address: Department of Botany, Ramananda College, Bish-nupur 722 122, Bankura, West Bengal, India.

of the reproductive phase in the development of such senescencehas been stressed by all of them.The rice plant seems to be a good experimental plant for

studying the mechanism of monocarpic senescence in annualswhere the sink is situated solely at the top, unlike leguminousplants where such sinks are present at several nodal positions. Thecharacteristic feature of this plant is that a special leaf, called the"flag" leaf, emerges as the last leaf during the reproductivedevelopment, but the initiation of senescence of this leaf ensuesmuch earlier than that of the two older leaves below it.

Thus, the typical sequential senescence observed in some otherplants is quite lacking in rice, and hence, this plant has beenselected to allow a different approach to the study of the mecha-nism of monocarpic senescence.

MATERIALS AND METHODSCuWlvation of Plants. Certified seeds (having 100%o germinabil-

ity) of rice (Oryza sativa L. cv. Jaya), collected from the StateAgricultural Farm, Burdwan, Govt. ofWest Bengal, were surface-sterilized in 0.1% HgCl2 for 1 min and then washed well in runningwater. Twenty-one-day-old seedlings raised from these seeds weretransplanted with one seedling per hill in earthenware pots (23-cm diameter and 26 cm high) containing loamy soil mixed withorganically rich, farmyard manure. Water on the soil surface ofeach pot was always maintained at a constant level.Measurement of Senescence of Leaves at Reproductive Stage.

The experiment was carried out with the leaf samples collectedfrom the plants during the period from ear emergence (90 days)to harvest (130 days). The changes in the contents of CM, protein,and RNA and the activities of catalase, protease, and RNase inthe flag, second, and third leaves at intervals of 10 days were takenas a senescence index. Enzymes were extracted from leaves (1 g)with 0.1 M phosphate buffer (pH 6.5) at 0 C, centrifuged at 10,000gfor 25 min, and the supernatant was used as the enzyme sourcefor the assay of the above enzymes. The activities of protease,RNase, and catalase were colorimetrically measured in the mannerdescribed by the present authors in details elsewhere (3). Theenzyme activities were expressed according to the formula of Fickand Qualset (8).

Defoliation Experiments. Any one from the three leaves insuccession of 100-day-old plants was removed in three differentsets of experiments, each consisting of 10 plants. The Chl andprotein contents of the remaining attached leaves in each set ofexperiments were estimated at intervals of 7 days up to 128 days.The unexcised plants served as controls.

Defruiting Experiments. Complete and half-ears were removedfrom two different sets of experiments containing 10 plants each.Contents of Chl and protein from the three leaves in each exper-iment were followed at intervals of 10 days up to the time ofharvest. Intact plants served as controls.

Spray Application of Kinetin. Aqueous solution of kinetin (100,ug/ml in 0.5% Teepol used as surfactant) was sprayed onto leavesof 10 plants at the rate of 5 ml/plant at the postflowering stage(96 days old) consecutively for 4 days. Control plants were simul-

340

MONOCARPIC SENESCENCE IN RICE

taneously sprayed with distilled H20. The senescence index men-tioned earlier was measured at intervals of 10 days from the timeof last spray up to harvest.

32p Translocation from Leaves to Grains. The three leaves fromkinetin-treated and control plants were separately fed with radi-ophosphorus (0.11 iCi/mmol) and the radioactivity (cpm/g dryweight) retained by the fed leaf and translocated to the grainswere recorded in a Geiger-Muller counting system. The detailmethodology has been described in another paper (2).Measurement of Senescence of Isolated Leaf Tips. Three-cm

tip portions of the leaf from each of the three successive leaveswere collected separately from 90-, 110-, 120-, and 130-day-oldplants and their senescence behavior during incubation in waterin the dark was followed.

Extraction and Estimation of ABA-like Substance(s). Semi-quantitative estimation of ABA-like substance(s) from the leavesof intact and defruited plants and partial purification of theextracted ABA were done (30) from 10 g freshly harvested leafsamples. These were then chopped, immersed in 80%o (v/v) meth-anol, and refrigerated for 24 h. The supernatant was decantedafter thorough washing with fresh aliquots of 80%o methanol andthe extract was freed from Chl by passing through activatedcharcoal. The fitrate was evaporated to dryness under partialvacuum at a maximum temperature of 35 C. The residue wasreconstituted in 33 ml ofwarm distilled H20, the pH was adjustedto 2.5 with 1 N HCI, and the free ABA-like substance(s) wasextracted three times with ether in a separating funnel shakenvigorously for 90 s; the layer was allowed to partition for at least2 min. The bulked ether containing free ABA-like substance(s)was dried with 90 g anhydrous sodium sulphate for 2 h andallowed to dry under an exhaust. The residue was redissolved in0.2 ml methanol and diluted to 1 ml with distilled H20 and keptat 2 C until used in bioassay.The ABA-like substance(s) in the extract was assayed following

the degradation of protein in the isolated leaf tips (first leaf)collected from 30-day-old rice plants (cv. Jaya). Protein wasestimated by Folin phenol reagent after 24-h incubation of theleaf tips in 1 ml of the test solution. The effect of synthetic (±)ABA on protein degradation of leaf tips of rice was found to berepeatable and sensitive (data unpublished), hence this bioassaywas adopted.

RESULTS AND DISCUSSION

The differential senescence behavior of the youngest flag leafsubtending the ear becomes clear if one follows the changes insome biochemical parameters and in the activities of enzymes

I-3:I(ALai

%-U)

zLI-z0U

4l0

3-0

2.0

I*0

having functional significance in senescence during the reproduc-tive development. It is now well known that the senescence processis characterized by the degradation ofChl, protein, and RNA (23),with a concomitant increase in the activities of a number ofhydrolytic enzymes (16, 26), and a decrease in catalase activity(1 1).Up to the age of 110 days (grain formation period) the flag leaf,

being youngest, contains higher levels of cellular components thanthose of the two older leaves (Fig. 1). This pattern of age-relatedmacromolecular degradation becomes completely reversed whenthe plant enters the stage of grain development (110-120 days).At this stage the cellular contents of the flag leaf are at theirrelative minimum. During this period the loss of cell constituentsin the flag and the third leaf proceeds at a faster pace, resulting inpartial senescence of these leaves at 130 days (Fig. 1). The mostsignificant event is that the period of faster degradation in the flagleaf almost coincides with that of the grain development and thatthe senescence of the second leaf is more delayed than that of theothers at 130 days. Thus, the senescence of rice leaves may not bea simple function of leaf age at the grain development period.The higher breakdown of protein and RNA in the flag leaf,

particularly at the grain development period, correlates withhigher activities of protease and RNase and lower activity ofcatalase. But, in the present study such a relationship of enzymicactivity with leaf age has not been observed in all of the leaves atthe time of grain development. At this stage activities of proteaseand RNase were higher and catalase activity lower in the flag leafcompared with the other two leaves (Fig. 2). This suggests that thesynthetic competence of the second and the third leaf is stillmaintained even when the youngest flag leaf has started senescingat this developmental stage. However, the degradations of bothprotein and RNA in the second and the third leaf are not alwaysproportional to the activities of protease and RNase, respectively,in those leaves (Figs. 1 and 2). Similar observations are not scarcein the literature (26, 27).Although there is strong evidence that some phase of reproduc-

tive development initiates senescence in monocarpic annuals (13-15, 19, 24), the nature of such involvement and the manner inwhich it participates are still conjectural. The cause of differentialbehavior of the flag leaf in the present experiment may be attrib-uted to its close proximity to the reproductive part (ear), whicheither draws in more metabolites from the nearest source or sendssome sort of senescence stimulus to it. However, in the presentcase, the latter possibility seems more remote because the secondleaf does not senesce next to the flag leaf which is clear from thedata of 130 days (Fig. 1).

RNA

15~ 3:

ChE

ss ~ ~ --~I- --8*.YELLWEtO

% ----80¾ YELLOWEDJn I FAF AR cA:5 u0

*- PLANT AGE (DAYS) -bFIG. 1. Changes in contents of chlorophyll (CHL), protein (PR), and ribonucleic acid (RNA) of the three leaves of rice (cv. Jaya). Data were

recorded at intervals of 10 days from ear emergence to harvest (plant age 90-130 days).

341Plant Physiol. Vol. 65, 1980

BISWAS AND CHOUDHURI

C

.'I. I

LAG LEAF ',.ND LEAF ',%

%I|

300 I

III a

200 'aI +. II

II '

100*-3RDLEAIFT . "a~ ~~I0.ELWD

150

125

- 75

50

25

90I00 110 120 130 9d K)0 110 120 130 3I00 110 120 130

-4 PLANT AGE (DAYS)FIG. 2. Changes in activities of catalase (C), protease (P), and RNase (R) of the three leaves of Jaya rice. Data were recorded as in Figure 1. Enzyme

activity was expressed as [(AA x Tv)/t x vJ, where A is the absorbance of sample after incubation minus the absorbance at zero time control, Tv the totalvolume of the filtrate, t the incubation time in min, and v the volume of filtrate incubated.

DEFOLIATED >§ DEFOLIATEDU.. ,,1 1 1 1 ,

3RD LEAF 0 81 YELLOWED

0.0:

3.0i

2.0

DEFOLIATED

00 107 114 121 128 100 107 114 121 128

- PLANT AGE (DAYS ) -

FIG. 3. Changes in Chi content ofthe three leaves of intact (a), and theremaiing leaves of the defoliated plants ofJaya rice. The flag leaf (b), thesecond leaf (c), and the third leaf (d) were removed separately at the plantage of 100 days. Data were recorded at intervals of 7 days from thedefoliation time up to 128 days.

A higher rate of breakdown of the cell components occurs inthe second leaf than in the unexcised control after removal of theflag leaf (Figs. 3a and 4a), but this leaf senesces later than thethird leaf (Figs. 3b and 4b). The higher rate of breakdown in thisleaf may be to compensate partially the role of flag leaf as an

B251 J.DEFOLIATED

C.l d

ui

CLa

00

-,3RD LEAF 801/. YELLOWED

25 DEFOLIATED j~DEFOLIATED

0oo 107 114 121 128 100 107 114 121 128

- PLANT AGE (DAYS)FIG. 4. Changes in protein content of the three leaves of intact (a), and

the remaining leaves of the defoliated plants of Jaya rice. Methodologysame as in Figure 3.

effective source of metabolite transport. But the earlier senescenceof the third leaf may be a normal consequence of aging. Theinitial higher contents of Chl and protein of the two remainingattached leaves after removal of any one indicate their increasedsynthetic activity, possibly due to higher sink demand (9). Thisphase of synthetic activity is transitory and soon a more rapiddecline of cell components starts in these leaves. This can be

l50

125

a~-

a~-

LAJ

NzuJ

100

75

50o---o FL

2T .--- 2Ip

I - I 111- I

T

342 Plant Physiol. Vol. 65, 1980

Plant Physiol. Vol. 65, 1980

further supported from the experiments where the second and thethird leaves have been separately removed (Figs. 3c, 3d, 4c, and4d). As a result, the attached flag leaf senesces much earlier thanthat of the intact control, pointing to the possibility of higher

CHL [a] PR

5.0- 75

2 9011-3 0 3

t IER ER 50 3:

39 I1.0 LIPLANT -O FLAG LEAF

&iJ U.

U- ~~~~~~~~~~~~25.

FIG5 CHL io t c r4(E - 100t

0at i.0of0cc ~~~~~~~~~~~~~750

-JI

50

10 HER HER2ND LEA -43RD LEAF

25

90 110 13O 90 110 1SOPLANT AGE (DAYS)

FIG. 5. Changes in contents of chlorophyll (CHL) and protein (PR) ofthe three leaves of Jaya rice. (a) The whole ear (ER) and (b) the half of theear (HER) were removed at the plant age of 100 days. Data were recordedat intervals of 10 days from the time of removal.

CHL PR

100*-4.0

LL3*0 75t-

E

2-0

2.10 KIN KIN

NESCENCE IN RICE 343

export of metabolites as one of the important causes for theinduction of earlier senescence of the flag leaf.The removal of inflorescence modifies the senescence behavior

of the three leaves so dramatically that the senescence of all of thethree leaves is delayed and a sequential mode of senescence isachieved even at the plant age of 120 days (Fig. 5a), when theleaves of intact control plants have showed an opposite trend.Thus, the removal of major sink may have facilitated the accu-mulation of cytokinin in the leaves (21, 25) and resulted in thedeferral of senescence of all of the leaves, but most strongly of theflag leaf. The removal of half of the ear (Fig. 5b), even thoughdelaying senescence slightly, could not change the senescencebehavior ofthe flag leaf (Fig. 1), suggesting a positive contributoryrole of the fruits in inducing differential flag leaf senescence. Itmay be noted that compared to control, the kinetin treatment ofleaves has much delayed the decline of cell components (Fig. 6)and retarded the activities of protease and RNase and increasedthe activity of catalase (Fig. 7), thus changing the senescencepattern of the three leaves, which has now been found to be afunction of leaf age. Thus, kinetin treatment mimics the action ofear removal and probably delays senescence by augmenting thesynthetic ability and/or metabolic sustenance of the leaves.The results show that the flag leaf remains metabolically most

active at the grain formation stage, as indicated by its higherretention and transport capacity of 3P at that time (Table I). Themaximum transport of 32P to the grains at this time is possibly dueto higher sink demand at this stage. With the dwindling rate of32p transport from the flag leaf with aging, the other two leavesshow greater involvement in the transport process, perhaps tocompensate the demand for metabolites still retained by the grainsat the top. The second leaf retains its metabolic capacity until theplant age of 130 days, when the flag and the third leaves havealmost senesced. The maximum transport of metabolites to grainsis an earlier event in the flag leaf which perhaps leads to quickersenescence of this leaf at the later period.

Higher cytokinin level in the developing fruits (10) strengthensthe sink capacity of the fruits (22) which reaches a maximum levelin rice at the time of grain formation (20). A 32P translocationstudy reported here also supports this. This is also clear from theobservations that kinetin treatment enhances the metabolic activ-ity of leaves by increasing their metabolite retention capacity and

RNA 50

25 _<:D~~~~~~~~~~~~~~~~~~~~~~-20

E

15

z

WiLl~~~~~~~~~~~~~i

.%T T~ i ---'.ooo FLAGLEAF'325S &-' 2ND LEAF 80

I I I II I' I I* I

100 110 120 130 90 100 110 120 130 90 100 110PLANT AGE (DAYS)S-

FIG. 6. Changes in contents of chlorophyll (CHL), protein (PR), and ribonucleic acid (RNA) of the three leaves of Jaya rice. Plants were sprayedwith kinetin (KIN, 100 mg/l) at the postflowering stage (plant age 96 days). Data were recorded at intervals of 10 days from the time of treatment.

I KINI4 .*-A

MONOCARPIC SE?

BISWAS AND CHOUDHURI

1-

1-L:

II.

NzL&

- PLANT AGE(DAYS)FIG. 7. Changes in activities of catalase (C), protease (P), and ribonuclease (R) of the three leaves of Jaya rice. Methodology same as in Figure 6.

Enzyme activity was expressed in the same way as in Figure 2.

Table I. Changes in 32P Retention and Translocation in Control andKinetin-treated Plants of Different Ages during Reproductive DevelopmentOne ml of 32P as buffered orthophosphate (0.11,IICi/mmol) was fed

through the tip ofeach leaf separately for 24 h and the radioactivity (cpm/min.g dry tissue) was measured in the fed leaves (retention) and in thegrains (translocation).

Leaf Po- Plant Age (Days)Treatment ..sition 110 120 130

RetentionFlag 2192±241 496±37 28±72nd 2112 ± 179 968 ± 85 105 ± 193rd 512±30 832±78 14±4

Control TranslocationFlag 207± 35 47± 12 10±62nd 95± 17 115± 18 71 ±153rd 65± 11 125± 15 9±2

RetentionFlag 3657 ± 256 1864 ± 194 480 ± 422nd 3085 ± 211 1078 ± 145 761 ± 393rd 1425 ± 166 712 ± 46 125 ± 22

Kinetin TranslocationFlag 117 ± 22 172 ± 19 88 ± 152nd 110 ± 33 121 ± 14 105 ± 183rd 91± 10 85±7 13±5

delaying the time of maximum transport of 32p (Table I). Thus, itcan be argued that the delayed senescence of the flag leaf due tokinetin treatment may be an effect of late translocation of metab-olites, persistence for a longer duration, and greater participationof the subsequent leaves in the process of mobilization.The pattern of senescence of the isolated tips from the three

leaves follows essentially the same trend (Fig. 8) as shown by theleaves in attached condition, suggesting that the senescence-in-ducing substance, if any, once formed in the flag leaf, sustains itseffect even in the isolated condition.The concept of cytokinin-ABA balance controlling the whole

plant senescence is at present gaining momentum (1). Reports thatstressed conditions increase the ABA levels in leaves abound inthe literature (18, 31). The possibility of such an augmentation ofstress-induced ABA formation in the flag leaf of rice due tonutrient exhaustion has gained support from our results of endog-

j60-<40 -

20 1 ' 1

280

60-

FIG 8.Cagsi ahoohl CL adpoen(R otnso h

40--U20,Z

1 23456 123456123456 123456

DAYS AFTER INCUBATION

100 110 i120 130

PLANT AGE (DAYS)

FIG. 8. Changes in chlorophyll (CHL) and protein (PR) contents of the

excised tips of the three leaves of Jaya rice. Leaf tips were collected fromthe flag (-4), second (A A), and third( leaves of plantsof different ages (100-130 days) and incubated in the dark in distilledwater. Data were expressed as per cent decrease from the initial.

Table II. Effect ofABA -like Substance(s) on Protein Content of Rice LeafTips

Leaf tips, collected from the first leaf of 60-day-old plants, were dark-incubated in ABA-like substance(s) extracted separately from three leavesof intact and defruited plants of different ages. Critical difference (CD) isthe value of difference between T and L which is just significant at thesignificance level P = 0.05.

ABA-like Plant Age in Days (T)

Treatment Substance(s)from Leaf 90 110 120 130

(L)

Intact plant Flag 1.8 8.6 21.1 24.42nd 7.5 16.5 17.9 18.73rd 8.4 19.2 19.8 20.6

Defruited plant Flag 5.4 7.6 17.52nd 10.1 14.5 15.93rd 14.5 17.6 20.2

CD at 5% (T x L) 0.85 2.67 1.66 1.45

50- tKIN\ 2ND LEAF"s} 0--o 3RD LEAF

3 90I ^10 1

130 90"100 110 I4

344 Plant Physiol. Vol. 65, 1980

MONOCARPIC SENESCENCE IN RICE

enous ABA-like substance(s) in the three leaves during the repro-ductive period (Table II). A steady increase in ABA-like sub-stance(s) in the flag leaf at the time of grain development (120days), preceded by a substantial amount of metabolite transportfrom it during grain formation stage (I 10 days), possibly leads tothe earlier senescence of the flag leaf. The removal of ear lowersthe endogenous ABA-like substance(s) and defers flag leaf senes-cence. This supports the hypothesis that the rise in ABA-likesubstance(s) in the flag leafmay well be due to the stress developedas a result of nutrient depletion, thereby inducing its senescence.This is further strengthened by the findings that in wheat ABA ismetabolized in the maturing grains and not redistributed withinthe plant (17), in barley ABA is formed at the time of grainmaturation and not at the time of grain development (28), andthat in the Alaska pea, ABA appears only during the final phaseof fruit development (5). The delayed senescence by kinetintreatment may be explained by the fact that the cytokinin cancounteract the senescence-promoting effect of ABA (6, 7). Thedeferred senescence of the second leaf is possibly due to itsprolonged synthetic activity, lower rate of metabolite transport,and lower level ofendogenous ABA-like substance(s). The presentstudy strongly advocates that senescence in rice is controlled by abalance between ABA and cytokinin and that it is induced byABA-like substance(s) formed by a deprivational stress developedin the leaves as a consequence of maximum transport of metabo-lites to the grains, rather than a signal coming from the fruits.

LITERATURE CITED

1. ADDICOTT FT, JL LYON 1969 Physiology of abscisic acid and related substances.Annu Rev Plant Physiol 20: 139-164

2. BiswAs AK, MA CHOUDHURI 1978 Deferral of leaf senescence and increasedproductivity in rice. J Nuclear Agric Biol 7: 14-18

3. BISWAs AK, MA CHOUDHURI 1978 Differential behaviour of the flag leaf ofintact rice plant during ageing. Biochem Physiol Pflanzen 173: 220-228

4. DAVIES PJ, WM PROEBSTING, TJ GIANFAGNA 1977 Hormonal relationships inwhole plant senescence. In PE Pilet, ed, Plant Growth Regulation. Springer-Verlag, Berlin, pp 273-280

5. EEUWENS CJ, WW SCHWABE 1975 Seed and pod wall development in Pisumsativum L. in relation to extracted and applied hormone. J Exp Bot 26: 1

6. EL-ANTABLY HMM, PF WAREING, J HILLMAN 1977 Some physiological re-sponses to d,l-abscisin (dormin). Planta 73: 74-90

7. EVEN-CHEN Z, C ITAI 1975 The role of abscisic acid in senescence of detachedtobacco leaves. Physiol Plant 34: 97-100

8. FICK NG, CO QuALsET 1975 Genetic control ofendosperm amylase activity andgibberellin responses in standard-height and short-statured wheat. Proc Nat

Acad Sci USA 72: 892-8959. GEIGER DR 1976 Effects of translocation and assimilate demand on photosyn-

thesis. Can J Bot 54: 2337-234510. HANN H, R DEZACKS, H KENDE 1974 Cytokinin formation in pea seeds.

Naturwissenschaften 61: 17011. KAR M, D MIsHRA 1976 Catalase, peroxidase, polyphenoloxidase activities

during rice leaf senescence. Plant Physiol 57: 315-31912. LEOPOLD AC 1975 Aging, senescence and turn-over in plants. Bioscience 25:

659-75213. LEOPOLD AC, K NIEDERGANG, J JANICK 1959 Experimental modification of

plant senescence. Plant Physiol 34: 570-57314. LINDoo SJ, LD NOODEN 1977 Studies on the behavior of the senescence signal

in Anoka soybean. Plant Physiol 59: 1136-114015. MALIK NSA, AMM BERRuE 1975 Correlative effects of fruits and leaves in

senescence of pea plants. Planta 124: 169-17516. MARTIN C, KV THIMANN 1972 The role of protein synthesis in the senescence of

leaves I. The formation of protease. Plant Physiol 49: 64-7117. McWHA JA 1975 Changes in abscisic acid levels in developing grains of wheat

(Triticum aestivum L.). J Exp Bot 26: 823-82718. MizRAHi Y, AE RIcHmoND 1972 Abscisic acid in relation to mineral deprivation.

Plant Physiol 50: 667-67019. MOLISCH H 1928 Der Lebensdauer der Pflanzen. In EH Fulling, transl, 1938

The Longevity of Plants. Science Press, Lancaster20. ORITANI T, R YOSHIDA 1976 Studies on nitrogen metabolism in crop plants. XIV.

Changes in cytokinins in rice grains during ripening. Proc Crop Sci Soc Jap 45:429-435

21. PRocHAzKA S, H SCHRAUDOLF, J SONKA 1977 Transport of'4C-kinetin in intactand decapitated pea seedling (Pisum sativum L.). Z Pflanzenphysiol 81: 308-313

22. SEILER-KELIsTscH H, G MICHAEL, E WILBERG 1974 Relationships between grainweight and cytokinin activity in spring barley demonstrated by cutting off sideshoots. Angewandte Bot 48: 299-307

23. SHAw M, PK BHATTACHARYA, WA QUICK 1965 ChlorophylL protein and nucleicacid levels in detached senescing wheat leaves. Can J Bot 43: 739-746

24. SINcLAIR TR, CT DEWIT 1975 Photosynthesis and nitrogen requirements forseed production by various crops. Science 189: 565-567

25. SrrrON D, C ITAI, H KENDE 1967 Decreased cytokinin production in the roots asa factor in shoot senescence. Planta 73: 296-300

26. SRIVASTAVA BIS, G WARE 1965 The effect of kinetin on nucleic acids andnucleases of excised barley leaves. Plant Physiol 40: 62-64

27. STOREY R, L BEEVERS 1977 Proteolytic activity in relationship to senescence andcotyledonary development of Pisum sativum L. Planta 137: 37-44

28. WAGNER H 1974 The influence of kinetin on protein synthesis in ripening barleygrain. Angewandte Bot 48: 175-184

29. WITTENBACH VA 1978 Changes in proteolytic activity and the levels ofRuBPcasein the flag leaf of wheat during grain development and senescence. PlantPhysiol 61: S-25

30. WRIGHT STC 1975 Seasonal changes in the levels of free and bound abscisic acidin black currant (Ribes nigrum) buds and beech (Phagus sylvatica) buds. J ExpBot 26: 161-172

31. WRIGHT STC, RWP HIRON 1972 The accumulation of abscisic acid in plantsduring wilting and under stress conditions. In DJ Carr, ed, Plant GrowthSubstances 1970. Springer-Verlag, Berlin, pp 291-298

Plant Physiol. Vol. 65, 1980 345