Pertanika 9(3), 291 - 298 (1986)

Water Use of Young Citrus as a Function ofIrrigation Management and Ground Cover Condition

KAMARUDZAMAN ARIBIFarm Division,

Faculty of Engineering,Universiti Pertanian Malaysia,

43400 Serdang, Selangor,Malaysia.

Key words: Trickle irrigation; lysimeter; potential evapotranspiration; crop evapotranspiration;crop water use coefficients.

ABSTRAK

Kertas kerja ini menerangkan kesan perairan dan pengurusan penutup bumi ke atas sejatpepeluhan, tamanan (ET) pokok limau muda jenis Valencia di atas tanah pasir halus Arredondo.Enam·perlakuan yang berlainan telah digunakan: tiga paras potensi air tanah dan dua paras keadaanpenutup bumi. Keputusan yang diperolehi dalam perlakuan yang mempunyai rumput penutupmemerlukan lebih 50% air daripada perlakuan tanpa rumput penutup. Sejat perpeluhan berkaitsecara positif dengan jumlah perairan yang dib'erikan. Angkali penggunaan air tanaman (Kc) lebihtinggi daripada perlakuan tanpa rumput penutup. Angkah penggunaan air tanaman menurun apa­bzia upaya air tanah yang dikenakan menurun.

ABSTRACT

This paper describes the effect of irrigation and ground cover management on crop evapo­transpiration (ET) of young Valencia citrus trees grown on Arredondo fine sand. Six different treat­ment combinations were used: three levels of sod water potential and two levels of ground coverconddion. Results obtained with grass cover treatments required 50% more water than with no grasscover treatments. Evapotranspiration correlated positively with the amount of irrigation applied.Month(y crop water use coefficients with grass cover treatments were 50% higher than with no grasscover treatments. Crop water use coefficient decreased as soil water potential decreased.

INTRODUCTION

Water is important to crop in providing asoil environment for the development of rootsystem, and supplying water for plant use. Soilmoisture must be maintained in a range thatpermits absorptions of water by plant roots at arate comparable to evapotranspiration losses.

Evapotranspiration rate is affected by manyfactors, the most important of which are theamount of leaf area, stage of crop growth,climate and soil. The most important climatic

factor affecting evapotranspiration IS solarradiation.

A study of efficient use of irrigation waterand accurate prediction of crop water is animportant step in reducing cost of production ofthe crop. Therefore, knowledge of crop wateruse is essential for efficient irrigation manage­ment.

The use of trickle irrigation has receivedconsiderable interest in the past few yearsbecause of its ability to minimize water use, in­crease water use efficiency (Harrison et al., 1984)

KAMARUDZAMAN ARIBI

and efficiently substitute the conventional irriga­tion without adverse effects on crop performance(Bester et al., 1974). Using trickle irrigation,water can be saved because non-productivewater used by evaporation and weed growth

-between trees can be minimized, especiallyduring the early stage of growth (Fereres et al.,1982).

The objectives of this investigation were:

1. To determine water use by young citrus treesirrigat€d with trickle system under two diffe­rent ground cover conditions.

2. To evaluate crop water use coefficients as afunction of soil water potential and groundcover conditions.

MATERIALS AND METHODS

DescTlfttion ofExperimental Facilities

The experiment was conducted at the Irri­gation Research and Education Park in Gaines­ville, Florida. Two year-old Valencia citrus trees(Citrus sinensis L.) grafted on sour orange rootstock were transplanted in drainage-typelysimeters. The lysimeters were buried in the soilso that the soil inside and outside the lysimeterswas at the same ground-level. The soil wasArredondo fine sand (Hyperthermic, uncoatedTypic Quartzipsamments).

Automatic rain shelters were used to coverthe plants during rain, and were removed assoon as rainfall ceased. Calibrated rain gaugeswere installed at the center and corners of theexperimental area in case of shelter failures and/or drift of rainfall. Surface drainage wasconstructed around the area to prevent rainwater from running under the shelter and intothe lysimeters.

Cultural Pra.ctices

The buffer and lysimeter areas were plantedwith bahiagrass (Paspalum notatum). Weedswere controlled by hand.in the lysimeters thatreceived treatments without grass cover. Grassesin the lysimeters that received treatments with

grass cover were clipped to a height of 15 cmwhen the grass got taller than 30 cm. The grassesin the buffer area were irrigated by hand andmowed when necessary.

Citrus trees were transplanted bare root intothe lysimeters. First side-dressing fertilizer wasapplied two weeks after planting, and then everymonth with compound fertilizer, 6 : 6 : 6 (N : P ~

05

: K~O) plus 1.0% Mg, 0.02% B, 0.05% Cu,0.3% Fe, 0.1 % Mn and 0.1 % Zn at the rate of0.5 kg per tree by broadcast.

Malathion was applied for insect control asneeded. Other management practices werefollowed according to the Institute of Food andAgricultural Science recommendations Gulianand Jackson, 1980). The trees were allowed torecover for one month. The actual experimentwas started in September and ended inDecember, 1984.

Treatments

This experiment was arranged in a com­pletely randomized design with factorial treat­ments. It consisted of 6 different treatment com­binations (3 levels of soil water potential (SWP)and 2 levels of ground cover), replicated fourtimes. The treatments were

lOG = - 10 kPa soil water potential withgrass cover.

20G = - 20 kPa soil water potential withgrass cover.

40G = - 40 kPa soil water potential withgrass cover.

lONg = - 10 kPa soil water potential with­out grass cover.

20Ng = - 20 kPa soil water potential with­out grass cover.

40Ng = - 40 soil water potential withoutgrass cover.

Each lysimeter was irrigated independentlyat predetermined soil water potentl~rs of - 10kPa,- - 20 kPa and - 40 kPa which is equivalentto 24%, 44% and 50% soil water depletion fromfield capacity level, respectively (Figure 1).. Fieldcapacity was considered to be - 5 kPa SWP

292 PERTANIKA VOL. 9 NO.3, 1986

WATER USE AND IRRIGATION MANAGEMENT OF YOUNG CITRUS

_\000

DRAINFillER

unAINTRAP

Ii'~~.~~_: -- -:_WN_-r_-_o-f,L~t:: :: flu ::II II .1 I,II II II 'I

" 11 U ----l,\1lk 'I

II II I,

I, I,

I , III I I,

!I III I~~~I~ ;~) .J _ .. ,;'~,'

----.-_":."':.:..._~

SOlENOIDYAlVr

Fig. 2: Details of individuallysimeter soil waterstatus moniton'ng system.

II

-20 ~ ~~-.-_-.-JJ-\0 , ,

II', I'

"IIIIIII,IIIII,II

P,IOL-__.J..JLl.J... ...!.....: --'--:- ..4.

0.0 0.1 0.2. 11.3 o,~

VOLUMETRIC WATER CONTENT (mm3

nwn·3 )

oJ

(5III

6ooJ

'ii .Ion~oJ'li=z1&1~o~

a:1&1~

'lI

Fig. 1: The relationship between water content andsoil water potential of Arredondo fine sand(Clark, J982).

which corresponded to 0.11 - mm 3 mm - 3 soilwater content.

the soil in one dimensional manner at the rate of270 mm h - I and applied 7.0 mm, 12.0 mm and14.0 mm depth of water' per cycle for treatments-10 kPa, - 20 kPa and - 40 kPa SWP, res-pectively.

Irrigation SystemA ctual Evapotranspiration

The amount of actual evapotranspiration ofthe trees at different soil water potential werecalculated using the following water balanceequation:

The amount of irrigation applied to eachlysimeter was recorded on a small calibratedimpeller flow meter. Irrigation events for eachlysimeter were recorded on a:. 24-channe1 eventrecorder.

where ET actual evapotranspiration (mm),P amount of prec~pitation(mm),I amount of irrigation (mm),D amount of drainage (mm)!:J.S = changes in soil water storage (mm).

Calibrated automatic tensiometers wereused to initiate irrigations. Each lysimeter hadswitching tensiometers located at depths of 15cm, 30 cm and 60 cm (Figure 2). The tensio­meters were serviced every 2 weeks (Smajstrla etal., 1981) or as required by the presence of airbubbles in the tensiometer tubes.

The amount of water applied was controll­ed using an automatic timer-controller. Instru­mentation for the automatic irrigation controlwith tensiometers is described by Smajstrla et al.(1981). The automatic timer-controller was setto irrigate for a period of 4 minutes, 6 minutesand 7 minutes for irrigation initiated at - 10kPa, - 20 kPa and -40 kPa SWP, respectively.Fifteen emitters were used per lysimeter to wet

ET = I + P - D - !:J. S (1)

PERTANIKA VOL. 9 NO.3, 1986 293

KAMARUDZAMAN ARIBI

The following equation was used to cal­culate percent volumetric water content:

where (J = percent volumetric water content,and

CR = the ratio of one-minute measuredcount and standard count with theprobe in the shield.

The amount of drainage was measured witbcalibrated water trap cylinders. They wereoperated by a vacuum system which operatedcontinuously to extract water from the bottom ofthe lysimeters. It was set to extract water to fieldcapacity at the bottoms of the lysimeters.

a

(0.56 - 0.08 YEd

) (1.42Rs- OAIl/ARso

= !J.~+o

+ f).0+ 0 [(0.263) (Ea - Ed) (0.5 +0.0062U2)]

(4)

where ET = potential evapotranspiration ratep

(mm/day),slope of the saturation vapor pres­sure-temperature curve (mb/oC),Stefan-Boltzmann constant (11. 71X 10 -scallcm 2day/k)

o = psychrometric constant (mb/oC),R s total incoming solar radiation

(callcm 2day),R so = total daily cloudless sky radiation

(callcm 2day),A. = latent heat of vaporization of water

(callcm ~,

U 2 = wind speed at 2 meter height (km/day),

E a = saturated vapor pressure, man ofvalues obtained at daily maximumand daily minimum temperature(mb),albedo or reflectively of surface forR ,and

s

Ed = vapor pressure at dewpoint tempe-rature (mb). The detailed descrip­

tion of the parameters used was given by Jones etal. (1984).

ETp

short grass as the reference crop. The modifiedPenman equation is as follows:

(3)

(2)() = 37.00CR - 3.31

(J' = W X BS X 100BW

Soil water content in the top 15 cm wasmeasured with a surface moisture-density gauge,Troxler Model 3411-B. One-minute readingswere taken with the radioactive source placed atthe backscatter position. Details of the instru­ment specifications and capabilities were givenby Troxler Electronic Laboratories (1979) andSmajstrla and Clark (1981).

Changes in soil water storage were calculat­ed based on the soil water contents measuredweekly. A neutron probe was used to measurevolumetric water contents from 30 cm to 150 cmdepths in 30 cm increments. The followingcalibration curve was used to calculate thevolumetric water content:

Crop Water Use Coefficients

Crop water use coefficients were calculatedbased on the following equation as described byDoorenbos and Pruitt (1977) and Wright (1981):

The weather data used in calculating poten­tial evapotranspiration was obtained from theIrrigation Park weather station located about 50m south of the experiment,al area.

where () = percent volumetric water content,W = computed soil water content (kg/

kg),BS = oven dry soil bulk density (kg/m~,

andBW = density of water (kg/m~.

Potential Evapotranspiration

Daily potential evapotranspiration rateswere calculated based on the modified Penmanequation as given by Jones et al. (1984) using

K = ETc

ETp

(5)

294 PERTANIKA VOL. 9 NO.3. 1986

WATER USE AND IRRIGATION MANAGEMENT OF YOUNG CITRUS

RESU LTS AND DISCUSSION

Monthly K ( values were calculated fromweekly measured ET and daily ET summed over

p

a month.

r------------------,..9.0-:

-8.0 ~

z-1.0 Q. ~

a:iLII)z«a:~

~«>wwo

3.0 «a:w>

2.0 «>­oJ:r:

1.0 ~zo~

o pOlllnlAl £T• - 10 kP. SWP TREAT.

! - 20 kP. S WP TREAT.

-:14.0.§zQ~«a:iLII)z«a:~

o~ 2.0>wwCl«a:w> 1.0«>-oJ:r:~

zo~ 0.0 .....---_-.l'- ......_---.....J 0.0

SEPTEMBER OCTOBER NOVEMBER DECEMBER

MONTHS

dimensionless crop water use co­coefficient for young citrus trees,crop evapotranspiration fromwell-watered treatment ( - 10kPa treatment) (mm/day), andpotential evapotranspirationdetermined by Penman's method(mm/day).

ET

ET =p'

where K(

Daz'ly Young Citrus Evapotranspiration

Evapotranspiration for grass cover treat­ments and potential evapotranspiration (ET )

pare graphed versus time in Figure 3. Evapot-ranspiration data were calculated using thewater balance method given in equation (l).Potential evapotranspiration was calculatedusing the modified· Penman method as given inequation (4).

During the research period, ET p wasgreatest in September, being 3.26 mm day I inOctober, 1.75 mm day lin November, and 1.64mrr. day I in December which is equivalent to6.50 L/day, 3.50 L/day and 3.30 L/day respec­tively. Decreased ET were related to decreased

pnet radiation. Evapotranspiration followed asimilar pattern as ET , that is, ET greatest inSeptember and lowest in December and weresignificantly different at the 0.05 level. Theseresults were in close agreement with the resultsobtained by Pallas et at. (1967), who found thattranspiration correlated positively with radiationlevel.

Young citrus ET with grass cover for thethree SWP studied did not exceed ET (Figure

P3). The magnitude of young citrus ET wasdirectly related to SWP. More water was requir­ed to maintain the root zone at Lower SWP.Vzriuzdaev .(1968) reported that transpirationrates decreased in direct proportion to SWP.Lower ET rodes at the low SWP studied were

Fig. 3: Monthly average potential and actualevapotranspiration rates for young citrustrees with grass cover as a function of sodwater potential. Vertical bars represent ±standard deviations.

probably due to decreased mobility of water andits decreased availability to plants.

Figure 4 shows the time series of youngcitrus ET with no grass cover, with respect to

different SWP. Each data point represents themean of four measurements. The effect of SWPon ET, showed a similar ET pattern under grasscover treatments, that is, ET decreased steadilyfrom greatest in September to lowest inDecember, and ET decreased as SWP decreased.All the treatments were not significantly dif­ferent at the 0.05 level.

Young citrus ET with grass cover for thethree SWP studied did not exceed ET (Figure3). The magnitude of young citrus PET wasdirectly related to SWP. More water was requir­ed to maintain the root zone at Lower SWP.Vznuzdaev (1968) reponed that transpirationrates decreased in direct proportion to SWP.Lower ET rodes at the low SWP studied wereprobably due to decreased mobility of water andits decreased availability to plants.

Figure 4 shows the time series of young'citrus ET with no grass cover, with respect todifferent SWP. Each data point represents the

PERTANIKA VOL. 9 NO.3. 1986 295

KAMARUDZAMAN ARIBI

9.0

~>.

0.

PONTENTIAL ET ·0.0 ~~ 4.0 ~

E • - 10 kPa SWP TREAT.g Z

4- - 20 kPa SWP TREAT. '7.0 0z i=0

- 40 kPa SWP TREAT. «i= a a:« 6.0 0::a: III0:: zIII «z 5.0 a:« .t-a: 0t- ~

0 «~ 4.0 >« Z.0 w> ww (:lw

f-~+3.0 «

(:l a:« wa: >w ·2.0 «> 1.0 >-«

+! .1.0

~

>- x~ t-X ~.t-Z ~0 0.0 .0~

SEPTEMBER OCTOBER NOVEMBER DECEMBER

MONTHS

Fig. 4: Monthly average potential and actualevapotranspiration rates for young citrustrees without grass cover as a function of soilwater potential. Vertical bars represent ±standard deviations.

mean of four measurements. The effect of SWPon ET, showed a similar ET pattern under grasscover treatm.ents, that is, ET decreased steadilyfrom greatest in September to - lowest inDecember, and ET decreased as SWP decreased.All the treatments were not significantly dif­ferent at the 0.05 level.

According to Peters (1960), if evaporativedemand was less, one would expect the changesin ET to be less sensitive to SWP. Veihmeyer etai. (1960) found that, under lower evaporativedemand that occurs in early spring in California,reduction in ET was small as the soil dried. Thisis in agreement with the results obtained fromthis research.

Fifty percent reduction in ET in no grasscover treatments were probably due to themulching effect of the surface soil as it dried.This finding was in agreement with Tanner andJl,lly (1976). With low Leaf Area Index, evapora­tion comprises a large fraction of the total ET.At low surface water contents, evaporation ratesmight decrease in proportion to the watercontent remaining in the soil. As the surfacebecomes relatively dried, evaporation becomes

proportionate to the rate at which water wasbrought up into the surface soil by capillaryaction. Eventually, a surface mulch will formedas a result of capillary break, thus greatlyreducing the evaporation and consequently theET rate.

Increased water-use under grass coverconditions emphasized the advantages of clean­cultivation over grass-cover groves. Eliminationof grass appears to have potential in reducing ETfrom young citrL.s groves. Increased water usewhen irrigated at high SWP, suggests the impor­tance of irrigating a rate no higher than neces­sary for the degree of growth desired and tominimize deep percolation losses.

Crop Water Use Coefficients

Figure 5 shows the time series of crop wateruse coefficients (Kc) with respect to differenttreatments and under different lysimeter groundcovers: grass and no grass. Monthly average Kcvalues from well-watered treatments were usedto indicate monthly young citrus Kc for thepurpose of estimating crop water requirements.

Crop water use coefficients of young citrustrees with grass cover treatments were 50%higher than with no grass cover treatments.Larger differences in Kc (Figure 5) were likelydue to the variation in the evaporation rate fromthe soil surface and from the grass cover itself.Frequent irrigation under grass cover conditionsresulted in higher soil water content in the toplayer. Therefore, water was more readily avail­able for ET.

Crop water use coefficients were inDuencedby SWP and ground cover conditions. As shownin Figure 5, Kc values decreased as SWPdecreased. This was apparently due to a decreasein E1' as a result of increasing resistance to waterflow through the Soil-Plant-Atmospheric-Conti­nuum as SWP decreased. A small decrease in Kctoward the end of the research period in Novem­ber and December were apparently due to lessactive growth of young citrus trees during aperiod of cool weather. Treatments 40G and40NG exhibited a larger decease of Kc in Sep-

296 PERTANIKA VOL. 9 NO.3, 1986

WATER USE AND IRRIGATION MANAGEMENT OF YOUNG CITRUS

l.llO

~ 0.75

---·-----tr-- I---- ~-----r----::--·--t------- --:rL --.-+-------~_.... - - .r --. - -- - -. _~.-- ----. J- ---1

CONCLUSION

During the research period, ET was highestin September, about 3.26 mm day \ and lowestin December, about 1.64 mm day' 1, with adecreasing trend from month-to-month. Youngcitrus ET was lower than the ET . Amounts ofET for no grass cover treatmen~ were signifi­candy less than grass cover treatments. Ingeneral, the increase in ET under grass cover wasmore than 50%.

The following conclusions can be drawnfrom this experiment:

Crop water use coefficients varied as afunction of SWP and grass cover conditions.Crop water use coefficients were highest for highSWP treatments, during the period of highestevaporative demand, and under grass coverconditions.

llECHIUEIlOCTOBER IIOVEHDERHOIITIIS

0.00 I.- -'L-.. --'- ---'

SEPIEMOER

F£g. 5: Monthly average young dtrus water usecoefFdents w£th grass and no grass covercond£t£ons (-O-lOG,- e·- 20G, -40G, --0-- lONG, -- e-- 20NG, -- --40NG).Vert£cal bars represent ± standarddev£at£ons.

tember, during a period of higher ET p' Acco.d­ing to Smajstrla (1982) the ratios of ET and ETwere logarithmically distributed as a function oftime when ET was fairly constant. Denmeadand Shaw (1961) showed that the SWP at whichthe ratio of ET and ET decreased from amaximum depends on the E1~ . They found thatET decreased below ET of 3 - 4 mm day I

pwhen SWP in the root-zone was about - 200kPa. On the other hand, when ET was 6 -7 mm

p

day 1 ET decreased below the ET when SWP, p

was about - 30 kPa. This finding agreed with

those results.

1. Young citrus with grass cover required con­siderably more water to meet the evapora­tive demand compared to no grass cover.Therefore, if water supply is limited, cleancultivation should be considered to reducethe irrigation requirements.

2. Crop water use coefficients were influencedby SWP and ground cover conditions. Cropwater use coefficients decreased withdecreasing SWP, during cool weather, andunder no grass cover conditions.

ACKNOWLEDGEMENT

Crop water use coefficients can be used as aguide for other locations and other years withdifferent climatic conditions, provided soil type,stage of growth and irrigation management aresimilar. Crop water use coefficient values for nograss cover condition could be used to calculateyoung citrus ET which has grass in between thetrees and bare soil around it. Crop water usecoefficients of young citrus with low Leaf AreaIndex published elsewhere (Doorenbos andPruitt, 1977) indicated slightly higher Kc. Thesedifferences could be due to differences in soiltype, and the number of irrigations.

The work was carried out under the super­vision of Dr. Allen G. Smajstrla, University ofFlorida, Gainesville in fulfilment for the M. Scdegree.. The author is also grateful to theFaculty of Agricultural Engineering, Universityof Florida for the financial assistance and facili­ties provided.

REFERENCES

BESTER. D.H., D.C. LOTTER and C.H. VELDMAN.

(1974): Drip irrigation on citrus. In: Proc. 2ndInter. Drip Irrig. Congr., San Diego, Calif.University of Calif. Riverside. pp. 58 - 64.

PERTANIKA VOL. 9 NO.3, 1986 297

KAMARUDZAMAN ARIBI

DENMEAD. O.T. and R.H. SHAW. (1962): Availabilityof soil water to plants as affected by soil moisturecontent and meteorological conditions. Agron. J.54: 385 - 390.

DOORENBOS. J. and W.D. PRUITT. (1977): Guidelinesfor predicting crop water requirements. Irriga­tioT) and Drainage paper No. 24. Food and Agri­culture Organization of the United Nations.Rome. 144 pages.

FERERES. E., D.A. MARTINICH. T.A. ALDRICH. J.R.CASTEL. E. HOLZAPFEL and H. SCHULBACH.(1982): Drip irrigation saves money in youngalmond orchids. California Agriculture. 36(9)(0): 12 - 13.

JONES. J. W., L.H. ALLEN. S.F. SHIH. J.S. ROGERS,L.C. HAMMOND. A.G. SMA]STRLA and J.D.MARTSOLF. (1984): Estimated and measuredevapotranspiration for Florida climate, crops andsoils. IFAS. University of Florida, Gainesville.Technical Bulletin 840.

JULIAN. W.S. and L.K. JACKSON. (1980): Planting andcare of young citrus trees. Florida CooperativeExtension Services. IFAS. University of Florida,Gainesville. Fruit crops fact sheet. FC-3!.

PALLAS. J.E., B.E. MICHAEL and D.G. HARRIS.(1967): Photosynthesis, transpiration, leaf tempe­rature and stomatal activity of cotton plantsunder varying water potential. Plant Physiol. 42:76 - 88.

PETERS. D.B. (1960): Growth and water absorption bycorn as influenced by soil moisture tension,moisture content, and relative humidity. Soil Sci.Soc. Am. Proc. 24: 523 - 526.

SMA]STRLA, A.G. and G.A. CLARK. (1981): Surfacemoisture-density gauge evaluation for moisturemeasurement in sandy oil. ASAE paper No. 81­2030. ASAE St. joseph, Michigan 49085.

SMA]STRLA. A.G., D.S. HARRISON and F.X. DURAN.(1981): Tensiometers for soil moisture measure­ments and irrigation scheduling. Florida Coope­rative Extension Services, IFAS, Gainesville, Fla.Circular 487.

SMAJSTRLA, A.G., L.W. HUNTER and G.A. CLARK.(1982): A field lysimeter system for crop water useand water stress studies in humid regions. ASAEpaper No. 82 - 2085. ASAE St. joseph, Michigan49085.

TANNER, C.B. and W.A. JURY. (1976): Estimatingevapotranspiration and transpiration from a rowcrop during incomplete cover. Agron. J. 68:239 - 242.

TROXLER ELECTRONIC LABORATORIES. (1979): Sur­face moisture-density gauges, 3400-B series.Instruction manual. Author, Research TrianglePark, N.C.

VEIHMEYER, J.F., W.D. PRUITT and W.D. Mc­MILLAN. (1960): Soil moisture as a factor ofevapotranspiration equations. Trans. of theASAE 4: 365 - 369.

VZNUZDAEV. N .A. (1.968): Consumption of water byplants depending on the negative pressure of thesoil water. Trans. 9th Int. Congr. of Soil Sci. 1:109 -115.

WRIGHT. j.L. (1981): Crop coefficient for estimationof daily crop evapotranspiration. In: IrrigationScheduling of Water and Energy Conservation inthe 80's. Proc. Irrig. Scheduling Conf. ASAE.Chicago. Dec. 14 -15.

(Received 26June, 1986)

298 PERTANIKA VOL. 9 NO.3, 1986