Institute of Plant Sciences, Federal Institute of Technology (ETH), Zurich, Switzerland

In¯uence of Soil Moisture and Fertilizer Potassium on the Vegetative Growthof Mungbean (Vigna radiata L. Wilczek) and Cowpea (Vigna unguiculata L.Walp)

U. R. Sangakkara, M. Frehner and J. NoÈ sberger

Authors' address: Prof. Dr U. R. Sangakkara (corresponding author), Faculty of Agriculture, University of Peradeniya,Peradeniya 20400, Sri Lanka; Dr M. Frehner and Prof. Dr J. Nosberger, Institute of Plant Sciences, Federal Institute of

Technology (ETH), 8092 Zurich, Switzerland

With 2 ®gures and 3 tables

Received November 22, 1999; accepted May 11, 2000

Abstract

Tropical food legumes are grown in a wide range of envir-onments, and water stress is considered the principal envir-onmental factor limiting growth and yield. Potassiumfertilizer mitigates the impact of water stress in plants. How-ever, the bene®ts of potassium in overcoming stress in tropi-cal food legumes have not been investigated in comparativestudies. The purpose of this study was to determine the ben-e®ts of potassium in overcoming water stress in mungbeanand cowpea, two important tropical food legumes with dif-ferent adaptabilities to soil moisture regimes. The experi-ment carried out under controlled conditions placedemphasis on vegetative growth and selected physiologicalparameters. The impact of potassium was di�erent in thetwo legumes grown at optimal and suboptimal soil moist-ure. Potassium increased shoot growth of mungbean to agreater extent than in cowpea under suboptimal moistureconditions. The roots of cowpea showed a greater responseto potassium fertilizer than in mungbean under suboptimalsoil moisture. The plant water relations and photosyntheticrates of mungbean were improved to a greater extent bypotassium under suboptimal soil moisture than those ofcowpea. Although di�erences were observed in theresponses of the vegetative growth of these species to mois-ture and potassium, in overall terms potassium promotedgrowth of both species when subject to suboptimal soilmoisture. While ®eld studies are required to validate theresults, the application of potassium fertilizer can be consid-ered a signi®cant factor in overcoming soil moisture stressin these legumes commonly grown in tropical cropping sys-tems.

Key words: cowpea Ð moisture stress Ð mungbeanÐ photosynthesis Ð potassium Ð tropical foodlegumesÐ vegetative growth

Introduction

Food legumes are an important component of theagricultural sectors of developing countries due totheir capacity to produce signi®cant quantities ofprotein-rich seed, and to improve soil conditions bythe inclusion of organic matter and biological nitro-gen ®xation (Kahindi et al. 1997, Snapp et al. 1998).Hence, they are grown in a wide range of environ-mental conditions under intensive and extensivemanagement.Soil moisture is a principal environmental factor

limiting legume productivity in the tropics and sub-tropics (Carranca et al. 1999, De Costa et al. 1999).The lack of adequate soil moisture a�ects bothvegetative and reproductive growth of foodlegumes, resulting in signi®cant yield losses(Ramirez Vallejo et al. 1998). However, most stu-dies on legumes (Ramirez Vallejo et al. 1998, DeCosta et al. 1999) have concentrated on the impactof moisture stress on the growth and developmentof one species. Comparative studies on the e�ect ofsoil moisture on di�erent food legumes have notbeen carried out, although these species are grownunder the same environmental conditions.Fertilizer potassium (K�) in¯uences plant water

status and helps overcome soil moisture stress(Marschner 1995). Studies by Sangakkara et al.(1996) and Nandwal et al. (1998) have reported thebene®cial impact of fertilizer K� in reducing plantwater stress in beans and mungbean, respectively.However, comparisons of the bene®cial e�ects of

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K� in overcoming soil moisture de®cits on di�erentfood legumes have not been reported.Mungbean and cowpea are two important tropi-

cal food legumes grown extensively under marginalconditions. Cowpea is considered a drought-toler-ant species (Ogbuinya 1997, Sangakkara 1998),while mungbean requires adequate soil moisture forgrowth and development (Sangakkara et al. 1995).Fertilizer K� would therefore be expected to havedi�erent e�ects on these di�erent food legumes inovercoming moisture stress. To investigate this pos-sibility, an experiment was carried out under con-trolled conditions to determine the impact offertilizer K� on the morphology and selected phy-siological characteristics of mungbean and cowpeaunder optimal and suboptimal soil moistureregimes. Emphasis was placed on evaluating thevegetative growth (V3/4 stage) of these two species,which has a substantial impact on yield (Timsinaet al. 1994), under di�erent soil moisture and K�

fertilizer regimes.

Materials andMethodsThe experiment was carried out at the Division of Grass-land Science and Crop Physiology, Institute of PlantScience, Federal Institute of Technology (ETH), Zurich,Switzerland, from July to September 1998. Climate cham-bers (CGP Conviron Instruments Co Ltd, Winnipeg,Canada) were used to maintain the environmental condi-tions of 25/18 �C day/night temperature with a photoperiodof 16 h and a PAR of 450±500 mmolmÿ2 sÿ1 provided byTungsten and Halogen lamps. The relative humidity was60%+ 2%.

The fertilizer levels used were 0.1, 1.0 and 3.0mM K�

provided in the form of nutrient solutions containing1.5mM N and other essential elements (Sangakkara et al.1995). Soil moisture levels maintained throughout theexperiment were ®eld capacity ÿ 25% depletion (optimal)and above 50% depletion (suboptimal) of available soilmoisture.

PVC tubes (40 cm tall and 50mm in diameter) were ®lledwith 1.25 kg of washed river sand (grain diameter0.7±1.2mm). The water-holding capacities of the rootingmedium at ®eld capacity, 25% and 50%moisture depletionwere determined gravimetrically. Thereafter, the respectivesoil moisture regimes were maintained by weighing the potsonce every 2 days and adding the required quantities ofnutrient solutions.

Pre-germinated uniform seeds of mungbean (var MI5)and cowpea (Arlington) were planted in the pots, main-tained at the two soil moisture regimes, with the respectivenutrient solutions, and placed in the climate chambers.Thereafter, each pot was inoculated with 5ml of a brothcontaining Rhizobium leguminosarum on four occasionsat 3-day intervals. The respective nutrient solutions were

added after weighing each pot at 2-day intervals to maintainthe desired soil moisture regimes.

Each species was therefore grown with three K fertilizertreatments at two soil moisture regimes. The experimentaldesign used was a randomized block design, with eight repli-cates per treatment for each species. Each replicate had twoplants.

Sampling was carried out at the V3/V4 growth stages inboth species, which corresponded to 21 and 27 days afterplanting for mungbean and cowpea, respectively. Thephotosynthetic rate of each species was measured on themost recently fully expanded leaf of ®ve plants per treat-ment under saturated light conditions, 2 h after the onset offull photoperiod. A CIRAS-1 (PP Systems, Stotford, UK)with an open circuit gas exchange system with a leaf cuvettefacilitating the measurement of 2.5 cm2 of leaf area wasused for this purpose. The external source of light was pro-vided by a 20-W quartz lamp (approximately1500 mmolmÿ2 sÿ1. Shoot water potential was measuredsoon afterwards on six plants per treatment, using a Scho-lander pressure bomb apparatus (Scholander et al. 1965).The roots and shoots were carefully removed from all plantsand the following parameters recorded: leaf area (with aLiCor automatic leaf area meter; LiCor Inc., NE, USA),lengths of tap and primary lateral roots (using the gridtechnique; Newman 1966), and numbers of ®rst, second andhigher order roots (using the classi®cation of Fitter 1982).All plant parts were dried at 80 �C to a constant weight anddry weights of leaves, stems and roots were determined.

The data for each species were treated as one unit andsubjected to appropriate statistical analysis using a generallinear model (GLM) model using each plant as a replicate.Arc-sin and log transformations were carried out whenrequired to ensure normal distribution of data. Probabilityvalues were used to determine the occurrence of signi®cantdi�erences between treatments.

Results and Discussion

Shoot growth

Shoot growth of mungbean and that of cowpeawere a�ected di�erently by soil moisture andK� (Table 1). Generally, the development of shootswas enhanced under optimal soil moisture condi-tions, illustrating the requirement of adequate soilmoisture for better vegetative growth of grainlegumes. However, under optimal soil moistureconditions, the mean leaf area and shoot dry mattercontents of cowpea increased by 55% and 34%,respectively, when compared with growth at thelower moisture regime. In mungbean, the incre-ments of these two parameters were 43% and25%, respectively. This suggests that, althoughmungbean is a drought-susceptible species, the sup-ply of adequate moisture enhances growth of cow-pea, the more drought-tolerant species, to a greaterextent.

74 Sangakkara et al.

Potassium increased leaf area and shoot dryweights of both species. The impact of 3mM potas-sium on the leaf area of mungbean was morepronounced (180%) under optimal soil moisturethan that of 0.1mM potassium. The application of1.0mM potassium under suboptimal moistureconditions increased the leaf area of mungbean by103% compared to a 67% increase under optimalsoil moisture. Hence, the leaf area of mungbeanseemed to respond better than under optimal soilmoisture conditions. Under optimal soil moistureconditions the increment in leaf area was greatestwith 3.0mMMK. This suggests that lower rates of K�

help mungbean plants to expand their leaves undermoisture stress, while a greater quantity of K� isneeded at higher soil moisture contents.

The leaf area of the cowpea plants was greaterthan that of mungbean, which is a characteristic ofthe species. With increasing potassium the di�er-ence between the two species was reduced, suggest-ing that K� produced a greater increase in leaf areain mungbean. However, K� increased the leaf areaof cowpea signi®cantly under dry conditions. Thesupply of 1.0 and 3.0mM K� increased the leaf areaof cowpea by 73% and 183% under the lower soilmoisture regime over that at 0.1mM K�, while theincrements were 50% and 93% in the optimal soilmoisture regime. This clearly indicates that K�,

which helps plants overcome water stress by osmor-egulation in plant cells (Walker et al. 1998), pro-moted leaf development and expansion in thisdrought-tolerant species, to a greater extent undersuboptimal moisture.Potassium increased shoot dry weights of both

species, con®rming its bene®t in promoting growthof legumes (Bailey andLaidlaw 1998). The impact ofK� was greater at suboptimal soil moisture, whichshowed the bene®cial e�ect of this nutrient in miti-gating moisture stress, as shown in Phaseolus (San-gakkara et al. 1996). In mungbean, the applicationof 1.0 and 3.0mMK� increased shoot dry weights by60% and 157% in the lower soil moisture regime,when compared to increments of 30% and 64% inthe optimal soilmoisture regime. In cowpea, the sup-ply of 1.0 and 3.0mMK� increased shoot drymatterby 41% and 68% in the suboptimal and by 30%and 64% in the optimal soil moisture regimes. Thisindicated the importance of potassium in promotingshoot dry matter accumulation in the drought-sus-ceptible species, which was di�erent to the responseobserved with leaf area. Potassium also helps themore drought-tolerant cowpea to overcome moist-ure stress and promote shoot growth.Cowpea had higher speci®c leaf areas in all treat-

ments, due to thicker leaves or a greater accumula-tion of photosynthates. Furthermore, the speci®c

Table 1: Shoot growth of mungbean and cowpea as a�ected by soil moisture and fertilizer potassium

Soil moisturePotassium(mM)

Leaf area(cm2)

Shoot dry weight(mg)

Speci®c leaf weight(mg cmÿ2)

MungbeanSuboptimal 0.1 27.7 166 4.33(over 50% 1.0 55.0 266 3.23depletion) 3.0 72.8 349 3.08Optimal 0.1 40.2 235 4.15(®eld capacity ± 1.0 67.8 359 3.5525% depletion) 3.0 112.6 460 3.13Probability Moisture 0.007 0.041 0.006(P �0.05) Potassium 0.021 0.037 0.040

Interaction * * *CowpeaSuboptimal 0.1 30.6 173 5.71(over 50% 1.0 52.5 245 4.72depletion) 3.0 85.2 355 4.18Optimal 0.1 58.9 262 4.51(below 25%± 1.0 87.8 343 3.94depletion) 3.0 112.6 432 3.86Probability Moisture 0.032 0.028 0.006(P �0.05) Potassium 0.019 0.038 0.022

Interaction * * *

75E�ect of Potassium on Mungbean and Cowpea

leaf weight was greater under the lower soil moist-ure regime in both species, which is indicative ofleaf thickening or accumulation of photosynthatesto overcome drought stress (Amir and Sinclair1991) and is consistent with similar data on peanuts(Craufurd et al. 1999). Application of K� reducedspeci®c leaf weights in both species, and the impactwas greater in the lower soil moisture regime, espe-cially in mungbean. In cowpea, the e�ect of K� inreducing speci®c leaf weight was lower than that inmungbean in the low soil moisture regime, and wassimilar under optimal soil moisture conditions. Thisprovides evidence of the possible role of K� in themovement of photosynthates from leaves, espe-cially under soil moisture stress, and its greaterimpact in drought-susceptible species. Under thehigher soil moisture regime, there was no di�erencein the impact of increased rates of potassiumbetween the two species in terms of photosynthatemovement, a phenomenon not previously identi®edor investigated in comparative studies in tropicallegumes. Hence, the study indicated a possible rolefor potassium in photosynthetic movement fromleaves based on the reduction in speci®c leaf weightsin these two species, especially under soil moisturestress.

Root growth

Cowpea had a more extensive and heavier root sys-tem (Table 2), a characteristic of a drought-tolerantspecies, which facilitates the extraction of moisturefrom dry soils (Fitter 1996). In addition, the growthof roots was more extensive under the lower soilmoisture regime. This again is a response of plantsto low soil moisture, in which root plasticity isexhibited, and there is greater partitioning of drymatter to roots to overcome moisture stress.Increasing soil moisture reduced the mean length

of roots to a greater extent in cowpea than in mung-bean. Root weights also showed a similar phenom-enon. In contrast, in the low soil moisture regime,the mean length of primary laterals was greater inmungbean. This implies that partitioning of drymatter and development of the tap root werea�ected to a greater extent in cowpea than in mung-bean by soil moisture depletion. The drought-pronemungbean develops a more extensive lateral rootsystem to absorb soil moisture under dry condi-tions. This again highlights di�erences in root plas-ticity in these two species in relation to soilmoisture.Potassium increased root growth of both species

signi®cantly. The impact of K� was greater under

Table 2: E�ect of soil moisture and fertilizer potassium on root growth of mungbean and cowpea

Soil moisturePotassium(mM)

Length of tap root(cm)

Length of primarylaterals (cm)

Root weight(mg)

MungbeanSuboptimal 0.1 14.7 60.2 194(above 50% 1.0 25.5 86.5 210depletion) 3.0 32.5 103.2 224Optimal 0.1 12.0 45.5 116(below 25% 1.0 20.0 64.0 165depletion) 3.0 29.0 82.5 237Probability Moisture 0.031 0.022 0.037(P �0.05) Potassium 0.029 0.012 0.007

Interaction ns ns *CowpeaSuboptimal 0.1 21.5 100.5 192(above 50% 1.0 37.7 135.0 221depletion) 3.0 51.7 144.2 301Optimal 0.1 23.2 85.0 149(below 25% 1.0 29.7 102.7 178depletion) 3.0 33.0 116.0 229Probability Moisture 0.040 0.038 0.041(P �0.05) Potassium 0.037 0.018 0.031

Interaction * ns *

76 Sangakkara et al.

low soil moisture, especially in mungbean. This

suggests that K� promoted root growth in thedrought-susceptible species to a greater extent,allowing increased extraction of soil moisture.

The supply of 3.0mM K� increased the length ofthe tap roots of mungbean by 128% and 141% in

the suboptimal and optimal soil moisture regimes,respectively. In cowpea, the increments were 142%

and 43% in these two soil moisture regimes. Thisalso illustrates that K� has a greater in¯uence

under dry conditions than under optimal conditionsin cowpea, while the impact was similar in both soilmoisture regimes in mungbean. This trend is also

seen in the lengths of primary lateral roots and rootdry weights. Thus, the root systems of cowpea seem

to respond to K� under soil moisture stress, whilein mungbean root growth is also promoted under

adequate soil moisture.The morphometry and branching ratios of roots

of mungbean and cowpea (Figs 1 and 2) also illus-trate the plasticity of roots in response to moisture

and K�. As expected, cowpea had a greater numberof ®rst-, second- and third-order roots, which ischaracteristic of an extensive branching habit. In

both species, the numbers of roots of all categorieswas greater under suboptimal conditions, which is a

response of plants to dry conditions. However, theimpact of K� was greater (as shown by the regres-

sion equations) in suboptimal soil moisture regimesin cowpea, where K� increased the second- andthird-order roots to a greater extent than in mung-bean, resulting in the development of a more exten-sive root system. The opposite was true inmungbean, where K� promoted the development ofthe second- and third-order roots to a greater extentunder optimal moisture conditions. This impliesthat root branching of the drought-tolerant speciesis promoted to a greater extent than that of thedrought-susceptible species by K�. Such resultshave not previously been obtained and requirefurther study.

Physiological parameters

Cowpea had a higher water potential and a greaterrate of photosynthesis than mungbean in both soilmoisture regimes (Table 3). This illustrates thegreater ability of this species to store moisture andproduce carbohydrates under a range of environ-mental conditions, when compared to mungbean.However, the impact of increasing soil moisturewas similar in the two species in terms of waterpotentials. The reduction of the mean shoot waterpotential of mungbean and cowpea was 36% and34%, respectively, with increasing soil moisture tooptimal levels. This suggests that the two speciesincrease their plant water holding abilities to similar

Fig. 1: E�ect of soil moisture and fertilizer potassium on root number and branching in mungbean at the V3/4growth stage. 1, 2 and 3 indicate ®rst-, second- and third-order roots, respectively

77E�ect of Potassium on Mungbean and Cowpea

extents with a supply of adequate moisture. In con-

trast, the higher soil moisture regime enhanced the

mean photosynthetic rate of mungbean and cowpea

by 12% and 17%, respectively. This indicated that

the drought-tolerant species has a greater potential

to increase its photosynthetic capacity with the sup-

ply of water, which could be a result of its inherent

higher photosynthetic rates.

In all instances, K� reduced water potentials,

especially under suboptimal moisture regimes, thus

con®rming the in¯uence of this nutrient in mitigat-

ing moisture stress (Marschner 1995). However,

Table 3: Shoot water potential and photosynthetic capacity of mungbean and cowpea as a�ected by soil moistureand fertilizer potassium

Soil moisture(mM)

Potassium(bars)

Water potential(mmolmÿ2 cmÿ1)

Leaf CO2

exchange

MungbeanSuboptimal 0.1 ÿ 6.82 6.62(above 50% 1.0 ÿ 5.40 9.30depletion) 3.0 ÿ 4.01 11.12Optimal 0.1 ÿ 4.15 7.45(below 25% 1.0 ÿ 3.52 10.45depletion) 3.0 ÿ 2.67 12.55Probability Moisture 0.034 0.018(P �0.05) Potassium 0.041 0.034

Interaction * *CowpeaSuboptimal 0.1 ÿ 8.40 14.62(above 50% 1.0 ÿ 6.60 18.02depletion) 3.0 ÿ 4.82 21.17Optimal 0.1 ÿ 4.97 18.55(below 25% 1.0 ÿ 4.40 20.77depletion) 3.0 ÿ 3.65 23.64Probability Moisture 0.044 0.026(P �0.05) Potassium 0.038 0.031

Interaction ns *

Fig. 2: E�ect of soil moisture and fertilizer potassium on root number and branching in cowpea at the V3/4 growthstage. 1, 2 and 3 indicate ®rst-, second- and third-order roots, respectively

78 Sangakkara et al.

under conditions of suboptimal moisture content,the e�ects of application of 1.0 or 3.0mM K� weresimilar in the two species, implying that the speciescan extract soil moisture at similar rates. In con-trast, application of 3.0mM K� in the optimal soilmoisture regime reduced the water potential ofmungbean to a greater extent than that of cowpea.Thus the drought-susceptible species had a greatercapacity to take up water with increasing K� in thehigher soil moisture regime. This could be a conse-quence of the lower moisture-holding capacity ofmungbean (as shown by the reduced water poten-tials), when compared to cowpea.

Potassium increased the photosynthetic rate ofmungbean to a greater extent than that of cowpea,irrespective of the soil moisture regime. This againcould be due to the lower photosynthetic capacityof mungbean, which is more responsive to K�.Application of 1.0 and 3.0mM K� increased thephotosynthetic rate of mungbean by approximately40% and 60%, respectively, in both moistureregimes. This implied that K� a�ected the photo-synthesis of the leaves of this drought-susceptiblespecies similarly in the two soil moisture regimes. Incontrast, the impact of K� was greater under sub-optimal soil moisture than under optimal condi-tions in cowpea. Thus the photosynthetic rate of thedrought-tolerant species responded better to K�

under soil moisture stress. This again could beattributed to the drought-tolerant capacity of cow-pea, as the impact of K� in optimal soil moistureregimes is lower in terms of photosynthesis. HenceK� seems to play a greater role in enhancing thephotosynthetic capacity of drought-tolerant speciesunder low soil moisture conditions, while thephotosynthetic capacity of drought-susceptible spe-cies responds to this nutrient at all moistureregimes.

Conclusions

The present study compared the e�ects of soilmoisture and K� on vegetative growth and twoimportant physiological parameters in mungbeanand cowpea, two species of tropical grain legumeswhich are, respectively, drought-susceptible anddrought-tolerant. The growth patterns andresponses of the two legumes to the treatments weredi�erent, although they are grown in similar envir-onments in the tropics, which could a�ect yields.

Increased soil moisture promoted the growth ofboth species, although the response of most mea-sured parameters was greater in mungbean. Appli-cation of K� increased the leaf area of mungbean to

a greater extent under optimal soil moisture, and asimilar e�ect was seen in cowpea under suboptimalmoisture conditions. In contrast, K� increasedshoot dry weights and speci®c leaf areas of bothspecies to a greater extent under suboptimal moist-ure conditions than under optimal conditions.Optimal soil moisture promoted root growth of

mungbean more than that of cowpea, although thee�ect of K� in mitigating soil moisture stress wasmore marked in cowpea. The impact of K� in pro-moting root growth in both species was clearly evi-dent in both moisture regimes.The e�ects of soil moisture and K� on shoot

water potentials and photosynthetic rates were dif-ferent in the two species. While increasing soilmoisture reduced shoot water potentials similarlyin both species, the impact on photosynthetic rateswas greater in cowpea. Potassium reduced theshoot water potentials of the two species to similarextents in the suboptimal soil moisture regime. Incontrast, K� reduced the water potential of mung-bean to a greater extent than that of cowpea in theoptimal soil moisture regime. This indicated thatthe drought-susceptible species had a greaterresponse to K� even under optimal soil moistureconditions.Potassium also increased the rate of photosyn-

thesis of mungbean to a greater extent than that ofcowpea at both soil moisture regimes, which couldre¯ect the lower capacity of this species to ®x atmo-spheric CO2. However, the impact of K� in promot-ing photosynthesis was greater in the suboptimalmoisture regime, suggesting it had an e�ect throughmitigating water stress.Overall, the results highlight the di�erences in

vegetative growth and physiological parametersbetween these two important tropical grainlegumes. Their responses to water and potassiumvaried. K� enhanced most measured parameters ofthe drought-susceptible species, mungbean, to simi-lar extents in the two soil moisture regimes. Theimpact of K�was greater in the suboptimal mois-ture regime in cowpea, the drought-tolerant species.Although ®eld studies are required to con®rm thepotential applicability of these ®ndings, the resultssuggest that K� is more important for cowpeaunder dry conditions than under optimal condi-tions, promoting vegetative growth and optimizingphysiological parameters which in¯uence subse-quent seed yields. In mungbean, K� was found toplay a major role in promoting vegetative growthand optimizing physiological parameters in optimaland suboptimal soil moisture regimes, thus valid-

79E�ect of Potassium on Mungbean and Cowpea

ating its use in all soil moisture regimes to optimizeyields.

Zusammenfassung

Ein¯uss der Bodenfeuchtigkeit und von KaliduÈ ngerauf das vegetative Wachstum vonMungbohne (Vignaradiata L. Wilczek) und Augenbohne (Vignaunguiculata L. Walp)

Tropische Speiseleguminosen werden in einem weitenBereich von Umwelten angebaut und Wasserstress wirdals die grundsaÈ tzliche Umweltbeein¯ussung, die zu einerLimitierung des Wachstums und des Ertrages fuÈ hrt,gesehen. KaliumduÈ ngung mildert den Ein¯uss von Was-serstress auf die P¯anzen. Allerdings, die Eigenschaftenvon Kali hinsichtlich der Verringerung des Stressein-¯usses auf tropische Speiseleguminosen sind bisher nochnicht verglichen worden. Der Zweck der Untersuchungwar es, die guÈ nstigen Wirkungen von Kalium hinsicht-lich der Vermeidung von Wasserstress bei Mungbohneund Augenbohne, zwei bedeutende tropischen Speisele-guminosen mit unterschiedlicher AdaptationsfaÈ higkeitgegenuÈ ber Bodenfeuchtigkeitsbedingungen, zu untersu-chen. Das Experiment wurde unter kontrollierten Bedin-gungen in GefaÈ ûkulturen durchgefuÈ hrt, wobeiinsbesondere das vegetative Wachstum und bestimmtephysiologische Parameter beachtet wurden. Der Ein¯ussvon Kalium war bei den beiden Leguminosen, die unteroptimalen und suboptimalen Bodenfeuchtigkeitsbedin-gungen angebaut wurden, unterschiedlich. KaliumerhoÈ hte das Wachstum der Sprossachsen von Mung-bohne in einem hoÈ heren Umfang bei suboptimalenFeuchtigkeitsbedingungen. Die Wurzeleigenschaften derAugebohne zeigten eine staÈ rkere Reaktion gegenuÈ berKaliumduÈ ngung unter suboptimalen Bodenfeuchtig-keitsbedingungen. Die P¯anzen-Wasser-Relationen unddie Photosyntheseraten von Mungbohne wurden ineinem hoÈ heren Ausmaû durch Kalium unter suboptima-len Bodenfeuchtigkeitsbedingungen im Vergleich zurAugenbohne gefoÈ rdert. Obwohl Unterschiede in denReaktionen des vegetativen Wachstums dieser beidenArten gegenuÈ ber Feuchtigkeit und Kalium beobachtetwurden. Es kann insgesamt festgestellt werden, dass dasWachstum beider Arten gefoÈ rdert wird, wenn sie untersuboptimalen Bodenfeuchtigkeiten angebaut werden.WaÈ hrend Felduntersuchungen benoÈ tigt werden, um dieErgebnisse zu bestaÈ tigen, kann die Anwendung vonKaliumduÈ nger als ein signi®kanter Faktor in der Ver-meidung von Bodenfeuchtigkeitsstress dieser Legumino-sen, die weit verbreitet in tropischen Anbausystemenauftreten, gesehen werden.

AcknowledgementsThe senior author (U.R.S.) acknowledges the Institute ofPlant Sciences, ETH, Zurich, Switzerland for ®nancing thiswork. Gratitude is expressed to the Royal Society of theUnited Kingdom for a fellowship, tenable at the University

of Reading, and to Professor P. D. S. Caligari for facilitiesprovided during the preparation of this paper.

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81E�ect of Potassium on Mungbean and Cowpea