J. AKtonomv & Crop Sdenee 177, 165—173 (1996)

[ ©1996 Blackwell Wissen.<!chafts-VeHi^, Berlin

\ ISSN 0931 2250

^ Grassland Science and Cn^ Mysiek^, Institute (^ Plant .Sciences, BTH Zentrum, S092 Zutieb,

d

Response of Root Branching and Shoot Water Potentials of FrenchBeans (Phaseolus vtdgeais L.) to Soil Moistui% and Fertilizer Potassium

IJ. R. SANC.AKIC.\RA, U . A . HARTWIG and J. Nf »SBt K<.KR

Authdrs* addresses: Dr U. A H\RT«I<: and Prof. Or J. NoSBiiRCrat, Institute of Plant .Sciences, liTH Zentrum,ai92 Zurich, Switzerland; Prof Dr i;. R. SANG.^KKARA, F-aculry o f Agriculture, Umvcrsity of Peradeniya, Sri l.anka

V-'ah 4 tahla ami exe fipm

Raekrd FrhnuBy 2}. 1996: aaepitd jam 17, 1996

Abstract

(WiensK-c branching patterns of rixits and the maintenance of adequate water within sh(K)rs enables plants toi>vercome water stress. However, information on the relationship herween fenitizer potassium, root branchingpjttems and shtxit water potentials o f ffKxl legumes grown under different soil moisture regimes is scarce. Thus, anexpcnment was conducted in a ph>^orron to ascertain the effect of fettili^er potassium on rtjoi branching patternsand shfKit water potentials of a popular tropical fixid k ^ m c s (Frcnchbeans Phasmtus VHi^ris U). The planK wereerown in a sand medium wirh 0.1.0.8 or 3.0 mM of poussium under a suboptimal and optimal soil moisture regime.

Root lengths and dry weights were enhanced hy potassium, especially under a suboptimal soil moisture regime.The branching patterns changed due to potassium, where the numbers of second and third order roots increasedunder both soil moisture regimes, aldiough the impact was grearer in plants grown with low foil nKjisrure. Plantwater contents measured m terms of shoot water potential, relative water contents, nitgid wei(^t:dry weight ratioand water uptake capacity were also increased by potassium. A positive rdarionship was observed hervieen nK>(brjnching patterns and water potentials with increasing potassium levels especially in plants grown undersuboptimal soil moisture ctmdidons. Shoot growth and nodulanon wa.s also pnimoted hy potassium. The ability ofplanis to develop a more cxtcnsKe branching pittem of roors by inducing a greater number of second and thirdorder nxns and chatigir^ the roor branching haliit fn)m a herringbone to a dicbotomous tspc to maintain a grearershiKK water potential especially under low soil moisture regimes is presented.

l&y words: Frenchbeans, soil moisture, fertilizer potassium, root branching, shoot water porential, shmit and rtx>t

{jou-th, nodulation.

Introduction R w growth of plants is very senstrK'e ro

r , , . , . changes in stiil moisture, nutrient supph- (tCrowd, and do' matter dismbutmn in plants is ^^J'^^^^ ^ ^^^,^^ (L,PIW: and Bl'^

T . l n . T ^ ' ,"'™^""""^^ ^^""^ 1991). Tbereforc. studies on toor growd, Ulus-1990). In the tropics wa.« stress J ^ f , dmitieral nutntjon ate considered the most g - T , r ,

1 '" ^«ess. Tbe absence of stressirnp<,rtant environmental paraixieters dtat deter- ^P""" >" «1^""" '" ^«ess. Tbe absence of stress• ..ne gto^vtb and yields of food crops (NRC faaor indices greater brancbii^tbereby reducing

l'''^). The successful growth of mos. food crops the branching rano {FiTTFR 1992).'•• the developit^ countries is diercfore deter- The adverse impact of envtronmental stress,

mintd by tbese environmental parameters especially diat of soil motsture and temperature,(I^ 1992). on plant giowtb is reduced by fertilizer potassium

v 0931 -2250/96/7703-01 (>5$11.50/0

166 SANCAKKARA, HAitrpinG and

Table 1. Root growth of Frenchbeans as affected by soil moisture and fertilizct potassium

Root length(cm)

Water contentPotassium

(tnM)

Root dty wt(mg)

HI H2 HI H2

Specific toot wt(mg/cm)

HI H2'

Over 50 % depletion(suboptitnal)

Bek)w 25 % depletion(tipamal)

interaction P = (tU).S)

0.)0.83.0

0.10.83.0

172a223b234b

132a209b2 1 %

0.451

20(.a24%280b

187a215b24Sb

0.C.73

79a95a

145b

76a100b128c

fl.l76

126a176b235c

12.5a182b216c

0.348

0.459a0.426a{».619l>

0.575a().478a0.584a

0495

0.611aO.7(li6b(i.839c

0.66fia0.846b0-87f)b

0421

HI and H2 correspond to V2/'V3 and V5 growth stages, respectively; Means within a column of a givamoisture regime followed by the same letter are not significantly different (P = 0.05).

(K*) (M.\KM;iiNKR 1995). Thus , studies bySAN(;AKK.^RA et al. (19'J5, lW6a) illustrate thatK * enhanced vegetative growth o f selected foodlegumes under suboptimal soil moisaire andsupra optimal temperature regimes. TTiis wasanributed t<) the role of K * in biocbcmicalpathways in plants (MI:NGKL and KiRKBY 1987,NtMWMM-.R 1995). Fur thermore, fertiliser K*enhanced plant water status ( A B D ALLA andAboiiL WAHAB 1995) and b i o k ^ c a l nitrogenfixation of legumes grown under moisture stress(SANfiAKKAR.\ et ai. 1995).

. \ l though the ni l t of K * in enhancing plantwater status is identified, there is a paucity ofinformation on the influence of this nutrienr onchanges in branching patterns o f r(H)ts as amechanism to overcome stress. T h e changes inbranching patterns of rrMjts subjected to waterstress as metliated by fertilizer K ' ' has not beenidentified. Thus , an experiment was carried our todetermine the effect of fertilizer K * on branch-ing patrems o f seedlings of naseolus vuharis L(Frcnchljean) and on plant water status whengnjwn under optimal and suboptimal soilmoisture conditions. Emphasis was placed uponthe determination of roor bratKhing patterns as amecharustn for overcoming water stress asinfluenced by K *.

Materials and Methods

The experinwnt was carried out at the Division ofGrassland Science and Crop Physiology of theInstitute of Plant Sciences, Federal Institute of

Technologi- (ETH) Zurich, Switzerland, from September to November, 1995. (Climate chambcn-(CGR (ijnviron Instrument (>i. lj:d., Winnipej:.Canada) were used to obtain the following envinwi-mental conditions: temperature 23/18 " day/ni^hi.with a phott^iertod of 116 h provided by a radiatiimof 450-500 nmol quanta PAR m " ^s using Tunp-tinand Halt^en lamps. The relative humidiu' i«-W/,, ± TA,.

The fertilizer K* levels used were 0.1, O.S an.i3.0 mM K, prtnHded in the form of a nutricr-solution containing 1.5 m M N other essential ekments (SANC,AK.K.\RA et al. 1995). Soil moisture levcKmaintained throughout the experiment were heWcapacity — 2 5 % depletion (opdmal) and aUwt50% depletion (suboptimal) of available «ii:moisture.

PVC tubes (4<K» mm high and 50 mm diameterwere filled with 1.25 kg of washed rivet sam)(diameter 0.7-1.2 mm). The water holding capacitie-<tf the sand at field capacity, 2 0 % ami 50"'-depletion were measured gravimetrically. Thertaftci.the respective nutnent solutions were added iwmaintain the desired soil moisture levels (appn>^-marely at field capsacity or below 50 % of availablesoil moisture) by weighing the pots daily.

Pregerminated uniform seeds of Frenehbean (cWade — Sri I..anka) were planted in PVC tubes, itwhich the stjil mCHSture was maintained at thedesired levels, and placed in gntwrh chambers. S*)!'"after plandrig, the sand in the tubes was inoculatedwith 4 ml of an inoculum broth containing fUr^t'l"""mfncii (Strain CIAT 899) at 3-day intervals on fnu'occasions. The respective nutrient solutions MWadded daily after weighing the pots to maintain thtdesired soil moisture levels.

Root Mofpholqgy and Water Potenriab in Fteitchbcans 167

The experiment, which had rwo soil moistureregimes and diree rates of K* fertilizer, was laid outas a randomized block design with three replicates.Fach replicate had 2.S pots.

Samplit was carried out at V2A'3 (14 days) andV5 (30 days) growth stages. At each sampling plantsof six pots were carefiiily removed and roots washed.The following measurements were made on eachplan which was treated as a replicate: shoot waterpotential — using a Scholander pressure bombapparatus (ScHOLANDtR et al. 19fi.S); leaf area perplant — using a LJ-cor autrmnatic area meter totalroot length — using the grid technique (NF.WMAKW)6); numbers of first, secondary- and higher orderbranches ofthe root system, using the description ofHrnra (1982); nodtiles per plant; shoot and TKHH dr\weights by drying al 80 "C for 48 h.

The shoot systems of four other plants werecarefully harvested ar the same growth stages andfresh wei^ts determined immediately. Thereafter,the shoots were immersed in distilled water for 24 hat .S 'XI in the absence of lighL These shoots wereweighed after removing excess water to determinethe luigid weight and desiccated for 48 h at 80 "X" todetermine dti* weights.

The numbers of primary, secondary and higherorder roots were used to determine the branchesratios as described by FITTF.K (1982). TTie fresh,lurpd and drj" weights of plants were used todetermine the following:

,, , . Fresh »t — Dr\' wtHelanve water content — ——r = x KXP

wr — Dr\' wt

VVarerretention capacity- =Tuqpdwt

Dr> wl

Viattt uptake capacity (mg mg"')

Turgid wt — Fresh wtDrywt

The data was subjected to statistical analysi.s using a<JLM model, usit^ each plant as a replicate to reducevanarion. Arc sin transformations were carried <xit'•n selected data to ensure normalit)'.

Results and Discussion

Total root lengths of Frenchbeans were fearer inplants grown under suboptimal soil n:ioistureconditions (Table 1), confirming earlier reportsrStT.iL\THwoRN et al. 1992, SANGAKKARA et al.l*'6b). Thus, mean total lengths of plants were^ - '•<> greater in the suboptitnal soil moisturerei'irne, at the second harvest, clearly indtcating"'^ greater exploitation of tbe soil volume to" •snrotne tnoisture stress QORDCJN ct al. 1983). In

contrast, mean root dry weights of plants grownunder both moisture regimes were similar ar bothharvests, although there were differences be-rween individual K^ treatments (Table 1).

The influence of K* was more pronounced inroot diry weights than in total ro<M lengths fTable1). At the second harvest, rhe increments in totalroot lengths due to the application of 3.0 mM Kover that observed with 0.1 mM K under optimaland suboptimal soil moisture r ^ ^ e s were 32 'Yaand 35 %, respectively, while those for r<x>t dryweights were 72% and 80 Vo for the sametreatments. The results confirm the role of K * mtranslocation of photosynthates (MAKS.HNKR1995) and illustrate that the nutrient has asignificant role in promotitig dry matter accumu-lation in roots of plants under dry conditions.The data also adds a further dimension to studiesof CAKMAK et al. (1994) which illustrate a greaterrole of K^ in increasing roor development oflegutnes grown under water stress by enhancingassimilate transport from sources to the sinks.The utilizatitin efficiency of K* in plants gntwnunder 3.0 mM K * as illustrated by root length ordr\' weights per unit (tnM) of K* is greater inplants grown under a low mr>isture regime (i.e.93 cm and 82cm per mM K." and 78 and 72 mgper mMK"" under suboptitnal and optimal soilmoisture regimes respccdvely) at the secondharvest.

In contrast, specific ixKit weights of plantsgrown under optimal and sub«>ptimal soilmoisture conditions were similar at the firstsampling (Table 1). The specific weights of plantsgrowth under a suboptimal soil moisture regimewas lower at the second harvest. This ctmfirmsthe greater expansion of roots per unit weight ofK*, thereby exhibiting a greater utilizationefficienc\- of this nutrient when plants arcsubjected to low soil moisture regimes. A furtherdimension to this observation is the greateraeration ofthe soil with a lower moisture conteni(ZoBEL 1993). This could also influence photo-synthate partitioning to develf^ a more robusrroot system to overcome moisture limitations.However, this needs verification.

Root hierarchy is influenced by stress factors(FlTThR 1982), which is again ilhistrated in thisstudy. Morphonietric analysis of root branchesh ^ g h t that the first order branches ansingfrotn the tap root are greater in plants grownunder a tow soil moisture regime (Tat4e 2). Incontrast, die second and third order roots ansingfrom the first and second order roots, respec-

168 SANtiAKKAIW, HAItTWIC and

Table 2. Effect of soil tnoisture and potassium fertilizer on branching patteai of Frenchbeans

Water Potassiumcontent (mM)

BranchHarvest 1

First Second TTiird

Harvest 2

First Second TTiird

Branchirig rado

HI H2

Over 50%depletion

Bck.w 25 "A,depletion

Interaction P

0.10.8

0.10.83.0

= (0.0.5)

31.2a32.8ab40.6b

20.8a24.7a31.4b

0.903

7.8a12.6b16.2b

n.2at2.4a16.8b

0.329

4.0a8.2b

12.4c

4.4a7.8b

10.6b

0.518

37.5a39.8a42.6a

32.0a38.8a

0.734

t l . l a18.6b25.6c

16.3a19.5a21.2a

0.421

6.7a12.8b18.6c

10.3a12.1a16.0a

0.235

6.813.523.05

3.992.842.69

39.7

56

15J

5.S

The branch t>TK:s arc classified as per method of TirrKR (1982); Means within a column of a given moisttitcregime followed by the same letter are not significantly different (P = 0.05); " HI and H2 corresp.-»nd to V2;V3 and V5 growth stages, respectively.

tively, arc greater in plants grown under anoptimal soil moisture regime, especially at thesecond sampling. Therefore, a greater branchingpattern of the r(H)r system was obser\'ed whenplants were grown with adequate soil moisture,thereby chariging the roor structure from ahcrringhonc to a dichotomous type. The resultsfurther showed that the changes in rootmorphology- due to soil moisture stress weresimilar to those observed with nutrient dcftcien-cies reported by iMTTiiR (1982), a concept notwidely reported.

Viithin a given soil moisture regime, applica-tion of K* changed root morphology. linhancedrates of K* increased numbers of all rctoibranches, especially of the higher orders in plantsgrown in a lower soil moisture regime. This againconfirmed the greater utilization efficiency of K^by plants gn»wn under suboptimal soil moistureregimes.

Application of 3.0 mM K* increased die firstorder roots of plant.s grown under suboptimaland opamal soil moisture regimes by 29 % arul.55 "/o wer that of plants grown with 0.1 mM K *in the first harvest. In the second harvest theincreases were 1 3 % and 28%, respectively, forthe first order roors of plants grown undersuhopdtnal and optimal soil moisture regimes.The impact of K'^ on the growth of first orderroots under a reduced soil moisture regime seetnsto be masked due to the lower number of theserf)Ots when plants are grown under diesecffliditions.

In contrast, in the first sampling, the increments in second and third order rt>ots due to thrappiicfltion of 3.0 mM K ^ over that of plantsgn>wn with 0.1 mM K* under a suhoptimalm(Hsture regjme are 128% and 200%, respec.tively, when compared to that of plans grownwith O.I mMK*. The increments in second andthird order nH)ts of plants grown wich3.0 mM K in the tlrst sampling under anadequate soil moisture regime over that of plantgrown with 0.1 mM were 5 0 % and 140%respectively. In the secomi sampling, the change?due to the application of 3.0 mM of K * over thatof 0.1 mM arc 127 % and 2(KI % in the secondand third order roots in plants grown undet asuboptimai soil mt>isture regime. The comp.vative increments in plants grown with adequatesoil moisture were 31 % and 60%, respecd\tk.Therefore, the changes caused by K in ihchranching patterns and the morphr)metr)- ofroots is greater under a low soil moisture regime,implying a significant effect of K on the grovrthof roots under this condition, which could beattributed to its role in the translocation <ifphotosymhaces firom sources to sinks (CAKMAK ctaL 1994). Analysis of the branching ratios (Fig. I,Table 2) highlights this phenomenon. ThebfMiching rados of plants grown under »suboptimal soil moisture r e ^ e declined tapioivwith increasing K * in contrast to that of plaofsgrown under an c^timal soil moisture regimt.The regression equations (Fig. 1) also confiri)this phenomenon. The greater rofe of K* "i

Rnoi MofphcAjgy and Water Potentials in Frenchbeans 169

2 T

s

Ii 1 • •

Controlkl V - -0.60171000 + 1.3476

I? = 0.8003

k2 y = -0.4539Lr)()0+1.3806»" = 0.7623

k3 1.50070.8786

1 2

Root order

'Al^—Loaarithmisch fk2)

k2•Logartthmisch (kl)'Looofithnnisch {k3)

I2 T Stress k l y =-O.8335UKX) + 14759

(?-0.8956

1.4988

- I -

I? = 0 8682

k3 V = -0.4846CJKX) + 1.58681?' = 0.6756

1 2

Root Older

k2 * k3 Logorithmisch(kT) logaiithmisch (k2) Logorithmisch (k3)

, li);- 1. The relationship berween rixir numbers and root orders iif Frenchliean as affected by optima)icnnirol) and suboptimaJ (stress) soil naoisture tegjmes and fertilizer potassium (kl, 1:2 and k^ represent 0.1,

; "-H and Xi) tnW potassiutn, respectively)

enhancing tbe branching panem to a grearerextent for extracting avaikble soil moisture isdcHtly presented by tbese resvilts.

The water potendal of plants gtou-n undersubt^timal soil moisture condiuons was signifi-cantly (68 %} lower than that of plants grownwill adequate soil moisture (Table 3). This'tla'sonship did not change sigruficandy with•"nc, illustrating the maintenance of thesedifiVrences in water potential throughout the^ceciative growth phase of this species. How-• 't-v, application of K* increa.sed the waterpi'ivntials of plants. At the first sampling, the"irrctnent in water potential of plants supplied'*"*' 3.0 mM K * was 65 % over tbat of plants

with 0.1 mM K at both soil moisture

regimes. At tbe second sampling, the increasewas approximately 67 "/n and 76 "/u m plantsgrown under a subt^timal and optimal soilmoisture rc^me. Wbile this again confirms tberole of K * in nnaintaining tbe .soil moislure statusof plants (MAR.SC;IINER J995), tbe greater magni-tude of increase in the water potential of jrfanf;gmwn under an optimal soil moisture regime isan interesting pbenomenon. However, due to thelack of a significant difference between rbc waterpotentials of plants grown wJtb 0.8 and3.0mMK* under a bigh soil moisture regime,tbe increment water potential due to K* inplants grown with adequate moisture is in theregion of SO %. Tbis increase is lower tban tbatof plants grown under conditions of suboptimai

170 SANCAKKAKA, HARTMG and Nt

Table 3. Moistute contents of Frenehbean shoots as influenced by soil moistiue and fertilizer potastium

Water potential Relanve water Tuigid wttdry(ban) Content (%) wt ratio

vcuc'

Water contentPotassium

(mM) HI HI H2 HI H2 HI H2-

Over 50% depiction(Suboptimal}

Below 25 % depletion(Optimal)

Interactirm P - (().(»5)

a i0.83.00.10.83.0

-6.6a-4.4b-2.1c-2.0a-1.7ab-0.7b0.001

-8.3a-5.6b-2.7c-3.1a-1.5b-0.7b0.001

7.V0a79.7b88.5c82.4a86.1 ab9().4b0.193

73.5a833b89.9c81.4a87.8b91.4c0.481

2.34a2.68b2.91c2.46a2.S8b2.69c0.067

2.29a2.66b2.75b2.68a2.76a2.87a0.169

0.364a0.339a0.220b0260a0.222ba.l66c0.574

0.4fHa

03")ah

o.r8c0.31 H

02l5at

0.162t

fl..S48

' VCTJC" (Water uptake capacit\') indicates the uptake of water per unit dry matter, ^ H1 and H2 correspond u-

V 2 A ' 3 and V5 growth stages, respectively; Means within a coiumn o f a given moisture regime foll<iiwed k

the same letter are not significantly different (P = 0.05).

soil moisture, where the application o f each

increment o f K* increased water potential

significant^. TTius, K"" is seen to have a prcater

impact under a sub<:ptiinal moistute regime. The

relative water content sipiifies the water contents

o f plants. Plants grown under a suboptimal

moisture regime had lower relative watct con-

tents. Application o f K * enhanced the relative

water c o n t e n t to a greatct extent in these plants

at both harvests. T h e magnitude o f increase in

plants grown with suboptimai soil moisture and

supplied with 3 . 0 m M K * when compared to

rhat o f plants grown with 0.1 m M K'^ was 22 %

at the second sampling in contrast to a 1 2 %

increment between plants grown with adequate

soil moisture. T h e ahilit\' o f plants to retain a

p-eater quandtj' o f moisture when supplied with

adetjuatc K" was evident especially under a low

soil mf)isnirc rc^me.

The t u t ; ^ weight:dry weight ratio ( T W : D W )

illustrates the water retention capacity o f plants

which is detemiined by the cell structures. Plants

grown under a high soil tnoisture regime had a

higher ratio which could be due to the lower

destruction to plant tissues by moisture deficit,

however, application o f K * reduced tiiis ratio,

especially at the first sampling. TTius, the ratio o f

plants grown under a suboptimal soO moisture

regime was enhanced by 25 % with 3.0 m M K"

when compared with pjants grown with

0.1 m M K*. Tl ie increment in plants grown with

adequate soil moisture and 3 . 0 m M K * was

sqjpioximately 1 0 % in tbe first harvest. This

could be due t o the tnaintcnance of cell strucmrt

witb increasing K * contents, whicb enables the

retention o f nioisture ei.-en under stress cijodi

tions. At the second harvest the differences win

reduced, especially in plants grown with adequan

soil moisture. This implies that the abilin- i>i

plants to absorb and retain moisture stabifeo

with time. T h e Water LIptake Capacity (Vil C

quantifies tbe capacity' o f plants to absorb J

greater quantity o f water per urut o f drj' weight !r

relation to turgid w e i g h t A bigher>XTJC iraiicatt

thar plants are subjected to a greater degree nt

moisture stress as these plants would absori' a

greater quantity o f water t o reach mtgid wcijtht

Thus , the mean W U C o f plants grown unctet .<

suboptimal soil moisture regime were appfoi!

matdy 4 6 % and 2 6 % greater than in th(w

grown under an optima) soil moisture regime «

tbe first and second barvests, respectively.

Application o f K"" reduced the W U C of

grown under a suboptimal soil moisture

to a greater e3a:ent, and the magnitude

with time. Application o f 3.0 m M K * to plant|

grown at tbe lower soil moisture r c p m e reduce*

the W U C by 56 % w b e n compared to thar of

plants grown with 0.1 m M K * in the sccitf'i

harvest. In contrast, the difference in '^^\

between plants supplied witb 3.0 m M K ^ i"^

0.1 m M K * under an optimal moisture iccjin'

was 4 8 % at tbe same barvest. This cica<B

presents the influence o f K * in maintaining; pi*"'

water status, especially under a low soil mcist'"'

regime, which could be amibuted to role •'? t^

Root Moipholf^' and Water Potentials in Frenchbeans 171

Table ^- Shoot gn)wth and nodulation of Frenchbeans as influenced by sail moisture and fertilizer

jmtissium

Water concent

l>er 50 % depletion(Suboptimal)

Btlow 25 % depled<m(Optimal)

Ititeracoon P = (0.05)

Potassium(mM)

0.10.83.00.10.83.0

Leaf area

( «

HI

5.1a17.3b26..SC17.6a28.3b33.1.3b0.200

H2

11.6a29.1b53.0c43..3a51.4b59.6c0.001

ShcHit dry v,i(mg)

HI

t40a148a195b17Ia2{t4b246co.2oy

H2

234a262b200c309a391b439c0.021

Shoot rRootratio

HI

1.77a1.56a1.34b2.23a2.02al>1.03b0.572

H2

3.57a1.85b1.27b2.60a2.15b2.04b0.33^

Nodulation(Nodules/plant)

HI

Oa3.8b12.4c013.8b20.0c(UI02

H2'

Oa7.5bI5.(>c018.5b26.6c0.(102

' HI and H2 correspond to V2/ \ ' 3 and V5 growth staj^es, respectively; Means, vkithin a column of a giveninoisnirc rej»ime followed by the same letter are not significantly different (P - 0.05).

auuient in maintainin|Lr CL-]I strucaire in plants:>Uitv:HNM 1995).

Development of correlations between plantttater status and rooting patterns presents anmtrrcsting phenomenon. Correlation coefficientsDcni'ccn root branching patterns (root bratichinjtratio) and shoot water potenttals utider different1 kvds are significant |r = 0.86 atid T — 0.71;P= 0.0.5)1 for suboptimal and optimfil soilmoisture regimes. Therefore, under a low soilmoisture regime tbe relationship berween waterpiHcntiaJ and nK>t branching pattern is affected toa greater extent by K * in alleviating plant water

Growth of Prench}>ean sbools as affected by«ii) moisture and fertilizer K * follow the trends"i roor grovpth and plant water relations (Table 4)JS these parameters affect the overafi dcvdop-mcm pr<x:ess of plants. Thus, leaf area, shoot dr\-'OKhts and sb<M>t: shoot ratios were increased to

»jrreaicr extent by K" especially when plants areP^'wn under a suboptitnal soil moisture r t ^ m e .This also ilfusttated a clear relationship betweenrix't and shoot growth as confirmed by earlierreports of the complementary effects of K* inwtrcoming water stress in this species (SANG.\K-^ w> et al. 1995) and other crops (IJNOHAUKR

N'odulation is mhibited in plants grown with'1 mM K fTable 4), irrespective of soil tnoisturcI'v This, as su^es tcd by S.\NCAKKARA et al.

) could be due to the reduction of rootwhich aie potential sites for inoculatitm.

With the application of O.K mM K* , n<)duktionwas initiated, although nodvde nutnbcrs werereduced in the low soil moisture regime. Thiscould be due to the dr\ing up of the upper laytr.sof the soil profile which is the site for profilenodulation (AbDi;i WAIIAB and AHO AI.I.A 1V9S).

However, K* enhanced nodulation and theimpact ts greater under a higher soil moistureregime. Thus, lK)th environmental factors deter-mine the success of ntHlulation and that ofbiological nitrogen fixation of this specie* asreported for fababean by .^BIMU. WAM.AB and ABD

AuA (1995). An adequate supply of K 'increases nodulation under a lower soil moistureregime. TTiis implies (be benefits <}f this nutrientin overcoming water stress ro promote rcK)tgrowth, root branching and also the smibioticpnxress in this important f<x)d legume which isgrown under suboptimal soil moisture regitnes inthe tropics.

Conclusions

The study was conducted to ascertain the impactof SO]) moisture and fertilizer potassium on rootbranching habits and shoot water potentials ofFrenchbeans, a popular food crop grown in awide range of tropical environments. Rootlengths were greater under a subopftmal moisturere^me, which facilities greater .liworption of soilwater. However, the impact of pota.ssium wasmore prominent in terms of enhanced r<x>t dryweights, especially under suboptimal soil moist-

172 SANCAKKAKA, HARTWIC antt

ure conditiotis. An aspect not evaloated in thisstudy was the impact of soD aeration on rootgrowth. However, it was assumed that at a givensoit moisture level, the soil aeradofl would besimilar in all potassium treatments. Due to thevery close correlation berween soil moisture andaeration (LIMEC and BUSSCHKI 1991), rootbratichittg habit could also be affected by thisphenomenon. The greater branching of roots inplants grown under a h^h soU moistuiie re^^mecould also be associated with lower soil aeration.In contrast, greater aeration seem to induce thedevelopment of a sturdier, low branched rootsystem, as shown by greater specific root weights.However, this tieeds further study.

The root hranching habits and ratios illustrateda change from a herringbone to dichotomoustype with increasing soil moistute and potassium.Plants proceed a more branched root systemwhen K was appbed, especially tmder a low soilmoisture regime.

Shoot water potentials were ako etihanced byK. The relative water contents, TW:DW ratiosand WUF. indicated that potassium increased theretention of moisture within the plant especiallyvinder a low soil nH)isture regime. This clearJypresented the role of K in maintaining osmoticpotential of plants to facilitate greater v^etativegrowth.

The lack of noduktion in the absence ofpotassium as reported hy SA.NCJ.'VKKAIIA et al.(1995) was again evident, irrespective of soilrooisture regimes. Application of 0.8 K inducednodulation, and the impact was greater under theoptimal soil moisture regime. Both soil moistuteand K played a significant role in promotingnodulation. The mechanism requires furtherstudy, although K cotitd stimulate photosynthatetnnsport to roots, as nodule activity is reduced inthe absence of adequate K (ABDKL WAHAB andABD AUA 1995). Akhou^ die results needconfirmation under field conditions, the applica-tion of K in fertilizer mixtures, especially ininfertile soils, seems to he a prerequisite topromote root branching Mid enhance waterpotentials of tropical food legumes. This phe-nomenon becotnes more evident under low soilmoisture legimes, which often Bmit productivityand sustainaUliiy of legumes in the tropics.

SpfoBwasseqwteitttals von Gaftenbohno){Phaseobts Vtik^am L.) gegenuber Bodnt.feuchtigiEeit und Kaliuntidungcr

Eine umfangreiche Verzweigung der Wurielcund die Aufrcchterhaltung einer ausreichendenWasserversorgui^ im Sprofisystem setzen ifcPllatizen in dk Lage, WasseistreB zu ubenvindca.Informationen bezii^ch der Beziehiu -nzwischen Kaliumdunger sowie Wurzdverzttfigung^mustem und SproGwasserpotentialen vor.Kdmert^^utninosen, die unter verschiedermBodenwasserbedii^;ungen wachsen, sind selteaEs wurde daher ein Experiment in einemPhytotron durchgefuhrt, um den BinHuf) vonKaliumdunger auf die Wutzelverzweigutipmuster und das SproRwasserpotential dner vrrhreiteten tropischer Komerl^uminose (Gmoibohne, Pbaseeius vn^^ms L.) zu untersuchen. DirPflanzcn wurden in einem Sandmedium mit 0,1,0,8 bzw. 3,0 mM Kalium unter suboptimalen undoptimalen Bodenfeuchtigkeitsbedingungen arigczctgen. Die Wurzellangen utid -gewichte wurdendurch Kalium erhoht, insbesondere bei subopdtnalen Bodenwasserbedirtgutigen. Das V«zweigungsmuster zeigt als Folge der KaiiuirbehandJung VeraiKlerutigen, wobei die Anafcder Seitenwuizeln zu-eiter und dritter Ordnung irbeiden Wasserbehandlungen erhoht wurde, (Awohl der FJnnuA bei geringer Bodenfeuchtif^ei'^ 8 e r war. Die Pfianzenwasseigehalte, gemcswials Sproflwasserpotential, relative WassergehalicVerhaltnis von Ftisch- zu Trockengewicht HWasseraufnahmekapazitat, zeigten sich mit (fc'Kaliumbehandlung erhoht. Es kwinte einetive Beziehung zwischen dem Wurzdver7we^gungsmuster und dem Wasserpotential nutzunchmetKkn Kaliutnkonzentratiotten, insbeson-dere bei Pflanzen unter suhopdmalen Feuchtitkeitsbeditigurigen, nachgewiesen werden. Sp(!wachstum und KnoUchenanlage wurden dufCKaUum gefordert. Die Fahigkeit der Pflanzen, ofumfangreicheres Verzweigungsrnuster als Fleiner groBeien AnzaM von Wurzeln zwciter uacdritter Ordnung zu entwickeb und das W 'verrweigutigsverhalten von einer 'Gratenform'Ji'dnem dichotomen Typ zu andem, diirfte Grund-iage eines hoheten %roBwasserpotentials, i"*besondere uncer tiiedtigen to^

hedittgui^en, sein.

Ziaainnieiifa8sung

Reakdon der Wurzelvetzweiguiig und desAckjiowledgeiiieiits

The authors are gntefiil to the Development CorpC'

jtoot Moipiidogy and Water Potentials in Ftencbbeans 173

lioo ot'Switzedind (DEH) fior financial support for theitiidy and to Dc D. MAUNOWSKI for assistance instadsdcil analysis.

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