Adaptive mechanisms of Blackgram (Vigna mungo (L.) Hepper) and pigeonpea (Cajanus cajan (L.) Millsp.) to water stress at different growth states

Abstract

The severity of water stress on plant growth and yieldndepends on (i) the history of water stress, (ii) the durationnand (iii) intensity of water stress, and (iv) the timing ofnwater stress relative to stage of growth. These aspectsnaffect also the plant's ability to recover from water stress.nA series of controlled-environment experiments was conductednat the Department of Agriculture, University of Queensland tonstudy the responses of blackgram (Vigna munqo (L.) Hepper)ncv. Regur and pigeonpea (Cajanus caian (L.) Millsp.) cv. QPLn207 to water stress. Particular attention was placed onnadaptive mechanisms that might be important in overcoming thenadverse effects of water stress, both during and afternperiods of water stress.In a preliminary experiment (Experiment 1), leaf waternstatus during water stress was studied in four grain legumenspecies, blackgram, cowpea (Vigna unguiculata (L.) Walp.) cv.nRed Caloona, pigeonpea and soybean (Glycine max (L.) Merr.)ncv. Buchanan. These species were subjected to preconditioningnwater stress compared with a well-watered control prior tonwater stress from 27 days after sowing (DAS). Changes innrelative water content (RWC), leaf water potential ( PL ) ,nosmotic potential (pL ) and turgor potential (PL) differednamong species. Leaf water potential changed markedly innpigeonpea and soybean; in blackgram there was moderatenresponse; while there was no significant response in cowpea.nEffective osmotic adjustment, shown by changes innrelationships between RWC, PLnand pLwith different prestress conditions, was evident only in pigeonpea. Based onnthese results, blackgram and pigeonpea were chosen fornfurther study.nThe effects of water stress on blackgram and pigeonpea atndifferent growth stages was studied in a series of threenexperiments. In Experiment 2, the effect was investigated ofnwater stress during the vegetative stage (14 d beforenflowering) on leaf area development. The severe water stressnthat developed in this study reduced leaf number more thannleaf expansion in both species. In Experiment 3, the effectnwas studied of a water stress period (14 d) of varyingnintensity imposed at a single growth stage. It was only afternlow intensity water stress during vegetative growth that bothnspecies resumed adequate growth and produced seed yieldsncomparable with those of well-watered plants. This was due tonadequate maintenance of leaf area that contributed tonrecovery after the relief from water stress. The podnformation and flowering stages were the most sensitive tonwater stress in both species. Pigeonpea survival was promotednby partitioning of assimilates into vegetative plant parts.nBlackgram, on the other hand, partitioned assimilates intonreproductive organs. In Experiment 4, the effect was studiednof duration of water stress during two or three growthnstages. Water stress treatments resulted overall in ansignificant reduction in seed yield, but differences werenevident in the effect of moderate (28 d) and long (42 d)nduration water stress. In both species, plants subjected ton28 d of water stress during pod formation and pod fillingnproduced significantly higher pod number and seed yieldncompared with those subjected to earlier, moderate waternstress and water stress of long duration. Under prolongednwater stress, pigeonpea recovered after rewatering betternthan did blackgram. This might have been due to leaf damagenaffecting stomatal function in blackgram.nThe second series of three experiments was designed toninvestigate interaction between root and shoot growth andnwater uptake during water stress and recovery periods. InnExperiment 5, the effect of water stress on rooting patternnand leaf movement was conducted in sloping-face glass boxesnwith water stress imposed from 35 DAS. As the soil dried, andecrease in root growth occurred at about the same time thatnstomatal closure and leaf movement became evident. Leafnmovement in response to water stress was evident in bothnterminal and lateral leaflets in pigeonpea; in blackgram,nthere was response only in terminal leaflets. The effect ofnosmotic adjustment on leaf and root growth during waternstress and recovery periods was studied in Experiment 6 andn7. Pigeonpea conserved water through limited rootnproliferation at depth and maintenance of leaf and rootnturgor through osmotic adjustment. On the other hand,nblackgram had an extensive root system, and used water morenrapidly than pigeonpea. Difference in root anatomy betweennspecies illustrated that pigeonpea had higher root resistancento water flow than did blackgram.It was concluded that the adaptation of both speciesndepends on the intensity, duration and timing of waternstress. While both species are tolerant of water stress,nthere were differences in adaptive mechanisms. In pigeonpea, important mechanisms included: (i) high root resistance tonwater flow; (ii) slow shoot development; (iii) limitedninitial root development at depth; (iv) partitioning ofnassimilates into vegetative parts; (v) leaflet movementnduring water stress; and (vi) osmotic adjustment. Othernstudies have shown that dehydration tolerance is importantnalso in this species. By comparison, blackgram exhibitednrapid shoot and root development, low root resistance tonwater flow and little osmotic adjustment. Important adaptivenmechanisms in blackgram were: (i) developmental plasticity bynpartitioning of assimilates to reproductive organs; (ii) leafnmovement during water stress; and (iii) rapid recovery fromnwater stress provided that leaf shedding was not too severe.n