Influence of Soil Water Supply on the Plant Water Balance of Four Tropical Grain Legumes

The water balance of soybean (Glycine max), cowpea (Vigna unguiculata), black gram (Vigna mungo), and pigeonpea (Cajanus cajan) grown in pots was studied during a soil drying cycle. The response of the plants was analysed for three distinct stages of dehydration. In stage I, the rate of transpiration remained constant and equal to that of well watered plants even though soil water status fell by more than 50%. Stage II began when the rate of soil water supply to the plant was less than potential transpiration and stomates closed resultingjn the maintenance of plant water balance. When soil water content was expressed as a fraction of transpirable soil water, all species showed a transition from stage I to stage II at a fraction of transpirable soil water of about 0.3 to 0.2. As the soil water declined further, all species had a similar decrease in relative transpiration rate. Consequently, the responses of the four species in stages I and II were essentially identical, except that pigeonpea extracted a slightly greater amount of soil water. Stage III occurred once stomates had reached minimum conductance and water loss was then a function of the epidermal conductance and the environment around the leaf. Substantial differences were found among the four grain legumes in epidermal conductance. Soybean had the highest conductance, followed by black gram, cowpea and pigeonpea. Substantial variation in dehydration tolerance among the four grain legumes was also found: the ranking of dehydration tolerance based on the relative water content was pigeonpea > cowpea > mungbean > soybean. Differences among the four grain legume species in the duration of stage III which finished when plants died, were consistent with differences in epidermal conductance and in dehydration tolerance of leaves.