Effect of irrigation on soil-plant water relations and canopy photosynthesis in mung bean (Vigna radiata (L.) Wilczek)

Mung bean (Vigna radiata (L.) Wilczek) was grown on a sandy loam soil under four irrigation schedules, irrigation at cumulative pan evaporation (CPE) equal to 200 mm (I-200), 300 mm (I-300), 400 mm (I-400), and no irrigation after seeding (I-0) to study the changes in plant water relations and canopy net photosynthesis (Pn) during the hot dry season (April-July, 1980) in a semiarid environment. An increase in the frequency of irrigation resulted in higher soil water content, leaf water potential, osmotic potential, turgor potential, and water loss from crop canopy (ET). The leaf water potential recovered at later stages of growth in all irrigation treatments even with the reduction in the soil water content. Accumulation of sugars and organic acids constituted most of the osmotic potential. Lower osmotic potential than leaf water potential maintained positive turgor potential throughout the season irrespective of the irrigation schedule. Turgor potential increased with ontogeny due to the recovery in leaf water potential and the reduction in osmotic potential. Turgor potential was linearly related to leaf water potential. Diurnal, vapour pressure deficit (VPD) showed positive significant linear correlation with photosynthetically active radiation (PAR). In general, ET increased with the VPD but the reverse was true for stomatal diffusive conductance gs. Both gs and canopy net photosynthesis were higher, particularly during the grain-filling stage in the 1-300 than in the more frequently irrigated 1-200. Grain yield had a positive significant correlation with canopy net photosynthesis but the correlation was even higher with the leaf area index and gs. The study has highlighted that irrigation scheduling at 1-300 can improve physiological functions and productivity of mung bean in a semiarid environment.