Living with Limited Water: I. Shoot Hydraulic Resistance as Water-Saving Strategy and its Significance in Varietal Improvement of Rice

Abstract

Knowledge of the location and magnitude of resistance to water flow in a plant is fundamental for describing whole-plant response to water deficit as the water after absorption by roots has to pass through various magnitudes of resistance offered by the stem and leaf sheath tissues before reaching the transpiring surfaces, i.e., laminae. Therefore, the hydraulic resistance to water flux was studied in aboveground portions of excised rice plant (Oryza sativa L.) by incubating them with their cut ends under water. The findings suggest that the hydraulic resistance occurs in every organ of the plant, though in varying degrees, the order of its magnitude being greatest in the stem followed by leaf sheath and lamina in that order, i.e., stem > leaf sheath > lamina. Pressurization of water to force entry through the cut end into the detached plant was used to elucidate the temporal dynamics of shoot hydraulic properties in rice cultivars. For this, the time taken for unrolling of leaves that roll back following release of pneumatic pressure by "stop-flow" technique, i.e., pressure application and its release, was used as a yardstick for measuring shoot hydraulic resistance. were found to have maximal shoot hydraulic resistance as they took the maximum time, i.e., 67 and 64 min respectively, for complete 200 Hydraulic Resistance in Rice Cultivars 201 leaf unrolling on account of increased water status under pneumatic pressure. The cultivars 'Jal-priya' and 'Jal-lahari', suited to waterlogged ecosystem, took the least time (38 to 36 min) for leaf unrolling, indicating low hydraulic resistance thereof. The responses of cultivars 'NDRH-2' and 'NDR-359', suitable for irrigated conditions, were intermediate, which took 52 to 48 min for leaf unfolding under pressure. It was assumed that the more the time taken for leaf unrolling by a cultivar, the more was its shoot hydraulic resistance. Higher shoot resistance appears to be an important trait that could enhance the ability of plants to compete advantageously for stored moisture in the soil. It is suggested that this new stop-flow technique, a move meant for the beyond the confines of laboratory making a significant departure from other techniques of measuring shoot hydraulic resistance, could be used for screening large number of germplasm and breeding materials of rice to enable plant breeders to select and develop varieties having increased shoot hydraulic resistance, a water-saving trait, for sustainable crop production under reduced water supply ecosystem across the globe.