Effects of Panicle Water Potential on Water Stress Susceptibility in Rice

The efects of drought on the rice yield differ depending on the time when plants are subjected to the drought. It is therefore probable that water stress susceptibility, the degree of yield reduction due to drought, is also a function of plant development. Water stress susceptibility is defined as the fractional decrease of panicle weight per unit change of cumulative water stress (an integrated water stress with time). The objective of this study was to determine whether water stress susceptibility is correlated with the yield responses to drought. Wetland rice plants (cultivars Koshinishiki and Somewake) were grown in pots containing soil under polyethylene rain shelters. Water potentials of leaves and panicles, and grain yield were measured in plants subjected to short-term water deficit imposed at various stages of panicle development. Water stress susceptibility increased with panicle development prior to heading but decreased as reproduction progressed. A similar trend was also seen in the data of winnowed paddy number. Prior to heading panicle water potential under drought increased with panicle development, whereas the water potentials of panicles and leaves were similar after heading. These results indicate that changes in water stress susceptibility reflect the variability of grain yield under drought at least partly. The greater water stress susceptibility was due to a smaller number of winnowed paddies. In addition, the susceptibility before heading was related to the maintenance of a higher panicle water potential under drought.