Changes of heading time and panicle weight in rice [Oryza sativa] subjected to water stress during the early stage of panicle development

The increase of heading time in proportion to cumulative water stress (an integrated estimate of the degree and duration of water stress) was demonstrated in a wetland rice cultivar Koshihikari subjected to water stress during early stages of panicle development. Since the yield decrease of rice under drought is generally attended with the delay of the heading time, it was expected that panicle weight might be reduced by water stress imposed at the early panicle development stage. To prove this hypothesis, we conducted an experiment in which the relation between the panicle weight and cumulative water stress was examined. Potted plants of three cultivars differing in maturity (Koshinishiki, Koshihikari and Norin 18) were subjected to different water treatments at the spikelet differentiation stage of development on the main culms. Control plants were kept continuously submerged. In the drought treatment pre-dawn leaf water potential declined during the stress cycle. Such water stress caused delayed heading and decreased panicle weight per plant. The delay in the time of heading and the reduction in panicle weight under drought were proportionally related to the cumulative water stress. The reduction in panicle weight per plant per unit cumulative water stress was greatest in Koshihikari, followed by Norin 18 and Koshinishiki. Such ranking is also seen in the number of spikelets per plant data. The results also suggest that the reduction in the panicle weight by water stress during early stages of panicle development was associated with that in the number of spikelets.