Abstract
Rice panicles start to transpire just after heading. We can therefore expect that the water potential of panicles changes little prior to heading and varies in response to transpiration after heading. To test this hypothesis, we followed the ontogenetic change of plant water potential from the panicle initiation stage to maturity in a wetland rice cultivar, Koshihikari, grown in a submerged rice field. Diurnal changes were also studied on selected days. Leaves underwent a gradual decrease in midday water potential, which is associated with plant development. A similar trend was also found in panicles, except just before the exposure of panicles to the atomosphere when a sudden and brief increase in the panicle water potential was observed. Before heading, the water potential of panicles changed little diurnally, whereas it fluctuated markedly in concert with a change in potential evapotranspiration after heading. The water potential of panicles was consistantly higher throughout phenological development than that of leaves, except during the last stage of the grain-filling period. The results suggest that, before heading, the water potential of rice panicles appears to be coupled strongly with water flux for growth. After heading, the influence of transpiration becomes predominant. Superimposed on these is the increase in plant resistance to water transport that is associated with plant development.
