GENETIC CHARACTERISTICS IN DETERMINING THE PRIORITY OF SPIKELETS WITHIN THE PANICLE FOR GRAIN FILLING IN RICE

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There is the genetic difference in determining the priority of spikelets within the panicle for grain filling in rice. Under the low source/sink ratio, the early growth of grains at the lower positions within the panicle (inferior grains) is easy to delay in Sasanishiki and Koshihikari, but is difficult to delay in 92133 (China) and Akinishiki. Consequently, the percentage of grain filling is lower in the former varieties than in the later varieties. The physiological mechanism which accounts for this genetic difference was investigated. Shading after heading lowered the abscisic acid (ABA) levels in the inferior grains in Sasanishiki and Koshihikari, but had no effect in 92133 (China) and Akinishiki. There was no relation between the early growth of inferior grain and its endogenous cytokinin or free sugar levels. It was known that exogenous cytokinin promotes the early grain growth and grain filling.Therefore, it was concluded that the genetic difference in determining the priority of spikelets for grain filling was regulated by the endogenous ABA levels in the grains.Generally, ABA promotes the ethylene evolution. But ABA did not change the ethylene evolution from the panicle, and some chemicals changed the ethylene evolution without the changes of the early grain growth, suggesting that ABA promote the grain growth directly.Not only ABA but also cytokinin promotes the early growth of inferior grains. In this study, exogenous cytokinin or ABA and root cutting promoted the early growth of inferior grains. Endogenous cytokinin levels in inferior grains was increased by increasing the nitrogen level, but was decreased by root cutting. Endogenous ABA level was increased by exogenous cytokinin or ABA and root cutting, and was closely related to the grain growth. These results suggest that cytokinin promotes the grain growth through the increase of the levels of endogenous ABA.