ASSIMILATE PARTITIONING WITHIN FLORET COMPONENTS OF CONTRASTING RICE SPIKELETS PRODUCING QUALITATIVELY DIFFERENT TYPES OF GRAINS

Grain quality, soluble and insoluble carbohydrate contents, and activi ties of two key enzymes (sucrose synthase and invertase) involved in s tarch synthesis in the endosperm were measured in the components of th e fertile florets of spikelets located at the tip of the uppermost pri mary branch (top spikelet), and the penultimate position of the lowerm ost primary branch (basal spikelet) of rice panicles during the period begining some days before anthesis to maturity. The basal spikelet, w hich reached anthesis a week after the top spikelet, produced a partia lly filled poor quality grain in contrast to the good quality high den sity grain produced by the latter. Sucrose was the major translocatabl e sugar in the organs of the fertile florets of both top and basal spi kelets, and poor grain filling of the latter was not caused by deficie ncy of this material due to any resistance in supply in the external p rotective organs. The activities of the sucrose synthase and invertase were higher and lower respectively, in the endosperm cells of the top spikelet compared with the basal spikelet. We conclude that poor synt hesis of the starch leading to partial grain filling in the basal spik elet is due to a lower activity of sucrose synthase.