Assimilate Partitioning Within Floret Components of Contrasting Rice Spikelets Producing Qualitatively Different Types of Grains

Abstract

Grain quality, soluble and insoluble carbohydrate contents, and activities of two key enzymes (sucrose synthase and invertase) involved in starch synthesis in the endosperm were measured in the components of the fertile florets of spikelets located at the tip of the uppermost primary branch (top spikelet), and the penultimate position of the lowermost primary branch (basal spikelet) of rice panicles during the period begining some days before anthesis to maturity. The basal spikelet, which reached anthesis a week after the top spikelet, produced a partially filled poor quality grain in contrast to the good quality high density grain produced by the latter. Sucrose was the major translocatable sugar in the organs of the fertile florets of both top and basal spikelets, and poor grain filling of the latter was not caused by deficiency of this material due to any resistance in supply in the external protective 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 synthesis of the starch leading to partial grain filling in the basal spikelet is due to a lower activity of sucrose synthase.