Characterization of high-yielding rice cultivars with different grain-filling properties to clarify limiting factors for improving grain yield

Recently developed high-yielding rice varieties with extra-large sink capacity often have unstable grain filling. Therefore, understanding the factors that limit grain filling is essential for further improvement of rice grain yield. Because grain-filling is determined by the complex sink–source balance, grain-filling ability is very difficult to evaluate. In this study, we compared grain-filling-related traits of three high-yielding cultivars with high sink capacity. We found that the translocation of non-structural carbohydrates (NSC) from stem to panicle during early ripening and grain filling was significantly lower in Momiroman than in Hokuriku 193 and Teqing, whereas dry matter accumulation of the whole plant did not differ among the cultivars throughout ripening. The NSC-components, sucrose and starch were both remained higher in stems of Momiroman than other cultivars. ADP-glucose pyrophosphorylase (AGPase; EC 2.7.7.27) activity was not enhanced and α-Amylase (EC 3.2.1.1) and β-Amylase (EC 3.2.1.2) activities were not impeded in the stems. These data suggested lower sucrose translocation is responsible for lower NSC translocation in Momiroman, rather than too high starch synthesis and too low starch degradation activities. At early ripening, grain growth of the superior spikelets was slow in Momiroman even if carbon supply was increased by spikelet-thinning treatments. These results raise the possibility that low sink strength determines low grain filling in Momiroman, which delays grain growth and decreases sucking force of NSC from the stem