A quantitative measure for assimilate partitioning efficiency in rice (Oryza sativa L.)

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Grain filling in rice is determined by the balance between sink size and source supply. Partially filled or unfilled spikelets is produced when source supply per spikelet is insufficient. Cultivars with high assimilate partitioning efficiency, which produce fewer partially filled spikelets, would be preferable because the grains of partially filled spikelets have low market value. However, it is unclear whether there is a genotypic difference in assimilate partitioning efficiency, and there is no quantitative measure for this parameter. The objective of this study was to develop such a measure from the relation between grain-filling percentage (GFP) and percentage of filled spikelets (PFS) of cultivars with various grain-filling abilities. GFP is a dry-weight-based grain filling degree, i.e. the ratio of total hulled grain yield to the sink capacity. PFS is a spikelet-number-based grain filling degree, i.e. the proportion of the number of filled spikelets to the total number of spikelets per area. We investigated the factors associated with partitioning efficiency based on grain weight distribution during grain filling. We also determined the filling potential of empty spikelets, which are indistinguishable from unfertilized ones, under limited assimilate supply. Cultivars with a large (indica-dominant Takanari, japonica-dominant Momiroman) or moderate sink size (indica Kasalath, japonica Koshihikari) were grown in a paddy field and defoliated at full heading to limit assimilate supply. Grain weight distribution was investigated at 10, 20, and 30days after anthesis. Yield, yield components, and shoot dry matter were measured at maturity. We found an exponential relation between GFP and PFS, which can be transformed to Ln(PFS)=B +A×GFP, with regression coefficients A varying among cultivars. A lower regression coefficient shows a higher assimilate partitioning efficiency and production of fewer partially filled spikelets. In indica and indica-dominant cultivars, which have lower regression coefficients, the difference in the weight of fast- and slowly-growing grains was larger than in japonica and japonica-dominant cultivars, suggesting an association between assimilate partitioning efficiency and the initial grain growth rate of fast-growing superior spikelets. We found that empty spikelets caused by low assimilate supply were fertilized and had a filling potential. In conclusion, the results suggest the regression coefficient between GFP and Ln(PFS) differs among cultivars with different grain filling abilities, and could be used as a quantitative measure of assimilate partitioning efficiency in rice breeding.