Effect of irradiance on the partitioning of assimilated carbon during the early phase of grain filling in rice

Low irradiance in the early phase of grain filling in rice often results in a low grain yield, but its effects on the partitioning of previously or recently assimilated carbon within the plant or panicle have not been seriously examined. The objective of this study was to demonstrate the effect of shading during the different stages in the early phase of grain filling on the partitioning of previously or recently assimilated carbon among constituent organs and into superior and inferior spikelets of the panicle in rice (Oryza sativa L. 'Sasanishiki') plants using ¹³C as a tracer. Plants were grown either under low (shading) or moderate (non-shading) irradiance (120 and 800 μmol quantum m^-2 s^-1) for 3 or 4 d before or after the ¹³CO₂ feeding at heading, full-heading or milky stages during the early phase of grain filling. Four days after the ¹³CO₂ feeding, the proportion of labelled (previously assimilated) carbon partitioned into the panicle was 17 % higher in plants grown under low irradiance compared with plants grown under moderate irradiance at the full-heading stage (7-11 d after heading), while the proportion partitioned into the culm was 13 % lower. The light treatments for 3 d were conducted before the ¹³CO₂ feeding and partitioning of the labelled (recently-assimilated) carbon into spikelets was examined 6 h after feeding. The amount of labelled carbon partitioned into the spikelets of the secondary branch (inferior grains) in the plants grown under low irradiance was only 31 % when compared with plants grown under moderate irradiance at the full-heading stage, although the partitioning of labelled carbon into the apical spikelets of the primary branch (superior grains) was not affected by the light treatments. These results clearly indicate that preferential partitioning of assimilated carbon into the panicle occurs under low irradiance at around 7-11 d after heading and that the priority of superior spikelets for assimilated carbon intensifies. This phenomenon is thought to be an important strategy for such rice cultivars as used in this study to achieve a certain proportion of ripened grains even under light limited conditions.