Studies on Varietal Difference in Yielding Ability in Rice. Relationship between source-sink ratio and yield

The yielding abilities of rice varieties were examined among three groups : 10 European and American varieties (E group), 20 Japanese varieties (J group), and 16 high-yielding varieties bred in Asian countries (H group). Each yield capacity (NV) was expressed as a product of the number (N) and the volume (V) of unhulled rice. The vield contents (CW+ΔW) were expressed by the sum of reserved carbohydrates in the rice plant at the full heading date (CW) and the dry matter production amount during the ripening period (ΔW). The (CW+ΔW)/NV was regarded as the source-sink ratio. The average yield (Y) was highest in H group and lowest in E group. Both NV and CW+ΔW of H group were highest among the three groups. But ΔW of H group was not so large in comparison with that of J group. The (CW+ΔW)/NV of H group was lower than that of J group because the increase of NV was greater than that of CW+ΔW in H group. The transfer ratio of yield contents [Ef:Y/(CW+ΔW)] was highest in H group. The specific gravity of unhulled rice (S:Y/NV) of H group was as high as that of J group. These results suggest that the high yielding ability of H group is mainly attributable to its large NV and CW, and also to its high Ef. Consequently, it is considered that an enhancement of source-sink ratio with the increment of ΔW is necessary to improve the yield potential in H group.