Characteristics of Dry Matter Production Process in High-Yield Rice Varieties VI. Comparisons between new and old rice varieties

Differences in yield, dry matter production, and harvest index between new (registered after 1955) and old (before 1940) Japanese rice varieties were examined with reference to light-intercepting characteristics, single -leaf photosynthesis, and dry matter accumulation in the panicle. The average yield of new varieties (586 g/m2) was higher than that of old varieties (483 g/m2) owing to higher dry matter production during the ripening period. This difference was caused by a smaller decline in the leaf photosynthetic rate and by the lower canopy light extinction coefficient observed in the new varieties, as compared with the old varieties, during the ripening period. The new varieties had a higher harvest index as compared with the old varieties, but there were no differences in sink size between them. The higher dry matter production in the new varieties resulted in the accumulation of assimilate in the culm at the late ripening stage. Clearly, the sink size was not enough in the new varieties, i.e. the sink size should be enlarged by using the Indica or Indica/Japonica hybrid varieties (Nanjing 11, Milyang 23) in the breeding program.