STUDIES ON VARIETAL DIFFERENCE IN YIELDING ABILITY IN RICE .1. VARIETAL DIFFERENCE IN YIELD STRUCTURE

In order to investigate the breeding objective on the improvement of rice yield, the yield structures were analyzed and compared among a hundred varieties. By using the mean specific gravity (S), all varieties were divided into two types, I (S: above 1.10) and II (S: under 1.10). And by using the translocating ratio of reserves from the culm and leaf to the grains (T) on yield (Y), they were also divided into three types, A (T/Y: under 19.8%), B (T/Y: 19.8-35.0%) and C (T/Y: above 35.0%). A hundred varieties were classified into five types that had different yield structures by combining with I, II and A-C classes. The IC type having the highest yield was very large in the capacity of unhulled rice (NV), high in S and T/Y, but not so large in the assimilates produced after the full heading date (DELTA W) and the lowest in DELTA W/NV. In this type, S was higher than the other types at the same level of the percentage of ripened grain. It was conjectured that the most important breeding objectives to improve the yield were the magnification of NV for IA type, the increase of T for IIA type, the magnification of NV and the increase of DELTA W for IB and IC types and the increase of DELTA W for IIB type, respectively.