Abstract
Experiments on the growth response to nitrogen were conducted in 41 (1981) and 51 (1982) rice varieties including japonica, indica and japonica-indica hybrid. The ratio of the number of spikelet to straw weight of japonica-indica hybrid and semidwarf indica was larger than that of japonica. However, the ratio decreased greatly as a result of nitrogen application in these groups, although it did not change in japonica. The rate of increase in dry weight of the leaf blade due to nitrogen application was larger than that in dry weight of the culm and leaf sheath, and the ratio of the rate of increase in dry weight of the leaf blade to that in the number of spikelet was larger in japonica-indica hybrid and semidwarf indica than in japonica. Coefficient of variation of the number of spikelet in a hill of japonica-indica hybrid and semidwarf indica was larger than in japonica, and several varieties of these groups with a large mean number of spikelet per panicle had a large percentage of the number of spikelet on the secondary and tertiary rachis branches. Japonica-indica hybrid and semidwarf indica, which were thought to have high-yielding characteristics, had a large ratio of spikelet number to straw weight, but, this advantageous characteristic decreased when nitrogen was applied. Moreover, these groups also had disadvantageous characteristics for ripening. It was concluded that these characteristics should be improved for breeding high-yielding varieties by using japonica-indica hybrid or semidwarf indica as genetic resources.
