Abstract
Field experiments on the growth response to nitrogen were conducted for three years (1983-1985) using 49 rice varieties including japonica, indica and japonica-indica hybrid. Rates of increase in culm length and lower internodes as a result of nitrogen fertilization to japonica, japonica-indica hybrid and semidwarf indica were large, intermediate and small, respectively. The rate of decrease in dry weight of the leaf sheath and culm per unit culm length was larger in japonica than that in japonica-indica hybrid and semidwarf indica. The ratio of leaf area to the spikelet number was obviously smaller in japonica-indica hybrid and semidwarf indica compared to that in japonica. However, the ratio of the rate of increase in leaf area to that in the spikelet number due to nitrogen fertilization was larger in these groups than that in japonica. Furthermore, the rate of increase in single leaf area and specific leaf area of japonica-indica hybrid and semidwarf indica were larger than those of japonica. The obtained results suggested that one of the reasons of higher lodging resistance of japonica-indica hybrid and semidwarf indica was ascribed to a little changes in the culm length due to nitrogen fertilization compared to that of japonica. It was also suggested that these groups had a tendency of over-luxuriant growth due to nitrogen fertilization compared to japonica. This characteristic should be improved for breeding high-yielding varieties by using japonica-indica hybrid or semidwarf indica as genetic resources.
