Abstract
The effects of nltrogen fertilizer supply on the genetic variability of 12 quantitative characters in rice were investigated, uslng ten varieties of Indica and Japonica each. Four nitrogen levels were applied In a split-plot design experiment. In the analysis of varlance, 8(Indica) or 10 (Japonica) out of the 12 characters examined showed significant nitrogen effect. In most of the significant characters, F values for nitrogen effect in the analysis of variance were larger in Indica than in Japonica, but F for genotype × nitrogen interaction gave no such differences. Stability of a broad sense heritability at different nitrogen levels varied with the characters. Number of days from sowing to heading gave high and very stable heritability, while effective tiller percentage and filled spikelet percentage showed moderately or highly variable heritability. Most of the characters showed higher heritability in Indica than in Japonica. As judged by magnitude of coefficient of variation in the heritability and in the varlance component for genotype, Indica varieties were stable over different nitrogen levels relative to Japonica varieties. It was suggested that breeding materials should be subjected to selection at a proper nitrogen ievel which maximize heritability of the character concerned, unless the order of lines as regards the magnitude of characters differs with nitrogen levels.