Changes in grain nitrogen content and ripening characteristics of rice (Oryza sativa L.) with fertilizer application after flowering under different soil moisture conditions

Changes in grain nitrogen content with fertilizer application and soil moisture content were examined in relation to harvest index, top dry weight and straw nitrogen content. Five cultivars from each ecospecies, i.e. japonica cultivars, indica cultivars and javanica cultivars in which a synthesized big grain is included and five upland cultivars were used in the experiment under paddy condition, and three cultivars from the abovementioned ecospecies and three upland cultivars in the experiment under upland condition. The rice plants were grown under the cultivation conditions as that in Shounai area until flowering. After flowering, the rice plants were divided into three plots with three replicates and were grown under different fertilizer levels, i.e. 0N, 1N and 3N plots. A compound fertilizer (N : P : K = 13% each) was applied at the rate of 1kg/10a to the 1N plot and 3kg/10a to the 3N plot, while the 0N plot was left fertilizer free throughout the grain filling period. The fertilizer was applied five times at an interval of 7 days after the end of flowering. The grain nitrogen content of all the rice plants grown under upland condition was higher than that under paddy condition. The harvest index decreased with increase of fertilizer level and also decreased under paddy condition as compared with upland condition. The grain nitrogen content was negatively correlated with the harvest index and the straw nitrogen content, but positively with the top dry weight. Especially, the grain nitrogen content of the japonica and indica cultivars increased markedly with decrease of the harvest index and increase of the top dry weight. The ripening rate of the japonica cultivars was slower than that of the javanica and indica cultivars and the synthesized big grain strain. The faster ripening rate of indica cultivars resulted in more rapid decrease of the straw nitrogen content