Genotypic Differences In Grain Yields And Nitrogen Use Efficiency In Rice Cultivars

China accounts for 20% of the total production of rice (Oryza Sativa L.) in the world but accounts for 37% of total nitrogen (N) fertilizers consumed in rice production. The primary reason for so much N fertilizer use in rice production is the very high grain yield goal of Chinese farmers. The average rice grain yield in China is 6180 kg ha^-1, which is 65% higher than the world average (3750 kg ha^-1). Another reason why N fertilizer use is so high for rice production in China is that rice farmers currently have smaller land holdings than previous and want to ensure their rice crops always have a sufficient supply of N. This results in excessive N applications and the risk in negative environmental effects. Such practices have aroused concerns of societies both in China and abroad. Nitrogen uptake and utilization of crop are two important parts in N cycle of Agriculture and ecology. The efficiency of the N use by rice is central to its overall yield potential and environment effect, and efforts to improve N use efficiency (NUE) must be guided by a through understanding of the processes that govern NUE. Breeding selection of high N efficient cultivars is an effective way to increase N fertilizer efficiency and reduce N loss in arable fields. Nevertheless, the agronomic basis underlining it is still quite vague.In this study, three parts of field experiments were conducted: (1) Field screening experiment for high and low NUE in Jiangpu Farm, 60 km northwest of Nanjing Agricultural University in 2003. Total 177 Japonica rice cultivars (71 medium-duration cultivars and 106 long-duration cultivars) were grown and supplied with two N application rates (0 and 180 kg ha^-1). The experiment was arranged as split plots in a randomized complete-block design with three replications. Nitrogen application rates were the main plots, and rice cultivars were the subplots. (2) Field experiment at Jiangning Farm, 40 km south of Nanjing Agricultural University in 2004. The field experiment was conducted with seven N rates (0, 60, 120, 180, 240, 300, and 360 kg ha^-1) as main plots and four Japonica rice cultivars selected from above field screening experiment as subplots in three replications. The four cultivars belonged to long-duration Japonica rice with similar growth duration and plant height were Wuyunjing 7, Nanguang, and 4007 classified as high NUE...