Evaluation For Nitrogen Use Efficiency Of Different High-yielding Rice Genotypes

In order to gain higher grain yield, a vast amount of nitrogen fertilizer is applied in rice production. But this excessive nitrogen fertilizer application has resulted in a series of environmental troubles, such as groundwater pollution, eutrophication, acid rain and so on, and has thus increased the burden of the farmers. Therefore, improving nitrogen use efficiency and reducing application of nitrogen fertilizer has become the main concern for current researchers. It is generally considered that the best approach for improving nitrogen use efficiency is to screen and cultivate nitrogen efficient rice varieties. In this study,18high-yielding rice varieties or breeding lines were used to evaluate their nitrogen use efficiency and screen genotypes with high NUE under field conditions. The dry matter production, distribution and translocation of these rice genotypes with different NUE were evaluated. The relationships between carbon and nitrogen metabolism and between nitrogen accumulation and translocation were also investigated for revealing physiological mechanism contributing to the higher NUE of different genotypes. The main findings of this research are as follows:1) The results showed that there were significant genotypic variations among different rice genotypes in terms of grain yield and its components. Increase in application of nitrogen fertilizer increased productive panicles per unit area, but conversely reduced the spikelets per panicle. Although, grain yield among the three N rates was not significantly different, the NUE decreased with an increase in N rate during2011. This result indicated that reducing nitrogen rate improved nitrogen utilization efficiency, while the grain yield was not restricted.Nitrogen use efficiency of various rice genotypes exhibited significant genotypic differences. Nitrogen use efficiency for grain production (NUEg) showed significantly positive correlation with grain yield, spikeletes per panicle, grain filling and grain weight. However, its relationship with productive panicles per unit area was significantly negative. Nitrogen use efficiency for biomass production (NUEb) was also negatively correlated with the productive panicles per unit area, but was significantly positive correlation with spikelets per panicle. Using NUEg, NUEb and grain yield as indicators, the tested rice genotypes were classified into two groups by cluster analysis, i.e. high NUE type (such as YLY-6, LY-8and LYPJ, etc) and low NUE type (such as YD-6, YZ-889and HY-3, etc).2) The study showed that at PI stage, the dry weights of stems, leaves and whole plant did not vary among genotypes with different NUE. But dry matters of stems, leaves and whole plant in high NUE type genotypes accumulated rapidly from PI to HD. especially that of stems. At HD stage, dry weights of stems, leaves and whole plant in high NUE type genotypes were higher than those in low NUE type. Differences of dry weight among two NUE types were not significant after HD. At MS, dry weights of stems, leaves and straws in high NUE type genotypes were less than those in low NUE type. However, grain dry weight, biomass and harvest index in high NUE type genotypes were significantly higher compared to low NUE type. The results suggested that translocation of dry matter and its contribution to grain in high NUE type genotypes were higher than those of low NUE type. Dry matter translocation was positively correlated with grain yield, while its efficiency was also positively correlated with NUEg.3) At PI, HD and MS stages, nitrogen accumulation of various organs in rice showed significant genotypic differences. Nitrogen accumulation increased with an increase in nitrogen application. At PI stage, nitrogen accumulations in stems, leaves and whole plant among genotypes with different NUE were not different. Whole plant nitrogen accumulations from PI to HD of high NUE type genotypes was higher by16.8%and14.3%than those of low NUE type under medium and low N levels, respectively, however slightly lower compared to low NUE type genotypes under high N level. Nitrogen accumulations of stems, leaves and straws in high NUE type genptypes at MS were lower than those in low NUE type. But conversely, compared with low NUE genotypes, grain and whole plant nitrogen accumulations of high NUE type genotypes were higher. Nitrogen translocation and translocation efficiency were positively correlated with grain yield, but showed significantly negative relationship with NUEg and NUEb. The results suggested that increase in nitrogen translocation may result in higher grain yield and NUE.4) At heading stage, GS activity and Rubisco content of flag leaves of high NUE type genotypes were significantly lower than those of low NUE type under the medium and high N levels, but was not significantly different under low N level. Moreover, the GS activity of the flag leaves at heading stage was positively related with biomass.