Abstract:

Nitrogen harvest index (NHI), defined as ratio of nitrogen accumulation in economic organs to whole plant at harvest, is an important index for evaluating nitrogen utilization efficiency, but less information is available on its physiological mechanisms and regulation methods in rice cultivation. Pot experiment was conducted using three rice genotypes including 4434 (low NHI), Dianrui 302 (medium NHI) and Yuchi 231 (high NHI) to elucidate nitrogen metabolism characteristics in leaves, panicle internodes and grains during grain filling and their relationships to NHI. The results showed that change patterns of grain yield, harvest index and nitrogen accumulation in grains were consistent with that of NHI among three rice genotypes, and all the parameters were the highest in Yuchi 231. Highly significant differences were observed in nitrogen translocation amount after anthesis in plants with the order of 4434 < Dianrui 302 < Yuchi 231, while rate of N translocation and contribution rate of translocated N to grain had no significant difference among three genotypes. At maturity, the contents of total nitrogen, protein nitrogen and non-protein nitrogen in leaf-stems and grains exhibited the order of 4434<Dianrui 302< Yuchi 231 with significant genotypic differences for the parameters but non-protein nitrogen. Protein nitrogen accumulation in leaf-stems of Yuchi 231 was significantly lower than those of 4434 and Dianrui 302, while that was significantly higher in grains, which suggested that higher nitrogen accumulation in grains was a key characteristics of the rice genotype with high NHI. Moreover, the activities of glutamine synthetase (GS) and glutamate synthase (GOGAT) in leaves of Yuchi 231 were highly significantly higher than those of 4434 and Dianrui302 at filling, which would be in favor of the formation of free amino acids in leaves and its translocation to grain, and resulted in higher bleeding intensity and free amino acid content in panicle internodes and providing the substrate for nitrogen accumulation in grains. Meantime, activities of GS and GOGAT in grains were higher in Yuchi 231 than in 4434 and Dianrui 302 which accelerated more proteins synthesis in grains. Stepwise regression indicated that free amino acid content in bleedings of panicle internode was a main physiological index involving in nitrogen assimilation related to NHI. The above results suggest that higher post-anthesis nitrogen translocation amount, protein nitrogen accumulation in grains and free amino acid content in bleedings of panicle internode are more beneficial indicators for enhancing rice NHI in nitrogen management and genetic improvement.

Key words: Rice, Genotype, Nitrogen harvest index (NHI), Nitrogen metabolism