Performance In Grain Yield And Nitrogen Use Efficiency Of Super High-yielding Rice Varieties Under Reduced Nitrogen Fertilizer Input Condition

Rice is one of the most important cereal crops in China,and rice production plays an important role in ensuring national food security.Rice production in China has kept increasing since 1949,which is mainly due to the increase of rice yield rather than harvested area.Genetic improvement is one of the most important reason for the improvement of rice yield.Both super hybrid rice combining heterosis with an ideotype and indica/japonical hybrid rice using intersubspecific heterosis have high yield potential in rice production system.However,yield and N use efficiency（NUE）of theae varieties under reduced N input condition is still obscure.Therefore,we conducted field experiments to compare grain yield and NUE of indica super hybrid（Yangliangyou6 and Yliangyou1）,indica hybrid（Luoyou10 and Chuanyou6203）and indica inbred rice（Huanghuazhan and LvdaoQ7）in 2014 and 2015 at Wuxue county,Hubei province,China.The two N treatments including 0（N0）and 90（N90）kg N ha^-1 were set as zero and reduced input of N fertilizer treatments,respectively,and 180 kg N ha^-1 from local farmer’s practice was set as high N control.We also compared the performance in grain yield and NUE of Yongyou4949（indica/japonical hybrid variety）,Yangliangyou6（indica super hybrid variety）and Huanghuazhan（indica inbred variety）under reduced N input condition(100 kg N ha^-1).The purpose of this study is to understand grain yield and NUE of these super high-yielding rice varieties and explore the agronomic and physiological mechanisms in the formation of yield and NUE under reduced N input condition.The results of this study are as follows:（1）The grain yield of indica super hybrid rice was significantly higher than that of the indica inbred rice by 5.4%at N0,8.4%at N90,and 6.6%at N180（the averaged of2014-2015）,respectively.The grain yield was 3.7%and 8.8%higher in indica super hybrid rice than that in indica hybrid rice at N90 and N180,respectively.Relatively higher grain filling percentage and grain weight contributed to that indica super hybrid rice outyielded the indica hybrid and inbred rice.In terms of total dry weight and harvest index,the yield advantage of the indica super hybrid rice over the indica hybrid and inbred rice was mainly due to higher harvest index at N0,and both high total dry weight and harvest index（HI）at N90,and relatively high total dry weight at N180.（2）No significant difference was observed in total N uptake among the three types of rice at N0 and N90.However,indica super hybrid rice had higher total N uptake at N180.The indica super hybrid rice had higher NUE for grain production,N harvest index,agronomic N use efficiency etc.,at three N treatments.Relatively higher HI,lower leaf area index at heading stage and lower leaf N concentration at maturity stage contributed to the higher NUE of super hybrid rice.（3）Indica/japonical hybrid rice（Yongyou4949）produced significantly higher grain yield than indica super hybrid rice（Yangliangyou6）by 10.6%and indica inbred rice（Huanghuazhan）by 19.5%（the averaged of 2015-2017）.Significantly higher spikelets per panicle was the main reason for the formation of high grain yield,and higher efficiency of spikelets production in respect of dry matter,N and accumulated temperature was the key to the formation of large panicle in Yongyou4949.In terms of biomass accumulation and allocation,high yield of Yongyou4949 was mainly due to significantly higher spikelets per unit leaf area,HI and post-floweing dry matter accumulation.Longer grain filling period,slower leaf senescence,relatively higher leaf area duration and radiation use efficiency after heading,optimized canopy structure（lower extinction coefficient for light K_L）,and more synergetic distribution of light and N within the canopy（higher extinction coefficient for nitrogen K_N and the ratio K_N/K_L）at heading stage contributed to higher post-floweing dry matter accumulation in Yongyou4949.（4）In terms of N accumulation and NUE,although the total N accumulation in Yongyou4949 was not significantly higher than that in Yangliangyou6 and Huanghuazhan,Yongyou4949 had obvious advantage in NUE,and its NUE for grain production,N harvest index and partial factor productivity of applied N were 8.2%,6.2%and 10.6%higher than Yangliangyou6,and 18.4%,17.7%and 19.5%higher than Huanghuazhan,respectively.Higher NUE in Yongyou4949 was related to higher HI and the ratio K_N to K_L,lower leaf area index at heading stage and lower leaf N concentration at maturity.In conclusion,both indica super hybrid and indica/japonical hybrid rice produced high yield and NUE under reduced N input condition.The grain filling percentage and HI play an important role in yield formation and high NUE of indica super hybrid rice at zero and reduced N input treatmens.Larger sink size,higher post-heading dry matter accumulation,better source-sink coordination（higher spikelets per unit leaf area and harvest index）,optimized canopy structure,more synergetic distribution of light and N within the canopy contributed to high grain yield and NUE in indica/japonical hybrid rice（Yongyou4949）.Our results clarified the performance and mechanisms of grain yield formation and high NUE in super high-yielding rice varieties under reduced N input condition.Our results provided theoretical basis for high-yielding and high-efficiency rice breeding,and the guidance of optimal N management for farmers.