Characteristics And Mechanism Of The Japonica Rice Varieties Differing In Responses To Nitrogen Rates

Over input of nitrogen fertilizer accompanied by low nitrogen use efficiency(NUE)is a serious problem in the rice production of China.Excessive nitrogen fertilizer application and low NUE could not only increase the production cost,but also lead to an environment pollution and reduce the rice quality.Therefore,breeding and selection of N-sensitive varieties or N-efficient varieties(grain yield and NUE are higher at a lower N rate)is an important way to reduce nitrogen fertilizer application and improve NUE.However,the agronomic and physiological traits associated with N-sensitive varieties,as well as the regulation technology of synergistic improvement of grain yield and NUE in N-sensitive varieties and its mechanism remain unclear.In this study,the population traits,aboveground and root morpho-physiological traits of Japonica rice varieties differing in responses to nitrogen rates and their relationships with grain yield and NUE,and the effects of chemical applications on the grain yield of N-sensitive varieties and its mechanism were investigated.The main results are as follows.1.Grain yield,NUE and population traits of the Japonica rice varietiesTwelve tested Japonica inbred rice varieties including an Indica/Japonica variety with Japonica phenotype were grown in a paddy field under the N rates of 0 kg N hm-2(0 N),180 kg N hm-2(180N)or 200 kg N hm-2(200N)and 360 kg N hm-2(360N).The response characteristics of the tested Japonica inbred rice varieties to nitrogen rates and the relationship between the grain yield,NUE and population traits were investigated.The results showed that the Indica/Japonica hybrid variety Yongyou 2640 had the highest grain yield among all the tested varieties under the three N rates,mainly due to more total number of spikelets.The performance of grain yield and NUE of other 11 varieties varied with N rates,i.e.,Wuyunjing 30,Lianjing 7,Huaidao 13,Huaidao 5 showed higher grain yield and NUE at the N rates of 0,180 or 200 kg N hm-2,whereas Yangjing 4038,Ningjing 1 exhibited a lower grain yield and NUE at the N rates of 0,180 or 200 kg N hm-2.However,a higher grain yield and NUE were observed for Yangjing 4038 and Ningjing 1 at the N rate of 360 kg N hm-2.Taking grain yield?9 t hm-2 and agronomic NUE?15 kg kg-1 at 180N or 200N as thresholds,this study evaluated the N-sensitivity of rice varieties.Yongyou 2640,Huaidao 5,Huaidao 13,Wuyunjing 30,Wuyunjing 24 and Lianjing 7 were classified as N-sensitive varieties(NSVs),and Yangjing 4038 and Ningjing 1 were defined as N-insensitive varieties(NIVs)(the two varieties showed a higher grain yield and NUE at 360N).Compared with NIVs and at a lower N rate,NSVs produced a higher grain yield and NUE as a result of more total spikelets and greater filled grain percentage.Higher spikelet-leaf ratio and more productive tillers,greater efficient and high efficient leaf area index(LAI)at heading time,longer green leaf area duration after heading,larger dry matter accumulation at maturity and higher harvest index,which accounted for higher grain yield and higher NUE for NSVs.The above-mentioned population traits were positively and significantly correlated with grain yield and NUE,which could be used as indexes for the selection of N-sensitive varieties.2.Leaf morpho-physiological traits of Japonica rice varieties differing in response to nitrogen ratesTwo N-sensitive varieties(Wuyunjing 30,Lianjing 7)and two N-insensitive varieties(Yangjing 4038,Ningjing 1)were grown in a paddy field and in pots under two N rates including 0 kg N hm-2 and 0 g N pot-1(0N),180 kg N hm-2 and 0.74 g N pot-1(180N and 0.74N),and the differences in leaf morpho-physiological traits of Japonica rice varieties differing in responses to nitrogen rates were analyzed.The results showed that NSVs had greater leaf thickness,specific leaf weight,longer leaf length,smaller leaf inclination angle of the flag leaves,higher CO2 concentration at intercellular and carboxylation sites,better fluorescence parameters,and higher contents of ribulose-1,5-bisphosphate carboxylase/oxygenase,chlorophyll,nitrogen,and soluble sugars,leading to better photosynthetic performance in comparison to NIVs.Additionally,NSVs had a better canopy structure,as reflected by a higher ratio of the extinction coefficient(KN)to the light extinction coefficient(KL),resulting in enhanced canopy photosynthesis and dry matter accumulation.The results suggest that these improved leaf morpho-physiological traits and canopy characteristics are the important eco-physiological reasons for high yield and high NUE for NSVs.3.The characteristics of dry matter production,nitrogen accumulation and translocation and their mechanism in Japonica rice varieties differing in response to nitrogen ratesTwo N-sensitive varieties(NSVs:Wuyunjing 30,Lianjing 7)and two N-insensitive varieties(NIVs:Yangjing 4038,Ningjing 1)were grown in a paddy field under two N rates including 0 kg N hm-2(0N)and 180 kg N hm-2(180N),and the differences of dry matter and nitrogen accumulation and translocation and their mechanism in Japonica rice varieties differing in responses to nitrogen rates were investigated.The results showed that NS Vs had higher pre-anthesis dry matter translocation and post-anthesis dry matter accumulation,greater sugar-spikelet ratio and activities of the enzymes involved in starch synthesis in grains,more remobilization of nonstructural carbohydrates(NSC)from stems and sheaths to grains during grain filling,higher activities of enzymes in sucrose synthesis and the expression level of genes related to sucrose transport.NS Vs also exhibited higher nitrogen uptake,greater nitrogen translocation amount after heading,higher contents of cytokinins and nitrogen,higher activities of enzymes involved in nitrogen metabolism and nitrogen uptake and assimilation,and the expression level of genes encoding these enzymes.These better physiological performances are the important mechanism for the higher uptake and translocation of nitrogen,higher dry matter production from heading to maturity,and higher NUE in NSVs.4.Root morpho-physiological traits and soil microbial abundance for the Japonica rice varieties differing in responses to nitrogen ratesTwo N-sensitive varieties(Wuyunjing 30,Lianjing 7)and two N-insensitive varieties(Yangjing 4038,Ningjing 1)were grown in a paddy field under two N treatments including 0 kg N hm-2(0N)and 180 kg N hm-2(180N),and the differences of root morpho-physiological traits and microbial abundance expression in Japonica rice varieties differing in responses to nitrogen rates were investigated.The results showed that NSVs exhibited better root morpho-physiological traits including greater root length,root diameter,root length density,root oxidation activity,root bleeding rate,higher activities of enzymes in root nitrogen metabolism,higher contents of cytokinins,indole acetic acid,and acetic acid,tartaric acid,and malic acid in root exudates and the gene expression level related to nitrogen uptake and assimilation.In comparison with NIVs,NSVs had higher total amount of fungi and bacteria amount and greater abundance expression level of Ascomycota,Mortierellomycota,Acidobacteria,Chioroflexi,Actinobacteria and Firmicutes,which were positively and significantly correlated with root morpho-physiological traits.The abundance expression level of rhizosphere microorganisms and root morpho-physiological traits were also positively and significantly correlated with grain yield and NUE.The results suggest that the coordination of rhizosphere microorganisms and root morpho-physiological traits is an important root-soil interaction mechanism for higher yield and higher NUE for NSVs.5.Effect of chemical application on the grain yield and NUE of N-sensitive varieties and its mechanismTwo N-sensitive varieties including Wuyunjing 30 and Lianjing 7 were grown a paddy field with micro-plot under 180 kg N hm-2.To investigate the regulation effect of chemical application on rice grain yield and nitrogen uptake and utilization,three chemical application treatments including control(CK,spraying distilled water),spraying 6-benzylaminopurine(6-BA),and spraying an inhibitor of cytokinin synthesis,lovastatin(Lov).The results showed that 6-BA treatment significantly increased the grain yield and internal NUE of NSVs when compared with the control,and the results were reversed when Lov was applied.The improved grain yield and internal NUE under the 6-BA treatment were mainly due to the increase in filled grain percentage,greater post-anthesis dry matter accumulation,higher photosynthetic rate and root oxidation activity,higher contents of cytokinins and nitrogen in leaves,enhanced activities of enzymes in nitrogen metabolism and assimilation and the expression level of gene related to nitrogen metabolism and transport,as well as the cytokinis biosynthesis(e.g.,OsAMT1.1-2.3,OsGS1-2,OsIPTs),and lower expression level of genes in nitrogen transport(e.g.,OsNRT2.3b,OsAAP6,OsAAP8)during grain filling.The results indicate that cytokinis could increase the grain yield and NUE by enhancing nitrogen metabolism and accumulation and promoting dry matter production during the grain filling period.