Agronomic And Physiological Traits Of Rice Cultivars Differing In Responses To Nitrogen Rates

There are many problems in the rice production,and it is a common phenomenon that nitrogen use efficiency(NUE)is low and input is large.The problem of large amount of nitrogen fertilizer is more prominent in the high-yield rice producing areas of Jiangsu Province.Therefore,breeding and selection of N-sensitive cultivars or N-efhcient cultivars(grain yield and NUE are higher at a low N rate)is an important way to reduce nitrogen fertilizer application and improve NUE.In this study,12 Japonica rice cultivars(including an indica/japonica cultivar)suitable for planting in Jiangsu Province were grown in the field,setting three N rates treatments,respectively ON(N omission),200N(200 kg/hm2)and 360N(360 kg/hm2).The agronomic and physiological traits of rice cultivars differing in responses to nitrogen rates and their relationships with grain yield and NUE of N-sensitive rice cultivars were investigated.The main results are as follows:1.Differences in gain yield and NUE among the rice cultivarsAmong the 12 tested cultivars,i.e.,Yongyou 2640(an indica/japonica cultivar),Yangjing 4038,Nanjing 9108,Wuyunjing 24,Ningjing 5,Huaidao 13,Lianjing 7,Ningjing 1,Lianjing 9,Wuyunjing 30,Lianjing 11,and Huaidao 5,Under the three N rates,the yield of the indica/japonica hybrid Yongyou 2640 was the highest,the large total number of spikelets made the contribution to yield especially outstanding.The response of grain yield and NUE to N rates varied with cultivars for other 11 cultivars.At the lower N rates(0N,180N and 200N),the cultivars of Wuyunjing 30,Huaidao 5,Lianjing 7,Huaidao 13,and Wuyunjing 24 showed a higher grain yield and NUE,whereas the cultivars of Yangjing 4038 and Ningjing 1 produced a lower grain yield and NUE.At a higher N rate(360N),a higher grain yield and NUE were observed for Yangjing 4038 and Ningjing 1,but showed no significant difference with the cultivars of Huaidao 5,Wuyunjing 30,Huaidao 13,Lianjing 7 and Wuyunjing 24.Taking grain yield?9 t hm-2?partial factor productivity(PFP?grain yield/N rate)?46 kg kg-1 and internal NUE(grain yield/total N absorption in plants)?56 kg kg-1,at 200N as thresholds,Huaidao 5,Wuyunjing 30,Huaidao 13,Lianjing 7,Yongyou 2640 and Wuyunjing 24 were considered as N-sensitive cultivars,and the two cultivars of Yangjing 4038 and Ningjing 1 showing lower grain yield and NUE at 200N but showing higher grain yield and NUE at 360N were defined as N-insensitive cultivars.The yield advantage of N-sensitive cultivars at 200N was mainly due to a higher seed-setting rate and the higher total number of spikelets.The higher NUE of N-sensitive cultivars was attributed to internal NUE and PFP.2.Agronomic characteristics of N-sensitive cultivarsAnalysis of the agronomic characteristics was made using the four N-sensitive cultivars of Wuyunjing 30,Huaidao 13,Lianjing 7 and Huaidao 5nd two N-insensitive cultivars of Yangjing 4038 and Ningjing 1 at 200N.Compared with the N-insensitive cultivars,the N-sensitive cultivars showed more productive tillers,hiher harvest index and dry matter accumulation,greater efficient and high efficient leaf area index(LAI)at heading time,higher spikelet-leaf ratio,greater root length and root density,more number of lateral roots,indicating that the N-sensitive cultivars have higher sink-to-source ratio,biomass productivity,a conversion efficiency of dry mater production to grain yield and nutrient absorption capacity,and better canopy structure.These traits could be used as agronomic indicators for screening N-sensitive cultivars.3.Physiological characteristics of N-sensitive cultivarsIn comparison with the N-insensitive cultivars,the N-sensitive cultivars had higher activities of antioxidant enzymeS and H+-ATPase iI roUts,and higher IIntents of soluble sugar and amino acids,implying that the N-sensitive cultivars possess stronger resistance to stress and a higher activity of nutrient uptake.The N-sensitive cultivars had higher crop growth rate and net assimilation rate during the grain filling period,higher activities of nitrogen metabolism enzymes and contents of cytokinins in the canopy leaves,higher sugar-flower ratio at the heading time and more remobilization of nonstructural carbohydrate(NSC)from stems and sheaths to grains during grain filling,and N harvest index and higher harvest index at maturity.This is an important physiological reason for the higher yield and higher NUE for N-sensitive cultivars.4.Radiation utilization efficiency and light-nitrogen distribution characters of N-sensitive cultivarsCompared with the N-insensitive cultivars,the N-sensitive cultivars had higher radiation utilization efficiency,canopy light interception and amount of canopy radiation interception from panicle initiation to maturity.The N-sensitive cultivars also exhibited a higher nitrogen rduction coefficient(KN),higher light-nitrogen matching degree(KN/KL),leaf photosynthetic NUE and specific leaf nitrogen content.The results suggest that the N-sensitive cultivars have better radiation utilization and stronger activity of canopy photosynthetic productionIn summary,Huaidao 5,Wuyunjing 30,Huaidao 13,Lianjing 7,Yongyou 2640 and Wuyunjing 24 were sensitive to nitrogen rates,Yangjing 4038 and Ningjing 1 were insensitive to nitrogen rates.Higher root density,root length,more number of lateral roots,higher root oxidation activity,higher content of root amino acids and soluble sugar,and higher activities of antioxidant enzymes and H+-ATPase in roots were the root morphological and physiological traits of N-sensitive cultivars.Greater crop growth rate and net assimilation rate during grain filling,higher contents of cytokinins and activities of nitrogen metabolism enzymes,and higher sugar-flower ratio at the heading time,more remobilization of NSC from the stems and sheaths from heading to maturity were important physiological characteristics of the plant shoots.Higher light-nitrogen matching degree,nitrogen reduction coefficient,specific leaf nitrogen content,photosynthetic NUE,higher canopy light interception and canopy radiation utilization efficiency were the traits of light and nitrogen utilization of the canopy.All above traits were positively and very significantly correlated with grain yield,internal NUE,and PFP.Therefore,the above traits could be used for screening and breeding N-sensitive rice cultivars,and they also be used as diagnostic indexes and regulation targets for cultivating a population with high-yielding and high NUE in rice.