Effect Of Nitrogen Application And Plant Density On Grain Filling And Carbon Nitrogen Metabolism Of Spring Maize

Northeast China is the main area of maize production.Recently,with the reduction of available land resources and the increase of population quantity,the demand for maize yield has gradually increased.Thus,increasing maize grain yield became a necessary condition for ensuring food demand.The combination of suitable nitrogen（N）fertilizers and appropriately high planting densities of high yield varieties are the keys to optimizing the grain filling process and improving the yield level.Maize grain filling not only related to single grain weight,but also involves the competition and coordination of each grain for assimilates.Improving assimilation production（source）,strengthening reproductive organs（sink）,and optimizing matter transport systems（flows）play a key role in increasing grain filling and yields.In this study,the effects of different nitrogen levels,two high grain yield maize hybrids of different genotypes（Xianyu 335 and Zhengdan 958）and different planting densities(60000 and 90000plant hm^-2)on the physiological mechanism of grain filling were investigated in field experiments from 2017 to 2021.It aims to develop rational nitrogen fertilizer and density management schemes for specific high-yielding varieties.At the same time,under the optimized nitrogen density management measures,the effects of nitrogen fertilizer on grain filling,assimilation production and distribution and carbon and nitrogen metabolism among high-yield maize varieties in Northeast China were clarified,which provided theoretical support for regulating the grain filling process and improving yield.The main results are as follows:（1）Effects of N level and plant density on spring maize yield,grain filling,dry matter accumulation and distributionThe analysis results of the relative contribution rate of grain filling rate and grain filling period to kernel weight show that the enhanced N rate mainly increases the grain filling rates of gradual increase period（T2）and slight increase period（T3）.The increase in the grain filling rate at T2 stage was greater than that at T3 stage.The ¹³C tracer mark was performed on ear leaves at the silking stage,and when the harvest was sampled.Increasing in plant density did not reduce ¹³C proportion ratio in ear at maturity under medium and high N levels(225 kg N hm^-2 and 300 kg N hm^-2)compared with D1 conditions,N proportion ratio in ear show similar result.（2）Quantitative analysis of the contribution rate of the grain weight from different kernel position to individual and whole scale population of sink capacityThe grain weight,maximum grain filling rate,average grain filling rate,active filling period,zeatin riboside（ZR）,indole-3-acetic acid（IAA）and photosynthetic products（starch content and soluble sugar content）from different kernel position were showed a consistency greatly,namely:upper<middle<lower part.The quantitative analysis results show that under the condition of conventional planting density（D1）,increased N application mainly due to the increase of grain sink capacity in the middle part to enhance individual-plant scale and whole-population scale sink capacity.High planting density（D2）conditions,the increased N application mainly increase the sink capacity in the upper part contributing to enhance sink capacity of individual-plant scale.However,as for whole-population scale,the grain sink was enhanced for all kernel positions.（3）The relationship between source-sink characteristics,carbon transport and grain filling of spring maize under high plant densityCompared to the Zhengdan 958,Xianyu 335 increased grain yield,kernel weight per plant,and sink capacity（kernel number×kernel weight）by 11.4%,15.7%,and 7.4%,respectively.Moreover,Xianyu 335 performed higher ear leaf net photosynthetic in silking 0-40 d rate and sucrose synthase activities in grains than those in Zhengdan 958,and higher levels of sucrose phosphate synthase and soluble acid invertase activity were mainly exhibited in the middle of the grain filling stage,which contributed to increasing the proportion of grain in total dry matter,grain C content and leaf C transport efficiency by 4.3%,12.2%,and 52.9%under D2 conditions,respectively.Additionally,a greater area and number of small vascular bundle in ear of Xianyu335 resulted in 21.3%higher matter transport efficiency and 4.8%higher maximum grain filling rate than Zhengdan 958 under D2 conditions.In addition,the Xianyu 335 grains exhibited generally higher levels of indole acetic acid（IAA）,abscisic acid（ABA）,as well as ABA/GA3ratio after maize pollination relative to those from ZD958,thus regulating C translocation from leaves to grains.（4）Effects of nitrogen on physiological and metabolic of different spring maize hybrids at milking stageThe increased N rate significantly improved chlorophyll a,chlorophyll b,chlorophyll a+b and PSⅡeffective light quantum efficiency（Fv/Fm）.The increase in N application significantly increased the activity of ear leaf N metabolism enzymes activity such as nitrate reductase（NR）,activity of glutamine synthase（GS）,glutamate synthetase（GOGAT）and glutamate dehydrogenase（GDH）.Compared with Zhengdan 958,Xianyu 335 showed higher NR,GS,GOGAT and GDH enzyme activities in ear leaf at the 10 d day and 25 d day after silking of maize.In addition,the In the pre-grain filling period（0 d,10 d and 25 d after silking）total content of chlorophyll a+b in Xianyu 335 is relatively higher and the higher Fv/Fm andΦPSⅡin the later stage ensure Xianyu 335 with relatively strong ability to produce photosynthetic assimilate.（5）Response of root morphology-physiological characteristics and N use efficiency to N levelsThe results show that excess N(400 kg N ha^-1yr^-1)boosts maize root length,root surface area in silking stage,total N uptake,and total dry matter accumulation,compared with optimized N application(200 kg N ha^-1yr^-1).However,compared with optimized N,excess N limited maize root activity,nitrate reductase（NR）activity,root activity absorbing area at milking stages,and did not increase post silking N（Post N）uptake,post dry matter（Post DM）accumulation,and grain yield,even decreased N use efficiency（NUE）.Meanwhile,under optimized N conditions,Xianyu 335 had lower the root to shoot ratio and root length,whereas there is no difference in root biomass,and Xianyu 335 had higher root surface area,root activity,NR activity and root activity absorb area（RAA）than Zhengdan 958 in 0-40cm soil layer.In addition,Xianyu 335 had higher vegetative N remobilization and higher Post silking N accumulation,facilitating a greater NUE,further leading to higher grain yield compared to Zhengdan 958（8.5%）.In conclusion,under high density conditions,excess nitrogen did not improve root physiology,filling rate,accumulation of photosynthetic products and grain yield of the two cultivars.Therefore,under high-density conditions,the selection of high-yielding variety Xianyu 335 combined with optimized nitrogen application(200 kg N hm^-2)improved the physiological activity of roots,increased the accumulation of photosynthetic products and Post silking N accumulatio,and optimized the vascular bundle structure of the panicle system,so that the grain filling rate was significantly higher than that of Zhengdan 958,and a relatively high grain yield was obtained.