Study On The Response Of Source-Sink Characteristics To Nitrogen In Different Maize Cultivars With Low Nitrogen Tolerance Of Post-Silking

Reducing nitrogen（N）and increasing N efficiency are the necessary requirements of maize production development in China.Planting maize cultivars with low-N tolerance is an effective method to achieve high yield and high N efficiency.Coordinated source-sink relationship at late growth stage,including slow senescence of leaves and large storage capacity of grains,were the key characteristics to achieve high yield of maize.To enrich maize N nutrition theory and improve N efficiency,it is great significance to clarify the physiological mechanism in coordinating the relationship between leaf senescence and translocation of assimilates to grain of low-N tolerance maize cultivars.In this study,the low-N tolerant cultivar‘Zhenghong 311’（ZH311）and low-N sensitive cultivar‘Xianyu 508’（XY508）screened out in the previous period were used as materials,and four N levels,including 0 N-0 kg·hm^-2,150 N-150 kg·hm^-2,300 N-300 kg·hm^-2 and 450 N-450 kg·hm^-2were set,a 3-year（from 2017 to 2019）field experiment was conducted to study the effects of N level on leaf carbon and N metabolism and leaf senescence,grain carbon and N metabolism and grain filling,and plant metabolites accumulation and translocation characteristics at later growth stage of cultivars with different low-N tolerance.The study analyzed the source-sink characteristics of different low-N tolerance cultivars and their differences in response to N,to reveal the source-sink characteristics,yield formation characteristics and its physiological mechanism of low-N tolerant cultivar at late growth stage.The main results were as follows:1.Compared with XY508,ZH311 showed larger leaf area and higher contents of chlorophyll and N metabolites under low and medium N conditions(≤300 kg hm^-2 N).In the middle stage of grain filling（0-30 d）,the activities of chlase（CLH）and pheophytinase（PPH）of ZH311 were lower,the chloroplast structure was better,and the accumulation of soluble sugar and starch in leaves were lower than XY508.Then,the time of leaf senescence initiation in ZH311（about 30 d after silking）was later than that of XY508（about 20 d after silking）,and leaf senescence and metabolites degradation of ZH311 were both slower than that of XY508.This difference was more significant in the lower ear leaves of the two cultivars.The differences in chlorophyll degradation rates of ear leaves,third upper ear leaves and third lower ear leaves were 0.63%,4.50%and 4.64%,respectively.The results indicated that maintain larger photosynthetic area,better photosynthetic structure and slower degradation of metabolites under N deficiency environment,and then delaying leaf senescence process is an important characteristics of maize.2.N deficiency increased the metabolites translocation rate of pre-silking and decreased the metabolites accumulation rate of post-silking in stem and leaf.Under low N condition(≤150 kg hm^-2 N),there was no significant difference between the two cultivars in pre-silking metabolites translocation rate.However,the translocation amounts of dry matter and non-structural carbohydrate（NSC）in the leaf of ZH311 were significantly lower than that of XY508（except for 2018）,they were 23.02%and 42.09%lower,respectively,which delayed the leaf senescence of ZH311 and ensured its strong metabolites accumulation ability post-silking.In the face of low N stress,ZH311 could maintain a higher assimilate accumulation capacity while ensuring the adequate translocation of metabolites accumulation in pre-silking to the grain to reduce the yield loss caused by N deficiency.3.N deficiency accelerated the process of grain dehydration,reduced the activities of carbon and N metabolism enzymes in the grain,and finally affected the accumulation of carbon and N metabolites and grain filling process.Compared with XY508,in the face of low N stress(≤150 kg hm^-2 N),the grain of ZH311 showed a higher and more lasting N metabolism enzyme and sucrose-starch syntheses activities,made its grain N metabolites（soluble protein and free amino acid）,and carbohydrate（soluble sugar and starch）accumulation were higher,eventually improved the grain of grain weight.The difference between the two cultivars was more significant in the apical grain.The difference of grain weight between apical and basal-middle grain of two cultivars were 12.80%and 3.48%,respectively.The filling characteristics,carbon and N metabolism enzyme activity and metabolites content of ZH311 between the apical and the basal-middle grain had little difference.Meanwhile,higher auxin（IAA）,zeatin and zeatin riboside（Z+ZR）contents,lower gibberellin（GA₃）content and closer hormone coordination effect under low N stress were also important physiological mechanisms to ensure grain filling and starch accumulation in ZH311.The source-sink characteristics of high yield and high N efficiency,and the physiological mechanism of low-N tolerance in late growth stage of maize include:in terms of“source”,maintaining a longer functional period and a higher metabolic capacity of leaves to accumulate more assimilates and delay the senescence process of leaves is an important basis for maize to obtain higher grain yield.The contribution of lower ear leaves to grain yield could not be ignored.For“sink”,a higher and more permanent level of carbon and N metabolism is beneficial to grain filling,and appropriate hormone levels and close hormonal coordination are also important physiological mechanisms to ensure grain filling.Minimizing the difference of grain filling and metabolism level between apical and basal-middle grains is also the key to increase grain yield.In the metabolites translocation from“source”to“sink”,slower leaf senescence resulted in a higher metabolites accumulation of post-silking.Higher assimilate accumulation capacity of post-silking is an important factor for high yield and high efficiency of maize while sufficient metabolites translocation of pre-silking to grain.