| As one of the most important cereal crops in the world,maize has been seriously threatened by heat stress in recent years,and it is expected that the scale,duration and frequency of high temperature will increase in the future.High temperature affects plant growth and development at the whole plant level,tissue and cellular level and even subcellular level.In order to cope with the increasingly serious temperature change,there is an urgent need to develop maize varieties adapted to climate change,and through the relevant physiological,biochemical and molecular mechanisms to achieve this goal.In this paper,Suyunuo5 and Yunuo7 were used as materials to systematically study the effects of different types and degrees of high temperature on grain yield,matter accumulation and transport,grain filling,soluble sugar,starch and protein accumulation and related enzyme activity,endogenous hormone content and starch quality of waxy maize.At the same time,the field different sowing dates experiment was carried out for two consecutive years combined with pot experiment to provide field verification results for this study,so as to distinguish the difference between pot experiment results and field planting results.The main results are as follows:1 Physiological mechanism of high temperature stress types on grain yield and quality of waxy maize during grain filling stageHigh temperature stress during grain filling stage significantly decreased grain yield,among different types of HS,daylong high temperature(DNH)had the greatest effect,followed by daytime high temperature(DH),and night high temperature(NH)had the least effect.HS significantly affected grain filling and decreased grain dry weight,fresh weight,fresh grain volume and grain moisture content.Proteome and transcriptome analysis showed that genes and proteins related to grain development and starch accumulation were upregulated and down-regulated in 10DAP under HS,respectively,but on the contrary in 25DAP.Metabolic group analysis under high temperature stress showed that the metabolite accumulation patterns of 1ODAP and 25DAP were negatively correlated.Multi-group analysis showed that the signal pathways mediated by auxin(IAA),abscisic acid(ABA)and salicylic acid(SA)were more active in response to HS in response to 10DAP than in 25DAP,which confirmed that HS in the early stage of grain development had a follow-up effect on late development.HS significantly decreased the activities of sucrose phosphate synthase(SPS),sucrose synthase(SS),ADPG pyrophosphorylase(AGP),soluble starch synthase(SSS),starch branching enzyme(SBE),starch debranching enzyme(DBE),glutamate synthase(GOGAT),glutamine synthase(GS),and increased nitrate reductase(NR),resulting in a significant decrease in starch content and increase in protein content,and the degree of effect was DNH>DH>NH.HS leads to the increase of pores on the surface of starch grains,increases the average size and relative crystallinity of starch grains,and affects DNH and DH more than NH.HS significantly decreased the peak viscosity,trough viscosity,final and breakdown viscosity of starch,but had little effect on starch setback viscosity and gelatinization temperature.The retrogradation value of Suyunuo5 increased significantly under DH,and the HS of Yunuo7 increased significantly.On the whole,different HS types had effects on grain development and quality of waxy maize,and DNH had the greatest effect.2 Physiological mechanism of high temperature stress degree on grain yield and quality of waxy maize during grain filling stageCompared with the control(T0:28?/20?),high temperature stress at different temperatures during grain filling stage(T1:32?/24?,T2:36?/28?,T3:40?/32?)had significant effects on grain yield.The yield and 1000-grain weight of Suyunuo5 and Yunuo7 decreased significantly under T2 and T3.High temperature stress significantly affected grain filling process(grain fresh weight,fresh grain volume and grain moisture content),and T3 treatment had the greatest effect.The grain soluble sugar content and starch content of the two varieties decreased significantly under T2 and T3,and T3 had the greatest effect.The total protein content increased significantly under different high temperature treatments.The increase of T3 protein content of Suyunuo5 was higher than that of T1 and T2,while that of Yunuo7 was the highest under T2.Among the protein components,the albumin content decreased significantly under stress,and T1 had the greatest effect,and each treatment significantly increased the gliadin content.High temperature stress significantly decreased the activities of starch synthase(ADPG pyrophosphorylase,soluble starch synthase,starch branching enzyme,starch debranching enzyme),soluble sugar synthase(sucrose phosphate synthase,sucrose synthase)and increased protein synthase(nitrate reductase),and the impact was the largest in T3 treatment.T2 and T3 increased the average starch particle size and iodine binding capacity of the two varieties,and the increase was larger under T3.The viscosity characteristics of two varieties decreased significantly under high temperature treatment,the starch regeneration values of two varieties increased significantly under T2 and T3,and the starch enthalpy of two varieties decreased significantly under T3.On the whole,different high temperature stress had significant effects on grain filling process and quality of waxy maize,among which T2 and T3 had greater effects.3 Physiological mechanism of high temperature stress durations on grain yield and quality of waxy maize during grain filling stageCompared with the control(28/20?),the grain yield and grain weight of waxy maize decreased significantly at 2-6 days(HS5)and 2-11 days(HS10)after pollination at different high temperature stress(35?,27?)at grain filling stage,and the decrease increased gradually with the prolongation of stress time.High temperature stress significantly decreased fresh grain weight,fresh grain volume and dry grain volume,and the effect intensified with the extension of stress duration.In general,high temperature stress had little effect on soluble sugar content in late grain stage,but significantly decreased grain starch content and increased grain protein content,and the effect of HS10 was greater than that of HS5.Different high temperature stress time significantly decreased the activities of enzymes related to the synthesis of starch(AGP,SSS,SBE,DBE),soluble sugar(SPS,SS)in the late stage of grain development,but there was no significant difference among treatments.The activity of NR,an enzyme related to nitrogen metabolism,increased significantly,and decreased significantly under HS10.Under different high temperature stress durations,the number of irregular starch grains increased,the surface pores of starch grains increased,the average size of starch grains increased,and the crystal order increased,of which HS10 had the greatest effect.The peak and setback viscosity of Suyunuo5 decreased significantly under HS10;the breakdown viscosity increased by HS5 and decreased by HS10;the gelatinization temperature changed opposite to the breakdown viscosity;the enthalpy decreased significantly under high temperature stress;the peak temperature and conclusion temperature increased under HS5;the onset temperature increased under HS10 treatment.The starch viscosity of Yunuo7 decreased significantly under two high temperature stress durations;gelatinization temperature was less affected by high temperature stress,enthalpy was less affected by high temperature stress,gelation temperature(onset temperature,peak temperature,conclusion temperature),retrogradation enthalpy and retrogradation value increased under two high temperature stress.and the increase was larger under HS10.In general,different high temperature stress durations had significant effects on waxy maize grain endosperm development,and the effect of HS10 was greater than HS5.4 Physiological mechanism of different sowing dates on grain yield and quality of waxy maize during grain filling stageThere were significant differences in grain weight,grain number per ear and yield of waxy maize under different sowing date treatments[early sowing(ST1):sowing on June 11,suitable sowing(ST2):sowing on July 1 and late sowing(ST3):sowing on July 21].The ST2 of waxy maize yield,grain weight and grains per panicle were significantly higher than other treatments,and the lowest under ST3 treatment.With the advance of grain filling process,the overall ST2 of grain dry weight and fresh weight was the highest,followed by ST1,and grain moisture content(ST3)was the highest.The soluble sugar content of the two varieties was the highest in the later stage,and the starch content in different sowing dates was ST1>ST2>ST3,and the protein content ST1 was the highest.High temperature also affected the quality of starch grains.ST1 treatment significantly increased the proportion of irregular starch grains and the surface pores of starch grains,and there was no significant difference between ST2 and ST3.The average size of starch grains under ST1 was significantly higher than that of the other two sowing date treatments.The iodine binding capacity of the two varieties was the highest under ST1,and the ratio of short chain to long chain was the lowest under ST1 of Suyunuo5.There was no significant difference among the three sowing dates of Yunuo7.During sowing period,the peak,trough,breakdown,final and setback viscosity of waxy maize starch of the two varieties were the highest under ST2,followed by the lowest under ST3,ST1 treatment.The enthalpy of the gelation temperature(peak,start temperature and conclusion temperature)of waxy maize starch were the highest under ST1 and the lowest under ST3 treatment.On the whole,different sowing dates had significant effects on grain filling process and quality formation of waxy maize,among which ST3 treatment had a greater 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