Effects Of Post-Anthesis Heat And Drought Stress On Growth,Physiological Characteristics And Yield Of Winter Wheat And The Regulatory Mechanism Of Nitrogen Fertilization

Climate models predict more frequent and intense heat and drought stress in terrestrial ecosystems in the future.Extreme heat and drought stress have servere impacts on wheat yield by affecting the physiological and biochemical processes.To date,the response of winter wheat to individual heat and drought stress have received extensive attention,however,little research has been reported on the recovery of physiological functions in winter wheat after stress relief and its effect on yield formation.There are few studies on the regulatory effects of nitrogen fertilizer on winter wheat under individual and combined stresses after anthesis,and whether reasonable nitrogen application can alleviate the adverse effects of drought,heat,and combined stresses on winter wheat.In view of this,a winter wheat experiment was carried out in the Key Laboratory of Agricultural Soil and Water Engineering of the Ministry of Education in the arid area of Northwest A&F University from2019 to 2021,including two temperature levels（heat stress:36°C;suitable temperature:26°C）,two soil moisture levels（drought stress:45%-55%FC;well-watered:70%-80%FC）and three nitrogen applications levels（low N:1.11 g N/pot;medium N:1.48 g N/pot;high N:2.16 g N/pot）.This paper systematically studied the effects of heat,drought,and combined stress on the growth,physiological characteristics and yield formation of winter wheat and the regulation mechanism.The main research results are as follows:（1）The physiological response of winter wheat to heat and drought stress and stress relief was revealed,and the effects of heat and drought stress and stress relief on the growth and yield of winter wheat were clarified.Heat and drought stress significantly reduced photosynthetic rate,chlorophyll content,leaf water potential,nitrogen metabolism enzyme activities and membrane stability,and the adverse effects caused by combined stress were significantly higher than those caused by individual stresses.Heat,drought,and combined stress lead to massive accumulation of O₂^-and MDA in plants.Plants increased the activities of superoxide dismutase（SOD）,peroxidase（POD）,catalase（CAT）,glutathione reductase（GR）,and ascorbate reductase（APX）to reduce oxidative damage.Under heat and drought stress conditions,the increased contents of soluble sugar（SS）and proline（Pro）in leaves enhanced cell osmoregulatory capacity by reducing osmotic potential.After 6 days of heat and drought stress,the processes of photosynthesis,antioxidant metabolism,osmoregulation,and nitrogen metabolism in wheat plants were reversible,and the recovery effect gradually diminished with the duration of stress.Re-watering after 3 days of drought（45%-55%FC）,the growth of aboveground and roots of wheat plants showed overcompensation effects,whereas the wheat plants showed incomplete recovery after 12 days of combined stress（45%-55%FC+36℃）.High levels of O₂^-and MDA during the recovery period indicated that the recovery of temperature+water could not compensate for the oxidative damage suffered by wheat.Compared with individual stresses,combined stress had more severe adverse effects on grain filling process.Under 12 days of combined stress,the average grain filling rate,the number of grains per spike and the 1000 kernel weight decreased by 20.18%,26.95%,and 26.64%,respectively,compared with the control,resulting in a yield decrease of 36.06%.（2）The responses of the growth and physiological characteristics of winter wheat to nitrogen applications under heat and drought stress were elucidated,and the regulatory effects of appropriate N application on growth,physiological,and biochemical characteristics of wheat under adversity disasters conditions were clarified.Under individual heat and drought stress,N2 or N3 application significantly increased relative LRWC and LWP,while N1 application were more conducive to maintaining better leaf water status when exposed to combined stress.By comparison,N2 or N3 application significantly increased photosynthetic rate and maximum photochemical efficiency under individual heat and drought stress,which promoted aboveground growth of wheat.Heat and drought stress for 12 days,N2 application could maintain higher SOD,POD,CAT,GR,and APX activities in leaves.Higher leaf SS,SS and Pro contents provided continuous osmotic protection for wheat plants.When plants were exposed to heat or drought stress,the activities of nitrate reductase（NR）and glutamine synthetase（GS）in grain were significantly enhanced by N2 application.Heat,drought,and combined stress enhanced the activities of antioxidant enzyme and the contents of osmoregulatory substance in roots.Under individual heat and drought stress,appropriately increasing N application maintained higher root vitality and bleeding sap.The regulation effect of N2 application was more significant than that of N3 application,meanwhile,N2 application effectively removed the excess O₂^-accumulated in roots under stress conditions and reduced electrolyte leakage level,which was mainly attributed to the higher antioxidant enzyme activities and osmoregulatory substance contents under N2 application.Under combined heat and drought stress,the activities of POD,CAT,and APX in roots under low N application increased by 20.17%,28.75%,and 51.09%,respectively,compared with high N application,and the SS and Pro contents in roots increased by 22.67%and 16.19%respectively.（3）The effects of appropriate N fertilization on the yield formation and water and N use efficiency of winter wheat were investigated,and the regulatory effect of N fertilizer on wheat production under adversity disaster conditions was clarified.Low N application was the best fertilization scheme to maintain higher NR and GS activities in grains at the later stage of combined stress.Optimal fertilization regimens for NR and GS activity.The maximum and average grain filling rate decreased with the increase of N application under combined stress conditions.Compared with N1 application,N2 or N3application effectively alleviated the loss of grain filling rate,grain weight and yield under individual heat and drought stress.The average grain filling rate,1000-kernel weight and yield of the HF treatment increased by 9.09%,9.19%,and 12.92%compared with N1application,respectively.N1 application was more conducive to maintaining higher wheat productivity under combined stress.Low N application was more conducive to maintaining higher wheat productivity under combined stress.The 1000-kernel weight and yield of N1application under combined stress increased by 11.62%and 17.06%,respectively,compared to those under N3 application.Compared with the control,the reduction of NUE_g,NUE_b,and NHI under combined stress conditions was more significant than those under heat and drought stress.N fertilizers effectively regulated the impacts of adversity disasters on water and N use efficiency of wheat plants.Under post-anthesis heat stress,WUE_gand NUE_gunder N2 application increased by 5.80%and 10.74%,respectively.WUE_gand NUE_gunder drought stress conditions increased by 6.03%and 8.31%,respectively,compared with N1application.The stress sensitivity coefficient of yield composition and water and N use efficiency under combined stress was the lowest under N1 application.WUE_gand NUE_gof N1 application under combined stress increased by 11.70%and 18.66%,respectively,compared with N3 application.N1 application is a suitable strategy to compensate for the decrease of water and nitrogen use efficiency under combined stress conditions.（4）The sensitivity of physiological and biochemical traits to heat and drought stress was evaluated,and the key stress resistance traits of wheat plants under heat and drought stress were quantitatively selected.ROC analysis showed that GR and SS could accurately diagnose the individual heat and drought stress that plants was suffering or had suffered（AUC=0.812–0.965）,and POD and Pro had superior potential to identify the heat and drought that wheat was experiencing or had experienced（AUC=0.871–0.958）.Within a certain stress range,the physiological process of wheat plants under heat and drought stress conditions was reversible.Among the key wheat resistance traits,POD,GR,SS,and Pro were closely related to photosynthetic rate during the stress and recovery periods,especially GR and Pro.The recovery of these traits contributed to improving photosynthetic performance and thereby promoting wheat growth and development.SOD,POD and Pro of leaves and roots play an important role in the defense mechanism against heat and drought stress.In addition to SOD and Pro,GR and SS also actively participate in the antioxidant defense mechanism of combined heat and drought stress.（5）The relationship between growth and physiological traits of winter wheat was clarified,and the regulation mechanism of growth and physiological traits on yield was elucidated.During the stress and recovery periods,the photosynthetic rate increased with the increase of antioxidant enzyme activities,especially POD and GR.The improved photosynthetic rate during the recovery indicated the effective relief of oxidative stress.POD,GR activities and SS content decreased synergistically during the recovery,and GR and Pro decreased linearly with the decrease of O₂^-and MDA levels.The correlations between various physiological traits was higher during the stress than during the recovery period.The effective recovery of GS,Pro,P_n,and LRWC（R²>0.80）after stress relief provided an important guarantee for higher wheat yield.Under heat and drought stress and nitrogen fertilizer,the correlations between aboveground N accumulation（0.960）,photosynthetic rate（0.931）and leaf water content（0.923）and wheat yield ranked the top three.Path analysis showed that the F_v/F_mhad the greatest direct impact on photosynthetic rate and leaf water content.NR activity of grain had the greatest direct impact on aboveground N accumulation.Photosynthetic rate mainly indirectly affected yield through aboveground biomass,the number of grain per spike,and 1000-kernel weight.Leaf water content had a higher indirect effect on yield through aboveground biomass,seed setting rate,and 1000-kernel weight.Aboveground N accumulation mainly indirectly affected yield through the number of grain per spike and 1000-kernel weight.Among the growth,physiological,and yield traits,1000-kernel weight had the largest coefficient of determination（R²=0.447）,followed by the number of grain per spike（R²=0.398）and aboveground N accumulation（R²=0.368）.