Response Of Physiological Growth Of Winter Wheat To High Temperature And Drought Stress During Grain-filling Stage

In winter wheat grain-filling period,high temperature and dry weather often occur,which bring great security risks to grain yield.It is of great significance to study the response of physiological and biochemical characteristics,yield composition and water use efficiency of winter wheat to high temperature and drought during grain-filling stage,and to explore the potential of winter wheat to cope with high temperature and drought disasters and guide disaster reduction production.In previous studies,the time scale of day/night constant temperature is usually used to construct high temperature stress scenarios,which is inconsistent with the characteristics of daily temperature variation in the actual natural environment.In this study,the response mechanism of leaf photochemical changes,photosynthetic characteristics,leaf antioxidant capacity,yield characteristics and water use efficiency to high temperature and drought stress in grain filling stage was analyzed by setting different gradient levels of high temperature and drought stress.The main conclusions are as follows:（1）The content of superoxide anion radical in flag leaf of winter wheat increased continuously with the increase of temperature and stress time.Drought stress had no significant effect on MDA content.Short-term high temperature and drought stress had little effect on the change of reactive oxygen species（ROS）in winter wheat,and sustained high temperature and drought stress destroyed the dynamic balance of ROS in wheat plants.Superoxide dismutase（SOD）activity in flag leaves gradually increased to mitigate the adverse effects of ROS,and SOD activity showed a decreasing trend on the 10th day of stress.Under combined stress,POD activity and CAT activity of flag leaf of winter wheat remained relatively high,which was conducive to the gradual increase of removing excessive H₂O₂ in wheat plants.However,CAT activity began to decrease with the increase of stress time.（2）In the early stage of compound stress,the relative water content of winter wheat flag leaf was mainly affected by soil water condition.Under high temperature stress,the relative water content of winter wheat leaves decreased significantly,while the stomatal conductance and transpiration rate increased.Under drought stress,wheat could not maintain the balance of leaf temperature through water loss,and the function of flag leaf was harmed by stress.The superposition of high temperature and drought stress significantly affected the net photosynthetic rate.When winter wheat was subjected to drought stress,the net photosynthetic rate decreased with the increase of ambient temperature,and the increase of ambient temperature had no significant effect on the initial decrease of net photosynthetic rate.With the increase of stress time,the net photosynthetic rate decreased and the energy conversion decreased with the increase of ambient temperature.Short-term high temperature or drought stress had little effect on F_v/F_m.Prolonged high temperature stress resulted in a significant decrease in F_v/F_m.F_v’/F_m’and Fv/Fm were basically the same under high temperature and drought stress.The increase of F_v/F_m was conducive to the increase of F_v’/F_m’,and then improved the conversion capacity of photosynthesis.Under short-term high temperature stress,theΦPSⅡand q P were improved by drought exercise,but NPQ was decreased,and light energy consumption was increased.（3）Moderate drought exercise did not reduce the water use efficiency of plants under short-term high temperature stress at the filling stage.Continuous high temperature stress resulted in a serious decrease in WUE of winter wheat.Compared with single high temperature and drought stress,the reduction of WUE caused by combined stress showed a typical superposition effect.The effects of drought and high temperature on wheat yield were different.Temperature stress had significant effect on grain weight and grain number per spike,while water stress had little effect on grain number per spike.Under the combined stress of high temperature and drought,the yield of wheat was 30.7%when the temperature increased by5℃compared with normal temperature,and 61.2%when the temperature increased by 10℃.