Interactive Effects Of Heat Stress And Drought Stress On Photosynthetic Characteristrics Of Wheat

Wheat is a widely cultivated food crop in the world.High and stable wheat production is important for food security in China and globe.Since the industrial revolution,increasing variability of temperature and precipitation under global climate change warming result in more frequent extreme high temperature and drought events,which will threaten the wheat production globally.Temperature and water has been the main ecological factors affecting the wheat growth and development,and photosynthesis as the most important physiological process for crop yield formation in wheat,is very sensitive to high temperature and drought stress.As major environmental stress,heat stress and drought stress usually occur simultaneously and have significant interactive effects on wheat growth and yield formation.Therefore,a deep understanding of the interactive effects of heat stress and drought stress on wheat photosynthesis,will provide theoretical basis for adapting wheat photosynthetic production and yield formation to future climate warming and changing precipitation conditions,and also provide technical support for the improvement and optimization of wheat growth model under heat stress and drought stress conditions in the future.This study using two winter wheat varieties（Yangma23 and Xumai3 3）,conduced wheat pot experiment under different heat stress and drought stress treatments with the automatic artificial climate chamber to simulate the natural temperature daily variation,and the continuous weighing system to control real-time soil moisture content.The treatment stages included flowering and grain filling.There were four high temperature levels:T1（17/27℃）,T2（21/31℃）and T3（25/35℃）,T4（29/39℃）,and three drought stress levels:W1（75±5%）,W2（50±5%）and W3（25±5%）.The heat and drought lasted for 5 days.The objectives were to clarify the change patterns of leaf growth indicators under different heat and drought stress treatments,and to analyze the interactive effects of heat stress and drought stress on photosynthetic indicators and fluorescence parameters of wheat.The research results are as followed:（1）During heat and drought stress treatment period,wheat canopy evapotranspiration（ET）increased with higher temperature level,and decreased with higher drought stress level.As the treatments continue,wheat canopy ET increased under no-water stress conditions,but decreased under severe drought stress treatments,and leaf water potential（LWP）and chlorophyll content（chl（a+b））decreased.More decrease in LWP and chl（a+b）were observed with higher heat stress and drought stress level.After treatments,leaf growth parameters can be recovered with re-watering under mild heat stress and drought stress conditions,but not for severe heat stress and drought stress treatments.Leaf growth parameters decreased during the period after treatments,and more decrease of leaf growth parameters were observed with increasing heat stress and drought stress levels.The effects of heat stress and drought stress were more severe for treatments at grain filling than at anthesis,and leaves with lower positions were affected more than upper leaves by heat stress and drought stress.The ANOVA analysis indicated that there were significant differences in leaf growth parameters between different heat stress and drought stress treatments,and the interactive effects of heat stress and drought stress on leaf growth parameters after treatments were also significant.Generally,drought stress have more profound effects on LWP than heat stress,but the dominant stress factor for chl（a+b）,SLA,LDW,and LAI have changed from drought stress to heat stress with the proceed of grain filling.（2）As stress treatment continued,leaf net photosynthetic rate（Pn）,initial light utilization efficiency,light saturation point and light decreased due to stress,while leaf stomatal conductance（Gs）,transpiration rate（Tr）,intercellular CO2 concentration（Ci）increased first then decreased during the heat and drought stress treatment period.Treatments with mild heat stress and water stress have no effects on leaf fluorescence parameters,but the maximum quantum yield（Fv/Fm）,actual amount of quantum yield（ΦPS Ⅱ）,photochemical quenching coefficient（qP）decreased,and the coefficient of photochemical quenching（qN）increased under severe heat stress and water stress treatments.The effects of stress on leaf photosynthetic indices and fluorescence parameters increased with the increase in temperature level and water stress level.Heat stress and water stress affected leaf photo synthetic indices earlier than fluorescence parameters.Larger effects on leaf photo synthetic indices and fluorescence parameters were observed for stress treatments at grain filling stage than at flowering stage,and more effects on leaves with lower positions were observed than on upper leaves by heat stress and drought stress.After treatments,leaf photosynthetic indices and fluorescence parameters can be recovered with re-watering under mild heat stress and drought stress conditions,but not for severe heat stress and drought stress treatments,and the recovering effects were more obvious for stress treatments at flowering stage than at grain filling stage.With the proceed of grain filling,there were significant differences between different heat stress and water stress treatments,the canopy photosynthesis rate decreased with increasing heat stress and water stress levels,and the main limited factors for Pn have changed from stomatal factors to non-stomatal factors.The ANOVA analysis indicated that there were significant differences between different heat stress and drought stress treatments in leaf photosynthetic indices and fluorescence parameters after treatments,and the interactive effects of heat stress and drought stress were also significant.The results of correlation analysis between leaf net photosynthetic rate and leaf growth parameters and fluorescence parameters showed a better correlation between Pn and chl（a+b）than other parameters,indicating the decrease in chl（a+b）could be the main reason for the decrease in Pn after heat stress and water stress.