SIF Based Monitoring Study On Physiological And Ecological Characteristics Of Summer Maize Under Heat And Drought Complex Stress

The occurrence of heat and drought events is usually characterized by a large impact range and a long duration,which will cause serious impact on human production and agricultural production activities.With the intensification of extreme climatic events such as global heat and drought,how to accurately and efficiently monitor the response mechanism of crops to heatand drought has become a hot topic of current research.Based on the field experiment of summer maize under heat and drought stress,this study firstly explored the response characteristics of summer maize morphology,yield and its components under heat and drought stress.Then the response characteristics of physiological and ecological indexes of summer maize were analyzed.Finally,based on the inversion algorithm of solar-induced chlorophyll fluorescence(SIF),the variation characteristics of SIF in summer maize canopy at the daily scale and different growth stages were revealed and the mechanism of its association with physiological and ecological indexes,photosynthetic parameters and photosynthetically active radiation(PAR)were explored.The main research results are as follows:(1)In terms of different growth stages,the plant height of summer maize under all treatments showed a trend of increasing first and then decreasing.The leaf area of the upper leaf increased first and then decreased from the flowering stage to the maturity stage,while the middle and lower leaf increased gradually.The leaf area of upper,middle and lower leaves at flowering and filling stage showed the trend:middle leaf>lower leaf>upper leaf,while the leaf area at maturity stage showed the trend:middle leaf>lower leaf>upper leaf.Under the complex stress of heat and drought,plant height,leaf area,ear length,ear diameter,plant grain weight,100-grain weight and theoretical yield of summer maize were all inhibited to different degrees,especially under the complex stress of heat and severe drought.(2)On a daily scale,the net photosynthetic rate(P_n),transpiration rate(T_r)and stomatal conductance(G_s)of summer maize leaves presented the characteristics of"double peak"type.The P_n,G_s and T_r of summer maize leaves were inhibited by drought and heatstress,which ledto a downward trend to varying degrees.Among them,the effect of drought stress was greater than that of heat stress,the effect of severe drought stress was greater than that of light drought stress,and the effect of heat and drought complex stress was more serious than that of single stress on summer maize.The water use efficiency(WUE)showed an opposite change trend compared with P_n,G_s and T_r.The WUE of summer maize leaves under mild drought stress was the highest,followed by adequate water and severe drought stress.This phenomenon indicated that the moderate drought stress could increase the WUE of summer maize leaves,however the severe drought stress had a great inhibitory effect on the WUE of summer maize leaves,which could decrease the WUE.The response curve of summer maize leaves to light intensity changes showed the similar change trend.When the light intensity reached the saturation point,the net photosynthetic rate did not increase,but decreased gradually.For different growth stages,the chlorophyll content and leaf water content(LWC)of summer maize under all treatments showed a trend of gradual decrease.The decreasing rate of LWC was faster from T1(silking stage)to T2(grain-filling stage),but slower from T2 to T3(maturity stage).Both drought and heat stress inhibited chlorophyll content and LWC ofsummer maize leaves,the effects of severe drought stress were greater than those of mild drought stress,and the effects of combined heat and drought stress were more serious than those of single stress.(3)On the daily time scale,the SIF of summer maize canopy under all treatments showed a"single peak"pattern.In terms of different growth stages,the SIF of summer maize canopy showed a trend of T1(silking stage)>T2(grain filling stage)>T3(maturity stage).Both heat and drought stress could inhibit the SIF of summer maize canopy.The effect of drought stress was greater than that of heat stress,the effect of severe drought stress was greater than that of mild drought stress,and the effect of combined heat and drought stress was more serious than that of single stress.The canopy SIF can timely monitor the heat and drought stress information of summer maize,and the more serious the stress,the more severe the canopy SIF response.In addition,SIF was significantly positive correlated with LWC,chlorophyll content,P_n,G_s,T_r and PAR in summer maize canopy.