Response Mechanism Of Different Wheat Cultivars To High Temperature, Drought And Their Combined Stress

Crops often suffer from various abiotic stresses under natural conditions.Wheat(Triticum aestivum L.)is an important food crop in the world.It often suffers from high temperature and drought stress during the flowering and filling stages,which seriously inhibits its photosynthesis,thus affecting the yield and quality of wheat.At present,most studies on the effects of high temperature and drought stress on wheat are focused on the effects of single stress.The response of plants to multiple stresses is complex.Different combinations of stress factors can produce enhanced,overlapping or antagonistic effects.Therefore,different types of stresses can not be simply combined,and it is difficult to explain the response of plants to the combination of high temperature and drought stress with the results of single stress.In this experiment,four Slovak winter wheat cultivars Torysa,Ilona,Arida and Eva were used as experimental materials(Torysa is more sensitive to high temperature and drought stress;Ilona is more resistant to high temperature and drought;Arida is more resistant to drought;Eva is more sensitive to drought stress).The response mechanism of wheat cultivars with different tolerance to high temperature,drought and the combination of high temperature and drought stress were studied in order to provide theoretical basis for breeding wheat cultivars with high temperature and drought resistance.The main results are as follows:(1)Compared with Torysa and Eva,resistant cultivars Ilona and Arida could maintain higher relative water content(RWC)of leaves under high temperature,drought and the combination of high temperature and drought stress,and accumulate a large amount of proline.This indicated that the resistant cultivars Ilona and Arida could maintain better water status through osmotic regulation under stress conditions.(2)Compared with Torysa and Eva,Ilona and Arida maintained higher activities of antioxidant enzymes(APX,CAT,POD and SOD)under drought and the combination of high temperature and drought stress,and reduced excessive accumulation of O2·-,H2O2 and MDA.However,only SOD activity increased significantly in wheat leaves under single high temperature stress.Compared with Torysa and Eva,Ilona and Arida have higher antioxidant capacity and can reduce the damage of ROS to photosynthetic mechanism.(3)Under high temperature,drought and the combination of high temperature and drought stress,the photosynthetic rate(Pn)of wheat leaves decreased significantly,especially under the combined stress.Compared with Torysa and Eva,Ilona and Arida maintained higher stomatal conductance(Gs),calvin cycle key photosynthetic enzymes(s FBPase,GAPDH,Aldolase and Rubisco)activities,chloroplast Ca2+-ATPase and Mg2+-ATPase activities under high temperature,drought and the combined stress,so as to maintain higher photosynthetic rate of flag leaves.(4)Under high temperature,drought and the combination of high temperature and drought stress,Fv/Fm and PIABS of wheat leaves decreased significantly,while VJ and VI increased significantly,indicating that the electron transfer from QA to QB in linear electron transfer chain was blocked,and PS? photoinhibition occurred in wheat leaves.Compared with Torysa and Eva,Ilona and Arida could maintain higher Fv/Fm,PIABS,(37)PS?,NPQ and q P under high temperature and drought stress,alleviate the damage of excess light energy to PS? and maintain higher PS? activity.(5)Under high temperature,drought and the combination of high temperature and drought stress,(37)PS? and ETR(I)of wheat leaves decreased significantly,and PS? photoinhibition occurred in wheat leaves.Compared with Torysa and Eva,Ilona and Arida had higher Y(ND)and lower Y(NA)under stress condition,indicating that the accumulation of oxidized P700 in leaves of Ilona and Eva could alleviate the excessive reduction of photosynthetic electron transport chains in wheat leaves and decrease the photooxidation damage.(6)Compared with Torysa and Eva,the expression of some antioxidant genes(Cu-Zn SOD and APX)were up-regulated significantly in Ilona and Arida leaves under high temperature and drought stress,and the expression of some heat shock protein genes(HSP70 and HSP90)were also higher than that in leaves of Torysa and Eva under stress condition.(7)Under drought stress in the field,the net photosynthetic rate(Pn)of flage leaves of four wheat varieties decreased significantly in the late stage of drought,and the grain weight also reduced obviously.Compared with Torysa and Eva,the Pn and 1000-grain weight of Ilona and Arida decreased less in late drought period.In conclusion,there are significant differences in physiological and biochemical characteristics of different wheat cultivars under high temperature,drought and their combined stress,mainly in antioxidant,photosynthesis and gene expression.Ilona and Arida had better resistance to the combination of high temperature and drought stress,while Torysa and Eva were more sensitive to the combined stress.Compared with Torysa and Eva,Ilona and Arida could maintain higher net photosynthetic rate under high temperature and drought stress,which was related to higher stomatal conductance,key enzyme activities of calvin cycle and the expression of related genes.The photoinhibition of PS? and PS? in leaves of Ilona and Arida was significantly lower than that in Torysa and Eva.Its photoprotection mechanism was to alleviate the excessive reduction of photosynthetic electron transport chain and protect photosynthetic apparatus through the regulation of heat dissipation and linear electron transfer.Under drought stress in the field,Ilona and Arida had higher 1000-grain weight,which was related to higher net photosynthetic rate.At the molecular level,the combination of high temperature and drought stress could induce significant up-regulation of antioxidant genes and heat shock protein genes in wheat leaves to improve resistance to the combination of high temperature and drought stress.Under the same stress,the expression of antioxidant genes and heat shock protein genes in Ilona and Arida leaves was higher than that in Torysa and Eva.