Adaptive Mechanism Of Introduced Ornamental Trees To Heat And Drought Stresses In Ejina

Ejina is located in the desert area of Northwest China.Vegetation construction is mainly limited by heat and drought stresses.In order to enrich ornamental tree species,introduction of ornamental plant resources with excellent drought and heat tolerant genes in Ejina area plays very important role in tourism industry development.Therefore,Forsythia suspensa and Syringa reticulata var.amurensis were used as experimental materials,and four temperature treatment were set up according to extreme high and average temperature in Ejina: T1(CK): 24 ℃ / 18 ℃,T2: 31 ℃ / 22 ℃,T3: 38 ℃/ 28 ℃,T4: 44 ℃ / 34 ℃.At the same time,they were stressed by drought without watering for 16 days and samples were collected at 0(D1: 60%-55%),4(D2: 45%-40%),8(D3: 30%-25%),12(D4: 15%-10%),16(D5: 5%-0%)and 18(rehydration after the stress)days.Growth index,physiological index,carbohydrate,antioxidant enzyme activity,endogenous hormone content were measured after sampling.The main results are as follows:(1)Under single stress,mild,and moderate combined stresses,the morphological growth indexes of two plants decreased slightly.Under severe heat and drought stresses,the plant height,basal diameter and leaf number of Forsythia suspensa and Syringa reticulata var.amurensis decreased significantly.The ratio of root to stem increased initially and then decreased,and no increasing trend was found after rehydration.(2)Under the single stress and the combined stresses,the leaf water content of two plants decreased with the extension of stress time,and the electrolyte leakage(EL)and MDA content increased while chlorophyll content varied with the heat and drought stress degrees.EL of Forsythia suspensa decreased and the leaf water content increased after rehydration under moderate heat and drought stresses while the moisture content of Syringa reticulata var.amurensis was still lower than CK and EL was higher than CK.Leaf water content of two plants decreased to 20% in 12 days of stress,and EL reached to 90%.(3)Under single stress,the total soluble sugar and fructose content of Forsythia suspensa increased and the total sugar and fructose of two plants increased initially and then decreased with the stress time.Under severe combined stresses,the total soluble sugar of Forsythia suspensa showed the trend of rise-down-up,the Syringa reticulata var.amurensis showed the trend of down and up,the sugar content of Forsythia suspensa showed down –up trend,and the sugar content was down – up trend.The sugar content of Syringa reticulata var.amurensis decreased and the sugar content increased.Under all treatments,fructose content of Forsythia suspensa increased with the increase of stress days,while that of Syringa reticulata var.amurensis under drought stress,and then decreased initially and then increased under compound stress.(4)Under the stresses,the activities of anti-oxidative enzymes such as superoxide dismutase(SOD),peroxidase(POD)and catalase(CAT)increased.CAT activity of Forsythia suspensa increased significantly under four temperatures treatments.POD activity of Syringa reticulata var.amurensis was higher and increased significantly under four temperatures treatments.The enzyme activities of two plants decreased slightly after rehydration.(5)The contents of gibberellin and auxin increased initially and then decreased under single stress,mild,and moderate combined stresses,but decreased under severe stress.Abscisic acid content of Forsythia suspense and Syringa reticulata var.amurensis increased under all stresses.(6)Syringa reticulata var.amurensis mainly maintained high protective enzyme activities to cope with heat and drought stresses.Forsythia suspensa mainly responded to heat and drought stresses through osmotic regulation and endogenous hormone content.Under moderate heat and drought stresses,Forsythia suspensa was more tolerant to heat and drought stresses than Syringa reticulata var.amurensis Under long-term of severe heat and drought stresses,both plants could not survive well.Therefore,Forsythia suspensa is more suitable than Syringa reticulata var.amurensis to be introduced in Ejina for ornamental tree species.