In order to study the heat tolerance threshold and physiological and biochemical changes of leaves of different apple varieties,the new shoots of Orin,Fuji and Gala were used as the test materials under the artificial simulated high temperature environment.The time of symptom onset,heat injury rate and heat injury index were investigated,and changes in relevant physiological and biochemical indicators were measured.The main results showed that the heat tolerance threshold of the leaves of Orin,Fuji and Gala were 41,43 and 44℃ ,respectively. In the field,Orin leaves had the highest damage rate and damage index of 53.50% and 18.05,and Gala leaves had the lowest damage rate and damage index of 5.36% and 1.57,respectively.With the prolongation of high temperature,the chlorophyll contents of the three apple varieties showed a gradual decrease trend,the soluble protein content and the activities of superoxide dismutase and peroxidase showed a law of increasing first and then decreasing,and reached the peak value at 6 hours after high temperature stress.In summary,with the prolongation of high temperature stress, the photosynthetic pigments of apple leaves are degraded, the membrane peroxidation is aggravated,the antioxidant enzymes activity of apple leaves increase,and the physiological metabolism of
apple leaves changes significantly.