Abstract: Using‘Zhangqiu Dacong’scallion as control，changes of malondialdehyde（MDA）content，electrolyte leakage，chlorophyll fluorescence parameters and antioxidant enzyme activities were studied in leaves of heat-tolerant scallion，‘Chunwei’，‘Jiye Wanchou’And‘Changyue’，at 25–50 ℃. The results showed that the extent of decrease in the maximum photochemical efficiency of photosystem Ⅱ and of the increase in electrolyte leakage in leaves of the heat-tolerant scallion was less than that in leaves of the ‘Zhangqiu Dacong’scallion at high temperature，indicating that the thermostability of photosystem Ⅱ and cell membrane was higher in the heat-tolerant scallion than that in the‘Zhangqiu Dacong’scallion. At high temperature，the activities of superoxide dismutase（SOD），catalase（CAT）and ascorbate peroxidase（APX）increased in leaves of the heat-tolerant scallion and‘Zhangqiu Dacong’scallion，but the increase extent was greater in the summer scallion，which suggests that heat-tolerant scallion had a higher antioxidant capacity than the‘Zhangqiu Dacong’scallion did. Having higher antioxidant ability is one of the reasons for higher heat tolerance in the leaves of heat-tolerant scallion.

Key words: summer scallion, heat tolerance, chlorophyll fluorescence, antioxidant enzymes