Abstract:To understand the effect of high temperature stress on Pyrus trees, the changes in antioxidant substance, antioxidant enzymet and osmotic substance in leaves of 2-year-old ‘Huangguan’ (P. pyrifolia ‘Cuiguan’) and ‘Cuiyu’ (P. bretschneideri×P. pyrifolia) seedlings were measured, and the expressions of related genes at different stages were analyzed by qRT-PCR. The results showed that under high temperature stress, the chlorophyll content in leaves of ‘Huangguan’ and ‘Cuiyu’ reduced, while MDA content gradually increased. The activities of POD and CAT in two cultivars were both reduced, but those in ‘Huangguan’ were higher than in ‘Cuiyu’. The Pro content in two cultivars was increased with stress time. And the ASA content increased at first and then decreased in two cultivars. The contents of IAA and ABA decreased with stress time in two cultivars, but those in ‘Huangguan’ were higher than in ‘Cuiyu’. The expressions of related genes in leaves were consistent with the changes in contents of ASA, IAA and ABA. Therefore, the heat resistance of ‘Huangguan’ was stronger than that of ‘Cuiyu’, and the turning point of physiological change in two cultivars was the fourth day under high temperature.