Abstract: The parameters of photosynthesis，chlorophyll fluorescence and antioxidant systems during the day or at 13：00 were ex situ measured in Beijing on the leaves of 1-year-old seedlings of Rosa platyacantha from three different altitudes from Xinjiang. The results showed there were continuous declines of P_n after 9：00 on the seedlings of R. platyacantha from all altitudes，which should be due to stomatal limitation in the morning while non-stomatal limitation at noon. In comparison to the seedlings from low and medium altitudes，the impaired PSⅡ activity and heat dissipation were observed at 13：00 in the seedlings from high altitude，indicating a worse photoinhibition. Moreover，the antioxidant system including POD and APX activities，as well as AsA/DHA were enhanced at 13：00 but still led to the excessive ROS and lipid peroxidation in the seedlings from high altitude，which suggesting the significant  photodamage happened.

Key words: Rosa platyacantha, altitude；ex situ, photosynthesis, photoprotection