Abstract: Changes of physiological characteristic on photosynthesis in Malus seedling leaves during water stress were studied with polyethylene glycol(PEG 6000)treatment．The results were as follows：Under water stress．the binding of Chl-Pro was loosened with the changes of time and intensity of water stress．Net photosynthesis rate(Pn)，stomatal conductance(Gs)decreased，intercellular CO₂ concentration (Ci)increased，then decreased．The activities of SOD，CAT increased in the early period，then decreased．The 0₂^-. generating rate and contents of H₂0₂ increased during water stress，the content of antioxidants carotenoid(Car)、ASA decreased．The changing extents of these parameters in the drought tolerant variety sieversii were lower than the drought resistive variety M.hupehensis． Under water stress．there are high negative significant correlation between Pn and Chl-Pro O ，H₂O₂ in M.hupehensis (Pamp．)Reld．and M.sieversii (Ledeb．) R0em． seedlings． With the duration of water stress。the excessive accumulati0n 0f 0₂^-.。H₂O₂nduced the nonstomatal limitation．It played key role in the decline of the photosynthetic rate.

Key words: Malus, Water stress, Photosynthesis, Active oxygen, Nonstomatal limitation