Abstract:Superoxide dismutase (SOD)，catalase (CAT)，O2.- and H2O2 in apples (Malus pumila Mill.) fruits were investigated under the hydroponic condition with 0，1，10 mmo1/L CaCl2 and 5 mmo1/L EGTA, and expression patterns of SOD and CAT1gene were analyzed by real–time PCR. Addition of 10 mmo1/LCaCl2 to the culture solution with 12 h and 24 h exposure significantly improved the activities of SOD and CAT, respectively, and simultaneously decreased O2.- and H2O2 accumulation. Addition of 5 mmo1/LEGTA, as a specific chelator of Ca, induced O2.-and H2O2 accumulation correlated with decreases in SOD and CAT activities. The abundance of SOD gene was up-regulated by calcium addition after 12 h exposure in accordance with change of SODactivity, indicating that SODactivity was modified by SOD gene expression. However, expression of CAT1 gene was not in accordance with change of CATactivity. It seemed that CAT activity was modified by CAT1 gene expression particularly at the level of post-translational modification. It was concluded that calcium addition might protect apple fruit tissuesagainst oxidative stress by up-regulating gene expressions and activities of SOD and CAT, then preciselycontrolling O2.-and H2O2 accumulation.