Abstract: Four year old  Pinus tabulaeformis seedlings were planted in open topped chambers and treated with elevated O₃ (80 nmol·mol^-1) and/or CO₂ (700 μmol·mol^-1). After treated for three years, the malondialdehyde (MDA) content, reactive oxygen species, and antioxidant enzyme activities in the  current year needles were analyzed. In treatment elevated O₃, the activities of superoxide dismutase (SOD), ascorbate peroxidase (APX), catalase (CAT) and dehydroascorbate reductase (DHAR) increased significantly, being 43.1%, 18.9%, 283.5% and 142.6% higher than the control; and in treatments elevated CO₂ and its combination with elevated O₃, only APX and CAT activities were induced remarkably but still lower than those in treatment elevated O₃. Elevated O₃ increased the MDA content by 51.8%, whereas the combination of elevated O₃ and CO₂ reversed this increase efficiently. No significant differences were observed in the superoxide anion (O₂) generating rate and H₂O₂ content among all treatments. All the results demonstrated that elevated O₃ was able to effectively activate the antioxidant enzyme system in  P. tabulaeformis current year needles, while elevated CO₂ could mitigate the negative effects of elevated O₃ through the actions other than enhancing the antioxidant enzyme activities.

Key words: Heat hardening, Photochemical quenching coefficient, Non-photochemical quenching coefficient, Brassica olexacea