Abstract：
Quercus mongolica Fisch. ex Ledeb. is an economically and ecologically important oak species mainly distributed in East Asia. Our study suggested that it was susceptible to elevated O3. Using open top chambers, changes in photosynthesis, lipid peroxidation and antioxidant system were studied in Q. mongolica leaves fumigated with elevated O3 (≈80 nmol mol−1). During the O3 exposure, photosynthetic rate (Pn) and stomatal conductance decreased significantly, but the decrease in Pn hardly resulted from the stomatal limitation, as no decrease was observed in the intercellular CO2 concentration. In addition, significant decrease was noted in carboxylation efficiency, indicating that the CO2 fixation process was affected by O3, and it can be taken as the main factor leading to the decline of Pn, particularly in the initial 45 days of O3 fumigation. In the initial 45 days of O3 fumigation, ascorbate (Asc) content and activities of superoxide dismutase and ascorbate peroxidase were enhanced, but the enhanced antioxidant capability did not balance reactive oxygen species production, resulting in significant increase in malondialdehyde content at day 45. In the following O3 fumigation period, significant decrease in activities of antioxidant enzymes and Asc content were recorded in the leaves. Along with the decline of antioxidant capability, chlorophyll loss, increase in electrolyte leakage and decrease in maximum photochemical efficiency of photosystem II were also noted. According to the biochemical and photosynthetic changes, we suppose that leaf senescence was accelerated by elevated O3 in the prolonged fumigation period.