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Four-year-old trees grown in open-top chambers (OTCs) were exposed to elevated O-3 (80 nmol/mol, EO) or/and elevated CO2 (700 mu mol/mol, EC) levels. The object is to study the responses of photosynthesis and stomata in leaves of Mongolian oak (Quercus mongolica) to EO and/or EC for two successive growing seasons in an urban area. The experiment was conducted in the arboretum located at the center of Shenyang city. The results show that: (1) EO decreased P-n (Light-saturated net photosynthesis rate), g(s) (stomatal conductance) and T-r (transpiration rate). The stomata closure maybe associated with the direct effect of EO on guard cells. The decreases of F-0, F-m and F-v/F-m indicated that EO limits the capability of the plant to use photon energy and thus alters photosynthesis. (2) EC increased P-n and C-i (intercellular CO2 concentration) of leaves, but decreased g(s) and T-r slightly. Stomatal density decreased significantly at the end of the second season. Perhaps, EC was detected by mature leaves in the first season which signaled to the developing leaves in the next season, the stomatal developments of which altered accordingly. (3) Under EC+EO, EC relieved most of the O-3-induced adverse effects to P-n. The decreases of g(s) and T-r were mainly caused by EO. There was no significant difference of F-v/F-m detected compared to the control, EC can largely relieve the adverse effects of EO on the PSII reaction center. Changes of stomatal parameters were just like the changes under EC. Perhaps the morphological changes of the stomata were mainly caused by EC.