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CO-ORDINATION OF PHOTOSYNTHESIS AND STOMATAL RESPONSES OF MONGOLIAN OAK (QUERCUS MONGOLICA FISCH. EX LEDEB.) TO ELEVATED O3 AND/OR CO2 LEVELS

Four-year-old trees grown in open-top chambers (OTCs) were exposed to elevated O3 (80 nmol/mol, EO) or/and elevated CO2 (700 μmol/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 Pn (Light-saturated net photosynthesis rate), gs (stomatal conductance) and Tr (transpiration rate). The stomata closure maybe associated with the direct effect of EO on guard cells. The decreases of F0, Fm and Fv/Fm indicated that EO limits the capability of the plant to use photon energy and thus alters photosynthesis. (2) EC increased Pn and Ci (intercellular CO2 concentration) of leaves, but decreased gs and Tr 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 O3-induced adverse effects to Pn. The decreases of gs and Tr were mainly caused by EO. There was no significant difference of Fv/Fm 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.