Elevated ozone negatively affects photosynthesis of current-year leaves but not previous-year leaves in evergreen Cyclobalanopsis glauca seedlings

To assess the effects of leaf age/layer on the response of photosynthesis to chronic ozone (O-3), Cyclobalanopsis glauca seedlings, a dominant evergreen broadleaf tree species in sub-tropical regions, were exposed to either ambient air (AA) or elevated O-3 (AA + 60 ppb O-3, E-O-3) for two growing seasons in open-top chambers. Chlorophyll content, gas exchange and chlorophyll a fluorescence were investigated three times throughout the 2nd year of O-3 exposure. Results indicated that E-O-3 decreased photosynthetic parameters, particularly light-saturated photosynthesis rate, stomatal conductance and effective quantum yield of PSII photochemistry of current-year leaves but not previous-year leaves. Stomata! conductance of plants grown under ambient conditions partially contributed to the different response to E-O-3 between leaf layers. Light radiation or other physiological and biochemical processes closely related to photosynthesis might play important roles. All suggested that leaf ages or layers should be considered when assessing O-3 risk on evergreen woody species. (C) 2013 Elsevier Ltd. All rights reserved.