Elevated CO₂ counteracts the limitation by chronic ozone exposure on photosynthesis in<em> Fagus sylvatica</em> L.: Comparison between chlorophyll fluorescence and leaf gas exchange.

The interaction of elevated CO2 and enhanced chronic ozone (O-3) impact was analysed throughout the growing season in the photosynthetic response (chlorophyll fluorescence and leaf gas exchange) of beech saplings (Fagus sylvatica) which had been acclimated to CO2 supply during the year prior to the experiment. Both light and dark reactions (i.e. electron transport rate and photosynthetic capacity) of plants grown at ambient CO2 and twice-ambient O-3 concentrations were distinctly reduced by August. The O-3-induced decline was counteracted by elevated CO2 supply (i.e. ambient +300 ppm). Plants grown at high CO2 supply and ambient or twice-ambient O-3 concentrations displayed a photosynthetic performance similar to plants exposed to ambient CO2 and O-3 conditions. Responses in chlorophyll fluorescence were found to be consistent with those in leaf gas exchange