Interactions of chronic exposure to elevated CO₂ and O₃ levels in the photosynthetic light and dark reactions of European beech (Fagus sylvatica)

Young trees of European beech ( Fagus sylvatica ) acclimated for one growing season to ambient ( c. 367 μl l −1 ) or elevated CO 2 levels ( c. 660 μl l −1 ) were exposed during the subsequent year to combinations of the same CO 2 regimes and ambient or twice‐ambient ozone (O 3 ) levels (generated from the database of a rural site). By the end of June, before the development of macroscopic leaf injury, the raised O 3 levels had not affected the light and dark reactions of photosynthesis. However, acclimation to elevated CO 2 had resulted in lowered chlorophyll and nitrogen concentrations, whereas photosynthetic performance, examined over a wide range of parameters from light and dark reactions, remained unchanged or showed only slight reductions (e.g. apparent electron transport rate, ETR; apparent quantum yield of CO 2 gas exchange, Φ CO2 ; apparent carboxylation efficiency, CE; and photosynthetic capacity at light and CO 2 saturation, PC). In August, after the appearance of leaf necroses, plants grown under ambient CO 2 and twice‐ambient O 3 conditions declined in both the photosynthetic light reactions (optimum electron quantum yield, F v / F m , non‐photochemical energy quenching, NPQ, reduction state of Q A , apparent electron quantum yield, Φ PSI I, maximum electron transport rates) and the dark reactions as reflected by CE, Φ CO2 , as well as the maximum CO 2 uptake rate (i.e. PC). CE, Φ CO2 and PC were reduced by c. 75, 40 and 75%, respectively, relative to plants exposed to ambient CO 2 and O 3 levels. By contrast, plants exposed to twice‐ambient O 3 and elevated CO 2 levels maintained a photosynthetic performance similar to individuals grown either under ambient CO 2 and ambient O 3 , or elevated CO 2 and ambient O 3 conditions. The long‐term exposure to elevated CO 2 therefore tended to counteract adverse chronic effects of enhanced O 3 levels on photosynthesis. Possible reasons for this compensatory effect in F. sylvatica are discussed.