O3 flux-related responsiveness of photosynthesis, respiration, and stomatal conductance of adult Fagus sylvatica to experimentally enhanced free-air O3 exposure

Knowledge of responses of photosynthesis, respiration, and stomatal conductance to cumulative ozone uptake (COU) is still scarce, and this is particularly the case for adult trees. The effect of ozone (O-3) exposure on trees was examined with 60-year-old beech trees (Fagus sylvatica) at a forest site of southern Germany. Trees were exposed to the ambient O-3 regime (1 x O-3) or an experimentally elevated twice-ambient O-3 regime (2 x O-3). The elevated 2 X O-3 regime was provided by means of a free-air O-3 canopy exposure system. The hypotheses were tested that (1) gas exchange is negatively affected by O-3 and (2) the effects of O-3 are dose-dependent and thus the sizes of differences between treatments are positively related to COU. Gas exchange (light-saturated CO2 uptake rate A(max), stomatal conductance g(s), maximum rate of carboxylation VCmax, ribulose-1,5-bisphosphate turnover limited rate of photosynthesis J(max), CO2 compensation point CP, apparent quantum yield of net CO2 uptake AQ, carboxylation efficiency CE, day- and night-time respiration) and chlorophyll fluorescence (electron transfer rate, ETR) were measured in situ on attached sun and shade leaves. Measurements were made periodically throughout the growing seasons of 2003 (an exceptionally dry year) and 2004 (a year with average rainfall). In 2004Vc(max), J(max), and CE were lower in trees receiving 2 x O-3 compared with the ambient O-3 regime (1 x O-3). Treatment differences in VCmax, J(max), CE were rather small in 2004 (i.e., parameter levels were lower by 10-30% in 2 x O-3 than 1 x O-3) and not significant in 2003. In 2004 COU was positively correlated with the difference between treatments in Amax, gs, and ETIR (i.e., consistent with the dose-dependence of O-3's deleterious effects). However, in 2003, differences in A(max), g(S), and ETR between the two O-3 regimes were smaller at the end of the dry summer 2003 (i.e., when COU was greatest). The relationship of COU with effects on gas exchange can apparently be complex and, in fact, varied between years and within the growing season. In addition, high doses of O-3 did not always have significant effects on leaf gas exchange. in view of the key findings, both hypotheses were to be rejected.