O3 Flux-Related Responsiveness of Photosynthesis, Respiration, and Stomatal Conductance of Adult Fagus sylvatica to Experimentally Enhanced Free-Air O3 Exposure

Stuttgart

2007

S. 197-206

4 Abb., 3 Tab., 61 Lit. Ang.

Unselbständiges Werk

200144784

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 (O3) 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 O3 regime (1 x O3) or an experimentally elevated twice-ambient O3 regime (2 x O3). The elevated 2 x O3 regime was provided by means of a free-air O3 canopy exposure system. The hypotheses were tested that (1) gas exchange is negatively affected by O3 and (2) the effects of O3 are dose-dependent and thus the sizes of differences between treatments are positively related to COU. Gas exchange (light-saturated CO2 uptake rate Amax, stomatal conductance gs, maximum rate of carboxylation Vcmax, ribulose-1,5-bisphosphate turnover limited rate of photosynthesis Jmax, CO2 compensation point CP, apparent quantum yield of net CO2 uptake AQ, carboxylation efficiency CE, day- and nighttime 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 2004 Vcmax, Jmax, and CE were lower in trees receiving 2 x O3 compared with the ambient O3 regime (1 x O3). Treatment differences in Vcmax, Jmax, CE were rather small in 2004 (i.e., parameter levels were lower by 10 - 30 % in 2 x O3 than 1 x O3) and not significant in 2003. In 2004 COU was positively correlated with the difference between treatments in Amax, gs, and ETR (i.e., consistent with the dose-dependence of O3’s deleterious effects). However, in 2003, differences in Amax, gs, and ETR between the two O3 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 O3 did not always have significant effects on leaf gas exchange. In view of the key findings, both hypotheses were to be rejected.