Responses of young trees (five species in a chamber exposure) to near-ambient ozone concentrations

This investigation is intended to add to the quantitative information about the sensitivity of European deciduous tree species to environmentally realistic ozone (O-3) exposure in respect to the established critical level of an AOT40 of 10 ppm.h (6-month growing season, daylight hours). Cuttings (without leaves) of Fagus sylvatica L., Sorbus aucuparia, L., Carpinus betulus L., Fraxinus excelsior L., and cuttings and seedlings of Prunus serotina Ehrh. were exposed during one growing season either to filtered air, to which 50 % of ambient O-3 concentration was added, or to 50 % + 30 ppb O-3, resulting in a final AOT40 of 0.3 and 20.7 ppm.h, respectively. The foliage formed per tree varied between and within the species, but was not significantly modified by O-3, whereas the number of symptomatic leaves per tree significantly increased in the 50 % + 30 ppb O-3 regime. By mid July light-green spots appeared in the leaves (except C. betulus), which developed into stippling (F. excelsior), red (P. serotina) or necrotic spots by September. The CO2-assimilation rate decreased more with increasing visual symptoms (earliest in 20- day-old leaves) than with age-dependent leaf discoloration in F. sylvatica, F. excelsior, and P. serotina (not in C. betulus). The dark-adapted photosystem II quantum yield (Fv/Fm) slightly declined with leaf age, but a considerable reduction became apparent in 107-day-old leaves with O-3-symptoms only. By late morning yield reduction in light-adapted leaves were exaggerated in such leaves from the 50 % + 30 ppb O-3 regime. The above mentioned responses were not reflected in significant changes of the net biomass production during the experiment. The critical level, therefore, is based on visual and functional symptoms rather than on production.