Growth, photosynthesis and ozone uptake of young beech (Fagus sylvatica L.) in response to different ozone exposures

Beech seedlings (Fagus sylvatica L.) were exposed to episodes of O₃ in environmentally controlled growth chambers during one growing season. Three treatments were applied: charcoal-filtered air, charcoal-filtered air with the addition of 40 ppb O₃ for seven episodes of 5 days' duration (9000-1700 hours), and charcoal-filtered air with the addition of 100 ppb O₃ for seven episodes of 5 days' duration (9000-1700 hours). The accumulated exposure over a threshold of 40 ppb in the last treatment reached 13,911 ppb h. Throughout the growing season we measured growth as well as photosynthetic properties and related effects to external and calculated internal doses of O₃, using stomatal conductance (g_s) data. Growth, measured as diameter increment and biomass, was not significantly affected by the O₃ treatments. In the 100-ppb treatment, light-saturated CO₂ assimilation rates and chlorophyll content were significantly reduced, and the chlorophyll fluorescence parameter F_v/F_m was significantly reduced at times of high uptake rates and coincided with strong reductions of assimilation rates. O₃ uptake was lowered in the 100-ppb treatment due to reduced g_s. There was serious visible damage by the end of the exposure period in the 100-ppb treatment, while the treatment with 40 ppb O₃ did not seem to cause any significant changes.