Foliar responses to ozone of Fagus sylvatica L. seedlings grown in shaded and in full sunlight conditions.

Ozone [O3] is well known to affect photosynthesis on tree seedlings under experimental conditions. Under natural conditions for regeneration, young trees grow under a competitive environment that may alter light availability. Such conditions were simulated in an open-top chamber (OTC) experiment carried out in Southern Switzerland (Lattecaldo). The experimental set-up consisted of Fagus sylvatica (beech) seedling plots (planted within the same chamber) either in monoculture (Mono) or under competition with taller plants of Viburnum lantana (Mixed). These cultures were treated with ambient air (NF, not filtered, 92% of ozone in ambient air) and with charcoal filtered air (CF, 50% of ozone in ambient air). Measures of net photosynthesis (PN) and chlorophyll a fluorescence (with the analysis of the OKJIP transient) were performed over the growing season 2004, from June to September. Net photosynthesis (PN) was higher in Mono conditions. At the end of the season (September), PN was lower in the NF plots compared to the CF ones. Fluorescence parameters were affected much more by light conditions (Mono vs. Mixed) than ozone exposure, but ozone enhanced the action of high light. Marked effects of ozone were observed only in high light (Monoculture) conditions, at the end of the season. Effects of light were greater at the beginning of the season, suggesting the presence of acclimation mechanisms. Ozone stress is connected to the inactivation of the end acceptors of electrons, that produces an excess of excitation. High light, on the other hands, enhanced the activity of the end acceptors and net photosynthesis. The results suggest that shaded beech seedlings are more protected against adverse ozone effects and high light stress than light exposed beech seedlings.