Growth and physiological responses to ozone and mild drought stress of trees species with different ecological requirements.

An open-top chamber (OTC) experiment was carried out in Curno (Northern Italy) in 2004 and 2005 on seedlings of Fagus sylvatica (FS), Quercus robur (QR), and an ozone-sensitive Populus (POP) clone, to investigate the role of two stress factors: tropospheric ozone and water shortage. Treatments were: filtered air to achieve a 50% reduction of the environmental ozone concentrations (charcoal filtered, CF); and non filtered air, with a 5% reduction of the environmental ozone concentrations (non filtered, NF). Overall ozone exposure (AOT40) in open air (April-September) was 26,995 ppb•h in 2004 and 25,166 ppb•h in 2005. The plants were either watered (W) or not watered (dry, D). We investigated the above-ground biomass, tree-ring growth, stable carbon isotopes ratio, i.e. δ13C of tree-rings, and the photosynthetic parameter Driving Forces (DFABS), derived from chlorophyll a fluorescence analysis. Ozone induced growth reduction (in terms of biomass) in POP, and that reduction was more pronounced in D plots. A synergistic effect of ozone and drought stress was evidenced by DFABS in POP and QR, but not in FS. The water availability was revealed as the main factor influencing the isotopic ratio δ13C. In drought-stressed seedlings, the increase of δ13C value was accompanied by the reduction of stomatal conductance and increased DFABS. Fast-growing plant species with high water requirements are more susceptible to ozone and drought stress.