Plants of 15 Pinus sylvestris clones were grown for 10 weeks in three open- top chambers at 300 and 150 Myg O3 m3 and at ambient air respectively. CO2 (Pn) and H2O (E) gas exchange were compared among the clones during and after the fumigation, and in the following vegetation period. For the higher ozone concentration there are significant differences between clones for all investigated traits at almost all dates. The grand mean of all clones reveals a decrease of photosynthesis after the fumigation, followed by a continous increase, resulting in higher rates of previously ozone treated plants in the next vegetation period. The grand mean for the transpiration rates significantly surpasses even 12 months after exposure the mean of the control. The grand mean for the transpiration ratio (water loss in relation to carbon gain) is significantly higher in the ozone treated plants than in the control, most prominent 6 months after the treatment. The range of the clonal means for photosynthesis and the transpiration ratio is high, and it varies with the dates of the measurements. Also, the ranking of the clones at different dates varies, revealing considerable genotype environment interaction. With respect to the time scale different clones show specific characteristics of response. There are genotypes which do not react on the ozone stress, some decrease others increase photosynthesis. For the transpiration ratio beneath non reacting clones there are clones with an increase, others with a decrease between 6 weaks and 6 months after fumigation. The results indicate that genotypic variation should not be neglected in eco-physiological investigations and that this investigation increases the understanding of the function of forest ecosystems under stress.
