Impact of increased springtime O3 exposure on Scots pine (Pinus sylvestris) seedlings in central Finland.

Three-year-old Scots pine (Pinus sylvestris L.) seedlings were exposed to ambient or elevated ozone (O(3)) concentrations in open-air exposure fields in central Finland in 1995-97. Three different treatments were applied in 1996 and 1997: ambient air, elevated O(3) (1.3-1.5xambient) during the growing season (June-September) and elevated O(3) in March-September, i.e. the growing season including the springtime O(3) exposure. The ambient mean O(3) concentrations were 40% higher in springtime (March-May) compared to the concentrations during the growing seasons. Maximum O(3) concentrations were measured in April or early May, whereas a clear increase in the stomatal activity of the seedlings was observed by the middle of May. This suggests a low intake of O(3) by conifers despite the higher O(3) concentrations in spring. Stomatal conductance, and contents of chlorophyll and ribulosebisphosphate carboxylase/oxygenase (Rubisco) in current-year needles were not significantly affected by any O(3) treatment. Only a slight decrease in current-year shoot growth, slight increase in the abscission of 2-year-old needles and increased electron density of chloroplast stroma by springtime O(3) exposure suggest a rather small contribution of elevated springtime O(3) concentrations to total O(3) damage under current climatic conditions in Finland. However, the increases in springtime O(3) concentrations may enhance the cumulative effects of O(3) during long-term O(3) exposures.