Impact of increased springtime O-3 exposure on Scots pine (Pinus sylvestris) seedlings in central Finland

Three-year-old Scots pine (Pinus sylvestris L.) seedlings were exposed to a mbient or elevated ozone (O-3) concentrations in open-air exposure fields i n central Finland in 1995-97. Three different treatments were applied in 19 96 and 1997. ambient air, elevated O-3 (1.3-1.5 x ambient) during the growi ng season (June-September) and elevated O-3 in March-September, i.e. the gr owing season including the springtime O-3 exposure. The ambient mean O-3 co ncentrations were 40% higher in springtime (March-May) compared to the conc entrations during the growing seasons. Maximum O-3 concentrations were meas ured in April or early May, whereas a clear increase in the stomatal activi ty 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 significan tly affected by any O-3 treatment. Only a slight decrease in current-year s hoot growth, slight increase in the abscission of 2-year-old needles and in creased electron density of chloroplast stroma by springtime O-3 exposure s uggest a rather small contribution of elevated springtime O-3 concentration s 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. (C) 2000 Elsevier Science L td. All rights reserved.