Naturally regenerated Scots pines (Pinus sylvestris L.), aged 28–30 years old, were grown in open-top chambers and subjected in situ to three ozone (O3) regimes, two concentrations of CO2, and a combination of O3 and CO2 treatments From 15 April to 15 September for two growing seasons (1994 and 1995). The gas exchanges of current-year and 1-year-old shoots were measured, along with the nitrogen content of needles. In order to investigate the factors underlying modifications in photosynthesis, five parameters linked to photosynthetic performance and three to stomatal conductance were determined. Elevated O3 concentrations led to a significant decline in the CO2 compensation point (Г*), maximum RuP2-saturated rate of carboxylation (Vcmax), maximum rate of electron transport (Jmax), maximum stomatal conductance (gsmax), and sensitivity of stomatal conductance to changes in leaf-to-air vapour pressure difference (∂gs/∂Dv) in both shoot-age classes. However, the effect of elevated O3 concentrations on the respiration rate in light (Rd) was dependent on shoot age. Elevated CO2(700 μmol mol−1) significantly decreased Jmax and gsmax but increased Rd in 1-year-old shoots and the ∂gs/∂Dv in both shoot-age classes. The interactive effects of O3 and CO2 on some key parameters (e.g. Vcmax and Jmax) were significant. This may be closely related to regulation of the maximum stomatal conductance and stomatal sensitivity induced by elevated CO2. As a consequence, the injury induced by O3 was reduced through decreased ozone uptake in 1-year-old shoots, but not in the current-year shoots. Compared to ambient O3 concentration, reduced O3 concentrations (charcoal-filtered air) did not lead to significant changes in any of the measured parameters. Compared to the control treatment, calculations showed that elevated O3 concentrations decreased the apparent quantum yield by 15% and by 18%, and the maximum rate of photosynthesis by 21% and by 29% in the current-year and 1-year-old shoots, respectively. Changes in the nitrogen content of needles resulting from the various treatments were associated with modifications in photosynthetic components.
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