Effects of elevated O-3 and CO2 concentrations on photosynthesis and stomatal conductance in Scots pine

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 (O-3) regimes, two concentrations of CO2, and a combination of O-3 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 O-3 concentrations led to a significant decline in the CO2 compensation point (I*), maximum RuP2-saturated rate of carboxylation (V-emax), maximum rate of electron transport (J(max)) maximum stomatal conductance (g(smax)) and sensitivity of stomatal conductance to changes in leaf-to-air vapour pressure difference (partial derivative g(s)/partial derivative D-v) in both shoot-age classes, However, the effect of elevated O-3 concentrations on the respiration rate in light (R-d) was dependent on shoot age, Elevated CO2 (700 mu mol mol(-1)) significantly decreased J(max) and g(smax) but increased R-d in 1-year-old shoots and the partial derivative(s)/partial derivative D-v in both shoot-age classes, The interactive effects of O-3 and CO2 on some key parameters (e.g. V-emax and J(max)) 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 O-3 was reduced through decreased ozone uptake in 1-year-old shoots, but not in the current-year shoots, Compared to ambient O-3 concentration, reduced O-3 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 O-3 concentrations decreased the apparent quantum yield by 35% 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.