Combined Effects of CO2 and O3 on Antioxidative and Photoprotective Defense Systems in Needles of Ponderosa Pine

Ponderosa pine (Pinus ponderosa Dougl. ex Laws.) seedlings were exposed to near ambient or elevated CO₂ (average concentrations during the last growing season 446 versus 699 mol mol^–1), combined with low or elevated O₃ for three seasons. Ozone exposure during the last growing season (accumulated dose above threshold 0.06 mol mol^–1) was 0.05 versus 26.13 mol mol^–1 h. Needles of the youngest age class were harvested after the dormancy period. Ozone exposure decreased needle contents of chlorophyll a, chlorophyll b, and ascorbate, and resulted in a more oxidized total ascorbate and a more de-epoxidized xanthophyll cycle pool irrespective of the CO₂ level. Trees under elevated CO₂ had a more oxidized glutathione pool and lower chlorophyll a content. Contents of glutathione, tocopherol, and carotenoids were not affected by the CO₂ or O₃ treatments. There were no interactive effects between elevated CO₂ and elevated O₃ on any of the parameters measured. The results suggest that elevated atmospheric CO₂ concentration does not compensate for ozone stress by increasing antioxidative capacity in ponderosa pine.