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 μmol mol-1), combined with low or elevated O3 for three seasons. Ozone exposure during the last growing season (accumulated dose above threshold 0.06 μmol mol-1) was 0.05 versus 26.13 μmol 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.