Elevated CO2 does not provide protection against ozone considering the activity of several antioxidant enzymes in the leaves of sugar maple

Seedlings of sugar maple (Acer saccharum Marsch.) were exposed for 46 days to 700 ppm of CO2, 200 ppb of ozone, and 700 ppm of CO2 + 200 ppb of ozone. A significant increase in the activity of H2O2 scavenging enzymes, i.e. ascorbate peroxidase [EC 1.11.1.11] and catalase [EC 1.11.1.6], was measured due to the action of O-3 This increase was rather negatively affected by elevated CO2. A tendency of decreased activity of glutathione reductase [EC 1.6.4.2] and superoxide dismutase [EC 1.15.1.1] due to the action of O-3 was detected. Elevated CO2 does not provide enhanced tolerance to oxidative stress in the seedlings of sugar maple. Changes in the activity of antioxidant enzymes were more pronounced in the young leaves (developed during the experiment) than in the old leaves (developed before starting the experiment). Stimulation of chloroplastic FeSOD by elevated CO2 was observed, indicating oxidative stress in chloroplasts evoked by elevated CO2 level. This effect did not result in enhanced protection against the detrimental effect of ozone, most probably due to compartmentation of CO2 and O-3 effects within the cell.