The effects of high O3 (200 nl l-1 during the light period) and high CO2 (650 &mgr;l l-1 CO2, 24 h a day) alone and in combination were studied on 45-day-old sugar maple (Acer saccharum Marsh.) seedlings for 61 days in growth chambers. After 2 months of treatment under the environmental conditions of the experiment, sugar maple seedlings did not show a marked response to the elevated CO2 treatment: the effect of high CO2 on biomass was only detected in the leaves which developed during the treatment, and assimilation rate was not increased. Under high O3 at ambient CO2, assimilation rate at days 41 and 55 and Rubisco content at day 61 decreased in the first pair of leaves; total biomass was reduced by 43%. In these seedlings large increases (more than 2-fold) in glucose 6-phosphate dehydrogenase (G6PDH, EC 1.1.1.49) activity and in anaplerotic CO2 fixation by phosphoenolpyruvate carboxylase (PEPC, EC 4.1.1.31) were observed, suggesting that an enhanced reducing power and carbon skeleton production was needed for detoxification and repair of oxidative damage. Under high O3 at elevated CO2, a stimulation of net CO2 assimilation was observed after 41 days but was no longer observed at day 55. However, at day 61, the total biomass was only reduced by 21% and stimulation of G6PDH and PEPC was less pronounced than under high O3 at ambient CO2. This suggests that high CO2 concentration protects, to some extent, against O3 by providing additional carbon and energy through increased net assimilation.

-Two-year-old sugar maple (Acer saccharum Marsh.) seedlings were exposed in open top chambers to an extensive gradient of O 3 (0 to 300 nL.L −1 ) during 85 days under two light environments (20% and 80% of full sun at noon on a sunny day). The growth of truncated seedlings (with one flush of leaves) and episodic seedlings (with two flushes) was decreased as O 3 increased, especially the growth of the second flush which developed under the oxidative treatment. Visible leaf injuries developed during the season under high O 3 concentrations. Survivalist growth strategy of sugar maple, as seen by the root/shoot ratio, together with the enzymatic stimulations of glucose 6-phosphate dehydrogenase, phosphoenolpyruvate carboxylase and glutathione reductase allowed the seedlings to tolerate the O 3 doses. However, at the end of the season, the cumulative oxidative stress in the second flush of the episodic seedlings exposed to concentrations over 150 nL.L −1 O 3 was too large and exceeded the capacity of seedlings for detoxification and repair. carboxylation / detoxification / croissance / stress oxydatif / érable à sucre