Both sensitive and tolerant clones of aspen (Populus tremuloides) were exposed to ozone using four different exposure regimes under controlled environmental conditions. Based on data on ambient ozone from 10 cities in the USA, three treatments of 4-wk exposure to the same SUM06 (an accumulation of hourly O3 concentrations greater than 0.06 ml l−1) were constructed. The regimes allowed us to investigate: (a) the importance of long (3 wk, treatment 1) versus short (1 wk, treatment 2) duration of regimes with high peaks; (b) the effect of treatments with variable peak occurrence (treatments 1 and 2) versus uniform peak occurrence (treatment 3) during the exposure period. Nonfumigated control plants were maintained at ozone concentrations <10 nl l−1. Bifacial black necrosis, a typical symptom of ozone injury on aspen leaves, occurred on both clones after 2 wk exposure. Up to 60% of the leaves on the sensitive clone were injured, with an average of 6% of total leaf area injured. In the tolerant clone only 10% of the leaves were injured, with less than 1% of the total leaf area symptomatic. The severity of injury was consistently greatest in treatment 2, followed by treatments 1 and 3, respectively. The interval between peak exposures was less important than the occurrence of peaks versus a stable maximum concentration. Premature leaf abscission occurred in the sensitive clone. Measures of gas exchange demonstrated reduced photosynthesis under ozone fumigation, but exposure regime was not a significant factor. Concentrations of two antioxidants, ascorbic acid and glutathione, were almost always greater in the resistant than in the sensitive clone, but the differences were not statistically significant. The levels of these antioxidants in aspen leaves did not change with ozone fumigation or leaf age.

Ozone (O3)-sensitive and tolerant clones of aspen (Populus tremuloides) were exposed to O3 at 1.0, 1.7 and 3.0 times the concentration in ambient air at Ithaca, NY, USA (×1.0, ×1.7 and ×3.0) for 3 months. Visible foliar injury occurred within a few days of the initiation of treatments and developed continuously in ×1.7 and ×3.0 treatments. Leaf senescence began earlier in trees exposed to greater-than-ambient treatments. Gas-exchange variables were measured five times over the growing season. Net photosynthesis was progressively decreased and finally reduced by 40 and 70% on tolerant and sensitive clones, respectively, but stomatal conductance was not affected by the O3 treatments. As stomatal conductance (gs) values did not differ in the two types of aspen, we conclude that this is not the defense mechanism to O3 in aspen. At the end of the season, dry weight was reduced by up to 55 and 35% in sensitive and tolerant clones, respectively. Ozone effects were significant and linear on foliar injury, biomass and growth variables such as height, diameter of stem and total leaf area. The levels of two antioxidants, ascorbic acid and glutathione, were investigated to determine if a clonal difference occurred. Concentrations of the free-radical scavengers did not differ in the clones, nor did they seem to be induced by O3. A cumulative O3 index, SUM06 (the sum of the total μl l−1 of exposure when the O3 concentration exceeded 0.06 μl l−1), was highly correlated with most foliar injury variables and gas-exchange variables except gs in higher O3 treatments. The reduction of carbon gain due to O3 is discussed with regard to foliar damage and biomass decrease.