The response of clones of Populus tremuloides differing in sensitivity to ozone in the field

Ozone (O-3)-sensitive and tolerant clones of aspen (Populus tremuloides) were exposed to O-3 at 1.0, 1.7 and 3.0 times the concentration in ambient air at Ithaca, NY, USA (x 1.0, x 1.7 and x 3.0) for 3 months. Visible foliar injury occurred within a few days of the initiation of treatments and developed continuously in x 1.7 and x 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 br 40 and 70% on tolerant and sensitive clones, respectively, but stomatal conductance was not affected by the O-3 treatments. As stomatal conductance (g(s)) values did not differ in the two types of aspen, we conclude that this is not the defense mechanism to O-3 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 tw antioxidants, ascorbic acid and glutathione, were investigated to determine ii a clonal difference occurred. Concentrations of the free-radical scavengers did not differ in the clones, nor did they seem to be induced by O-3. A cumulative O-3 index, SUM06 (the sum of the total mu l l(-1) of exposure when the O-3 concentration exceeded 0.06 mu l l(-1)), was highly correlated xith most foliar injury variables and gas-exchange variables except g(s) in higher O-3 treatments. The reduction of carbon gain due to O-3 is discussed with regard to foliar damage and biomass decrease.