We exposed trembling aspen (Populus tremuloides Michx.) clones differing in tropospheric ozone (O3) tolerance in various open-top chamber studies for three growing seasons, and examined the effects of O3, CO2, and O3 + CO2 on growth and physiological processes. Ozone in the range of 80 ppm hr (Sum 00) per growing season decreased height, diameter, and stem and leaf biomass slightly in a tolerant clone but severely in a sensitive clone. Elevated CO2 (150 ppm over ambient) did not compensate for the O3 effects. Antioxidant enzyme analysis showed elevated SOD levels in the tolerant done but not in the sensitive clone following O3 exposure. Northern blot analysis indicated that the chloroplastic and cytosolic Cu/Zn SOD's were significantly increased in response to O3 in the tolerant but not the sensitive clone. Currently, we are conducting molecular analysis to determine the functional significance of SOD's in regulating O3 tolerance in aspen.