Differential ozone sensitivity interferes with cadmium stress in poplar clones

Information on plant responses to combined ozone and cadmium stresses are scarce and limited to herbaceous species. In this research, two poplar clones (I-214 and Eridano), differently sensitive to O3, were grown for 5 weeks in pots supplied with 0, 53.5, and 160.5 mg(Cd) kg-1(soil d.m.) and then exposed to 15-d O3 fumigation (0.06 mm3 dm-3, 5 h a day). The effects of the two stressors, alone or in combination, on Cd, Ca, Fe, and Zn accumulation in above- and below-ground organs, photosynthesis, leaf pigments, and accumulation of H2O2 and NO were investigated. Cadmium induced a reduction in stomatal conductance and a significant accumulation of H2O2 and NO in both clones and negatively affected the carotenoid content in I-214. Ozone, on the other hand, counteracted Cd accumulation in the above-ground organs and significantly increased the xanthophyll de-epoxidation state indicating photoinhibition in O3-treated plants. Surprisingly, O3 alone or in combination with Cd decreased H2O2 accumulation in I-214. The NO production was generally stimulated by Cd, whereas it decreased following O3 exposure in I-214. The overall data indicate that Cd and O3 induced clone specific responses. Moreover, when they were applied in combination, antagonistic rather than synergistic effects were observed.