Effects of combined ozone and cadmium stresses on leaf traits in two poplar clones

Information on plant responses to combined stresses such as ozone (O₃) and cadmium (Cd) is scarce in tree species. On the other hand, high O₃ concentrations in the atmosphere and heavy metal contaminations in water and soil simultaneously affect forest ecosystems. Toxic metals may exacerbate the consequences of air pollutants. In this research, two poplar clones, differently sensitive to O₃ (“I-214” O₃-tolerant and “Eridano” O₃-sensitive), were grown for 5 weeks in pots supplied with 0 and 150 mg Cd kg⁻¹ soil and then exposed to a 15-day O₃ fumigation (60 nl l⁻¹, 5 h a day) or supplied with charcoal-filtered air under the same conditions (referred to as control samples). The effects of the two stressors, alone or in combination, on Cd accumulation, photosynthetic capacity, ethylene emission and oxidative state were investigated in fully expanded leaves. Cadmium accumulation in leaves caused a reduction, but not complete failure, of photosynthesis in Eridano and I-214 poplar clones. The reduction in assimilation rate was more important following O₃ fumigation. Stomatal aperture after O₃ treatment, instead, increased in I-214 and decreased in Eridano. Overall, Cd treatment was effective in decreasing ethylene emission, whereas O₃ fumigation increased it in both clones, although interacting with the metal treatment. Again, O₃ fumigation induced a significant increase in ascorbate (ASA) + dehydroascorbate (DHA) content, which was strongly oxidised by O₃, thus decreasing the redox state. On the other hand, Cd treatment had a positive effect on ASA content and redox state in I-214, but not in Eridano. Although Cd and O₃ are known to share some common toxicity pathways, the combined effects induced distinct clone-specific responses, underlying the complexity of plant reactions to multiple stresses.