The ozone sensitivity of five poplar clones is not related to stomatal conductance, constitutive antioxidant levels and morphology of leaves.

Ground-level ozone (O₃) is an important phytotoxic air pollutant in China. In order to compare the sensitivity of common poplar clones to O₃ in China and explore the possible mechanism, five poplar clones, clone DQ (Populus cathayana), clone 84 K (P. alba × P. glandulosa), clone WQ156 (P. deltoids × P. cathayana), clone 546 (P. deltoides cv. '55/56' × P. deltoides cv. 'Imperial') and clone 107 (P. euramericana cv. '74/76') were exposed to four O₃ treatments. According to the date of the initial visible O₃ symptom and the slopes of O₃ exposure-response relationships with the relative light-saturated rate of CO₂ assimilation, we found that clone DQ and clone 546 were the most sensitive to O₃, clone 84 K and clone WQ156 were the less sensitive, and clone 107 was the most tolerant, which could provide a basis to select O₃ tolerant clones for poplar planting at areas with serious O₃ pollution. Elevated O₃ significantly reduced photosynthetic parameters, total phenols content, potential antioxidant capacity, leaf mass per area and biomass of five poplar clones, and there were significant interactions between O₃ and clones for most photosynthetic parameters. Elevated O₃ also significantly increased malondialdehyde content and total ascorbate content. The responses of total antioxidant capacity for poplar clones to elevated O₃ were different, as indicated by the increase for clone 107 and reduction for other clones under elevated O₃ treatment. Our results on the sensitivity of different poplar clones to O₃ are not related to leaf stomatal conductance, leaf constitutive antioxidant levels or leaf morphology of plant grown in clean air. The possible reason is little difference in leaf traits among clones within close species, suggesting that more properties of plants should be considered for exploring the sensitivity mechanism of close species, such as mesophyll conductance, antioxidant enzyme activity and apoplastic antioxidants.