Elevated O3 inhibited isoprene emission of a diploid and a triploid genotype of Populus tomentosa by different mechanisms

Ozone (O₃) pollution affects plant growth and isoprene (ISO) emission. However, the response mechanism of isoprene emission rate (ISO_rate) to elevated O₃ (EO₃) remains poorly understood. ISO_rate was investigated in two genotypes (diploid and triploid) of Chinese white poplar (Populus tomentosa Carr.) exposed to EO₃ in an open top chamber (OTC) system. The triploid genotype had higher photosynthesis rate (A) and stomatal conductance (g_s) than the diploid one. EO₃ significantly decreased A, g_s, and ISO_rate of middle and lower leaf positions in both genotypes. In the diploid genotype, the reduction of ISO_rate was caused by a systematic decrease related to ISO synthesis capacity, as indicated by decreased contents of isoprene precursor dimethylallyl diphosphate (DMADP) and isoprene synthase (ISPS) protein and activity of ISPS. On the other hand, the negative effect of O₃ on ISO_rate of the triploid genotype did not result from inhibited ISO synthesis capacity, but from increased ISO oxidative loss within the leaf. Our findings will be useful for breeding poplar genotypes with high-yield and lower ISO_rate, depending on local atmospheric VOC/NO_x ratio, to cope with both the rising O₃ concentrations and increasing biomass demand. They can also enlighten the incorporation of O₃ effects into process-based models of isoprene emission.