Large variability in ambient ozone sensitivity across 19 ethylenediurea-treated Chinese cultivars of soybean is driven by total ascorbate

The sensitivity of Chinese soybean cultivars to ambient ozone (O₃) in the field is unknown, although soybean is a major staple food in China. Using ethylenediurea (EDU) as an O₃ protectant, we tested the gas exchange, pigments, antioxidants and biomass of 19 cultivars exposed to 28 ppm·hr AOT40 (accumulated O₃ over an hourly concentration threshold of 40 ppb) over the growing season at a field site in China. By comparing the average biomass with and without EDU, we estimated the cultivar-specific sensitivity to O₃ and ranked the cultivars from very tolerant (< 10% change) to highly sensitive (> 45% change), which helps in choosing the best-suited cultivars for local cultivation. Higher lipid peroxidation and activity of the ascorbate peroxidase enzyme were major responses to O₃ damage, which eventually translated into lower biomass production. The constitutional level of total ascorbate in the leaves was the most important parameter explaining O₃ sensitivity among these cultivars. Surprisingly, the role of stomatal conductance was insignificant. These results will guide future breeding efforts towards more O₃^-tolerant cultivars in China, while strategies for implementing control measures of regional O₃ pollution are being imple-mented. Overall, these results suggest that present ambient O₃ pollution is a serious concern for soybean in China, which highlights the urgent need for policy-making actions to protect this critical staple food.