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 implemented.Overall,these results suggest that present ambient O3 pollution is a serious conc for soybean in China,which highlights the urgent need for policy-making actions to protect this critical staple food.