Quantification of ozone exposure- and stomatal uptake-yield response relationships for soybean in Northeast China

High ground-level O₃ is a new threat to agricultural production in Northeast China with the increasing ambient O₃ concentration. Little is known about its impacts on soybean production in this key agricultural region. Accumulated O₃ exposure-response and stomatal O₃ flux-response relationships were developed during two continuous growing seasons to evaluate O₃-induced yield reduction of four typical soybean cultivars in Northeast China. Results showed that critical levels of AOT40 (accumulated hourly O₃ concentrations over a threshold of 40nmol·mol^-1), SUM06 (sum of all hourly average O₃ concentrations over 0.06μmol·mol^-1) and W126 (sum of O₃ concentrations weighted by a sigmoidal function) in relation to 5% reduction in relative seed yield were 4.2, 7.6 and 6.8μmol·mol^-1·h, respectively. The effect of O₃ on plants was influenced by leaf position in canopy. An improved Jarvis stomatal conductance model including leaf (node) position fitted well with field measurements. The best linear relationship between stomatal O₃ flux and relative soybean yield was obtained when phytotoxic ozone dose was integrated over a threshold of 9.6nmol·m^-2·s^-1 (POD_9.6) to represent the detoxification capacity of soybean. POD_9.6 and the commonly used POD₆ in relation to 5% reduction in relative seed yield of soybean were 0.9mmol·m^-2 and 1.8mmol·m^-2, respectively. O₃ concentrations above ~38nmol·mol^-1 contributed to POD_9.6 and caused seed yield loss in soybean. Current annual yield loss of soybean at ambient O₃ was estimated to range between 23.4% and 30.2%. The O₃ dose-response relationships and corresponding thresholds obtained here will benefit regional O₃ risk assessment on soybean production in Northeast China.