Ozone exposure- and flux-based response relationships with photosynthesis of winter wheat under fully open air...

Five winter wheat cultivars were exposed to ambient (A-O₃) and elevated (E-O₃, 1.5 ambient) O₃ in a fully open-air fumigation system in China. Ozone exposure- and flux based response relationships were established for seven physiological variables related to photosynthesis. The performance of the fitting of the regressions in terms of R² increased when second order regressions instead of first order ones were used, suggesting that effects of O₃ were more pronounced towards the last developmental stages of the wheat. The more robust indicators were those related with CO₂ assimilation, Rubisco activity and RuBP regeneration capacity (A_sat, J_max and Vc_max), and chlorophyll content (Chl). Flux-based metrics (POD_y, Phytotoxic O₃ Dose over a threshold ynmolO₃m^-2s^-1) predicted slightly better the responses to O₃ than exposure metrics (AOTX, Accumulated O₃ exposure over an hourly Threshold of X ppb) for most of the variables. The best performance was observed for metrics POD_{1 (}A_sat, J_max and Vc_max) and POD₃ (Chl). For this crop, the proposed response functions could be used for O₃ risk assessment based on physiological effects and also to include the influence of O₃ on yield or other variables in models with a photosynthetic component.