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

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 (Asat, Jmax and Vcmax), and chlorophyll content (Chl). Flux-based metrics (PODy, 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 POD1 (Asat, Jmax and Vcmax) 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.