Evaluating tropospheric ozone effects on wheat productivity using the CERES-O3 model

Tropospheric ozone (O₃) is a major air pollutant that reduces crop productivity by altering photosynthesis, stomatal conductance, and carbon allocation. Process-based crop models are useful for assessing these impacts under changing environmental conditions. In this study, we developed and evaluated the CERES-O3 model to simulate and quantify O₃ impacts on wheat physiology and biomass production, using a flux-based approach. The model incorporates O₃ effects on carboxylation and electron transport rates, as a function of the phytotoxic ozone dose (POD₆). CERES-O3 was calibrated using data from a free-air fumigation experiment in Grignon, France, in 2009, with four O₃ treatments, and evaluated using independent field experiments from 2003 and 2004 and FR-Gri ICOS ecosystem data from 2006, 2009, and 2012. CERES-O3 reproduced the O₃-induced decline in photosynthesis and stomatal conductance. Agreement with ICOS GPP was good, with annual regression slopes ranging from 0.84 to 1.05 and R² values from 0.79 to 0.85. Simulated canopy stomatal O₃ deposition accounted for 33-60% of measured total ecosystem deposition during cropping periods. Grain yield and above-ground biomass showed variable agreement, with particularly large yield underestimation in 2006 associated with strong simulated water stress. The fitted POD₆-relative-yield relationship (RY = 1.003 - 0.041 POD₆, R² = 0.93) was consistent with established wheat flux-response relationships. Across ICOS years, the derived relationship indicated an estimated mean O₃ -associated yield reduction of 6.5%. Overall, CERES-O₃ provides an efficient framework for representing O₃ impacts on wheat productivity, while highlighting remaining uncertainties in canopy scaling and the representation of crop responses to water limitation.