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Abstract. A substantial body of empirical evidence exists to suggest that elevated O3 levels are causing significant impacts on wheat yields at sites representative of highly productive arable regions around the world. Here we extend the DO3SE model (designed to estimate total and stomatal O3 deposition for risk assessment) to incorporate a coupled Anet–gsto model to estimate O3 uptake; an O3 damage module (that impacts instantaneous Anet and the timing and rate of senescence); and a crop phenology, carbon allocation, and growth model based on the JULES-crop model. The model structure allows scaling from the leaf to the canopy to allow for multiple leaf populations and canopy layers. The DO3SE-Crop model is calibrated and parameterised using O3 fumigation data from Xiaoji, China, for the year 2008 and for an O3-tolerant and sensitive cultivar. The calibrated model was tested on data for different years (2007 and 2009) and for two additional cultivars and was found to simulate key physiological variables, crop development, and yield with a good level of accuracy. The DO3SE-Crop model simulated the phenological stages of crop development under ambient and elevated O3 treatments for the test datasets with an R2 of 0.95 and an RMSE of 2.5 d. The DO3SE-Crop model was also able to simulate O3-induced yield losses of ∼11 %–19 % compared to observed yield losses of 12 %–34 %, with an R2 of 0.68 (n=20) and an RMSE of 76 g m−2. Additionally, our results indicate that the variance in yield reduction is primarily attributed to the premature decrease in carbon assimilation to the grains caused by accelerated leaf senescence, which is brought forward by 3–5 d under elevated O3 treatments. 

Abstract. Ozone (O3) air pollution is well known to adversely affect both the grain and protein yield of wheat, an important staple crop. This study aims to identify and model the key plant processes influencing the effect of O3 on wheat protein. The DO3SE-Crop model was modified in this work to incorporate nitrogen (N) processes, and we parameterised the O3 effect on stem, leaf, and grain N using O3 fumigation datasets spanning 3 years and four O3 treatments. These modifications mean that the newly developed DO3SE-CropN model is the first crop model to include O3 effects on N processes, making it a valuable tool for understanding O3 effects on wheat quality. Our results show that the new model captures the O3 effect on grain N concentrations and the anthesis leaf and stem concentrations well, with an R2 of 0.6 for the increase in grain N concentration and an R2 of 0.3 for the decrease in grain N content under O3 exposure. However, the O3 effect on harvest leaf and stem N is exaggerated. Overestimations of harvest leaf N range from ∼20 % to 120 %, while overestimations of harvest stem N range from ∼40 % to 120 %. Further, a sensitivity analysis revealed that, irrespective of O3 treatment, early senescence onset (simulated as being ∼13 d earlier in the treatment with very high O3 vs. the low-O3 treatment) was the primary plant process affecting grain N. This finding has implications for the breeding of stay-green cultivars for maintaining yield, as well as quality, under O3 exposure. This modelling study therefore demonstrates the capability of the DO3SE-CropN model to simulate processes by which O3 affects N content and, thereby, determines that senescence onset is the main driver of O3 reductions in grain protein yield. The implication of the sensitivity analysis is that breeders should focus their efforts on stay-green cultivars that do not experience a protein penalty when developing O3-tolerant lines, to maintain both wheat yield and nutritional quality under O3 exposure. This work supports the second phase of the Tropospheric Ozone Assessment Report (TOAR) by investigating the impacts of tropospheric O3 on wheat, with a focus on wheat quality impacts that will subsequently affect human nutrition.