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. We modified the DO₃SE-Crop model to incorporate nitrogen (N) processes, and parameterised the O₃ effect on stem, leaf and grain N using O₃ fumigation datasets spanning 3 years and 4 O₃ treatments. Our results show the new model captures the O₃ effect on grain N concentrations, and anthesis leaf and stem concentration, well. However, the O₃ effect on harvest leaf and stem N is exaggerated. Further, a sensitivity analysis revealed that, irrespective of O₃ treatment, accelerated senescence onset was the primary plant process affecting grain N. 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.