The model NEMA (Nitrogen Economy Model within plant Architecture) simulates nitrogen fluxes within a wheat plant (Triticum aestivum) after flowering. Bertheloot et al. show that the model is able to predict the time and spatial patterns of the distribution of N between organs in close agreement with observations from field experiments. In simulations, root capacity to import carbon, rather than carbon availability, limits N acquisition and ultimately N accumulation in grains, while Rubisco turnover intensity mostly affects dry matter accumulation in grains. NEMA can thus provide the basis for comprehensive mechanistic modelling of N economy at plant and crop scales.