Annals of Botany

• Background and Aims Nitrogen partitioning within stands has been described fairly comprehensively, especially for C₃ plants in dense stands where the horizontal heterogeneity of foliage distribution is relatively small. Nitrogen has been shown to be distributed vertically and in parallel to light, maximizing carbon assimilation and stand productivity. Conversely, row crops such as maize (C₄ plants) are characterized by strong horizontal heterogeneity of foliage distribution, and a three-dimensional (3D) approach is required to investigate the combined effect of spatial distribution of nitrogen and light on canopy photosynthesis.

• Model The 3D geometry of maize canopies was modelled with varying densities and at different developmental stages using plant digitizing under field conditions. For lamina parts, photosynthesis was measured and nitrogen content per unit area (N_a) was described from analysis of nitrogen content per unit mass (N_m) and dry mass per unit area (M_a). Hyperbolic relationships between photosynthesis at irradiance saturation (P_max) and N_a were established as well as a linear relationship between dark respiration (R_d) and N_a, whereas quantum efficiency () was found to be independent of N_a.

• Key Results and Conclusions N_m, M_a and N_a were shown to change over time vertically (i.e. between laminae), which has been largely reported previously, and horizontally (i.e. within laminae), which has scarcely been described previously. Even if M_a played a major role in N_a, a strong relationship between N_a and M_a could not be demonstrated, whereas several previous studies have found that N_a was essentially related to M_a rather than N_m. From simulations of radiative exchange using a 3D volume-based approach and lamina photosynthesis using a hyperbola, it was shown that real patterns of N_a partitioning could increase daily crop photosynthesis by up to 8  % compared with uniform patterns of N_a, especially for the earliest stages of stand development.