Do variations in local leaf irradiance explain changes to leaf nitrogen within row maize canopies?

Abstract

Numerous studies have dealt with the relationship between leaf nitrogen content and leaf irradiance. However, most of them refer to dense stands presenting reduced horizontal heterogeneity of foliage distribution. Both gradients of leaf nitrogen and leaf irradiance related to canopy depth are significant under such conditions, and modelling radiative exchange using a turbid-medium analogy and dividing the canopy into vegetation layers is sufficient. Conversely, row crops such as maize are characterized by strong horizontal heterogeneity of foliage distribution and the one-dimensional (1D) approach may be unsuitable. We thus modelled the three-dimensional (3D) geometry of maize canopies with varying densities and at different developmental stages using plant digitizing under field conditions. The nitrogen content per unit area of each leaf part was obtained subsequently by nitrogen analysis. We next calculated radiative exchange using a 3D volume-based approach within the canopies in order to estimate local leaf irradiance on a daily integration scale. Vertical gradients in leaf nitrogen content per unit area observed in dense stands during the vegetative phase corresponded largely to those reported in the literature. We also identified significant gradients in nitrogen content along the leaves, which had not before been clearly demonstrated. Our study shows that local light climate during plant development plays a major role in leaf nitrogen distribution and remobilization. Moreover, brutal plant thinning involves rapid changes in leaf nitrogen partitioning. It is concluded that taking account of the 3D heterogeneity of nitrogen and irradiance distribution may have implications for modelling crop photosynthesis and production.