INTRA-LEAF GRADIENT OF ASSIMILATION RATE AND OPTIMAL ALLOCATION OF CANOPY NITROGEN - A MODEL ON THE IMPLICATIONS OF THE USE OF HOMOGENEOUS ASSIMILATION FUNCTIONS

A model on the allocation of nitrogen available for the construction of photosynthetic apparatus in leaves in different morphological positions is presented. The allocation pattern is calculated under the assumption that nitrogen distribution is optimised in order to maximise daily whole-plant assimilation. The solution is fairly sensitive to the assimilation function applied. It is shown that assimilation functions homogeneous in irradiance and nitrogen imply assimilation gradients and light-saturation characteristics of the whole canopy which contradict experimental findings. An equation for the calculation of electron transport rates as a function of the intra-leaf gradient of the photosynthetic photon flux density is presented. This inhomogeneous assimilation function leads to substantially different predictions of nitrogen allocation which reproduce a wider array of observed allocation patterns. The results presented in this paper support the thesis that the intra-leaf gradient of photosynthetic photon flux density and self-shading of the thylakoids need to be considered if the assimilation flux is to be modelled as a function of light as well as nitrogen availability on a mechanistic basis.