Maize radiation use-efficiency response to optimally distributed foliar-nitrogen-content depends on canopy leaf-area index

Canopy foliar nitrogen content is an important determinant of crop radiation use efficiency (RUE). Previous studies suggested that nitrogen-deficient maize (Zea mays L.) crops may face a trade-off between light interception and RUE. Quantifying the relevance of this trade-off, however, demands for analytical approaches on the response of canopy photosynthesis to foliar nitrogen content. In this study we set up a theoretical framework based on canopy photosynthesis modelling to determine for maize (i) whether RUE response to total canopy foliar nitrogen (TCFN) is affected by canopy leaf area index (LAI), within a range of conventional TCFN and LAI values, and (ii) the relative advantage, in terms of crop RUE, of optimal vs. uniform foliar nitrogen distribution within the canopy. Vertical profiles of optimally-distributed foliar nitrogen became steeper with increases in LAI and reductions in TCFN. In addition, the higher the LAI, the more pronounced the reductions in RUE in response to reductions in TCFN. Accordingly, the relative advantage of optimal vs. uniform foliar nitrogen distribution increased with canopy LAI and increased with the deficiency of foliar nitrogen. With a small effect of LAI, the optimal profiles of critical-TCFN (i.e. the minimal TCFN to achieve the 95% of the maximal RUE) appeared to be appropriately described by a single curve as a function of the light distribution profile within the canopy. Results of this study highlight the importance of coupling the canopy LAI to the nitrogen availability (or vice-versa) among the crop management decisions oriented to maximize crop production while reducing the negative environmental impacts of nitrogen inputs. We also discuss the potential utility of these results as reference values in diagnosis or monitoring the nitrogen status of maize.