Crop nitrogen demand and canopy area expansion in winter wheat during vegetative growth

The effect of nitrogen (N) on crop growth and productivity is mediated through effects on both light interception (green crop area) and radiation use efficiency (RUE). The effects of N nutrition on these factors were studied using data on green area index (GAI), above-ground dry matter and N uptake from growth analysis measurements in winter wheat from a number of experiments in Denmark with different application rates of N. Only measurements taken prior to anthesis were used in the statistical analyses. The N uptake was found to be proportional to GAI, and to have an additional curvilinear response to dry matter implying decreasing N concentration with increasing dry matter. This supports the hypothesis that nitrogen is associated with both the green surfaces of the crop canopy and with the dry matter component. A model of GAI expansion is presented incorporating three limiting factors: an exponential increase in GAI in thermal time, a minimum leaf area ratio, and a minimum ratio of GAI to N content in above-ground dry matter. This simple function has potential to be used as a tool for targeting timing and rates of N fertilisation in relation to a desired development of GAI. Such N application strategies should also consider the nitrogen nutrition index (NNI), which was defined based on the relationship between N uptake and both GAI and dry matter. The response of RUE to NNI showed a curvilinear response with a tendency for saturation at high NNI. The design of N application strategies should therefore ensure that low NNI is avoided during the most productive periods in the growing season.