A physiological explanation for the canopy nitrogen requirement of winter wheat
Nitrogen (N) fertiliser is one of the most important agronomic inputs and yet the application recommendations for winter wheat (Triticum aestivum L.) still remain imprecise. This increases costs both to the wheat grower and to the environment. An understanding of the canopy nitrogen requirement (CNR) is required before any improvements in fertiliser recommendations can be made. The CNR is defined here as the minimum amount of N required to produce and maintain a canopy and be efficient in light capture and conversion. This thesis aimed to provide a physiological explanation for this requirement in winter wheat, based on canopy structure and radiation geometry, to test the hypothesis that CNR can be predicted from canopy architecture. Variation in CNR was predicted from canopy architecture using data from the literature and a series of principles developed here. The prediction for the photosynthetic N requirement of the laminae and leaf sheath was based on the light distribution, modelled by a form of Beer's Law, and maximising N use efficiency (NUE). The structural N in the true stem was predicted from stem dry weight, stem area and the assumption that 0.3% of stem dry weight is structural N. Field experiments in 1997/8 and 1998/9 were designed to test these predictions by creating a wide range of canopy architectures through three seed rates (20, 320 and 640 seeds m-2) and two varieties (Soissons and Spark). In the 1998/9 experiment there was also a low fertiliser N treatment to reduce the amount of luxury N uptake. The CNR of any particular treatment was stable with canopy development and increased canopy size through depth and was an average of 2.2 g m-2; lower than the original proposed CNR of 3.0 g m-2. The CNR of the low seed rate canopy was also greater than the conventional seed rate and Soissons unexpectedly had a greater CNR than Spark. The results confirmed that the green area, light extinction coefficient (k) and incident light could explain the light flux and its distribution through the canopy. However, in the low seed rate and early growth, Beer's Law appeared not to hold because full ground cover was not achieved. Leaf N requirement decreased linearly with increased canopy size through depth and most canopies distributed N to maximise NUE. The results suggested that of the 50% of total N in the stem, 35% was photosynthetic N in the leaf sheath, 25% was structural in the true stem and the remaining 40% was transport, metabolic, storage and luxury N. The photosynthetic N requirement was overestimated in the prediction indicating that the leaf sheath had a lower N requirement than the lamina. Direct measurements of structural N requirement in the stem could not be made but there was supporting evidence for the relationship with canopy architecture. It is suggested that these principles could be used by growers to predict the CNR from canopy characteristics and by breeders to identify traits that could improve yield. Further detailed analysis of photosynthetic requirements and function of N in the stem would allow the development of a more quantitative prediction scheme, which would be the next step to greater precision in fertiliser recommendations.