Identifying traits to improve the nitrogen economy of wheat: Recent advances and future prospects

Nitrogen fertilizer represents a significant cost of production for the grower and may also have environmental impacts through nitrate leaching, use of fossil fuels for manufacture and application, and N2O emissions associated with denitrification. The development of N-efficient cultivars will be of economic benefit to farmers and will help to reduce environmental contamination associated with excessive inputs of N fertilizers. The physiological, metabolic and physio-chemico processes that may contribute to high N-use efficiency (NUE) and reduced N fertilizer inputs while maintaining an acceptable yield are reviewed and the prospects for application in breeding programs discussed. Promising N management strategies for high NUE are also considered. Traits operating from the cellular to the whole-crop scale, including root traits, nitrate assimilation, N partitioning, leaf and canopy photosynthesis, senescence, grain N remobilization and grain protein content and composition, are discussed in relation to their optimization in both feed and bread-making cultivars. Promising traits for selection by breeders to increase NUE are identified, and approaches for their integration at the whole plant level discussed. It is concluded that: (i) increased root length density (RLD) at depth, (ii) a high capacity for N accumulation in the stem, potentially associated with a high maximum N-uptake rate, (iii) low leaf lamina N concentration, (iv) more efficient post-anthesis remobilization of N from stems to grain, but less efficient remobilization of N from leaves to grain, both potentially associated with delayed senescence, and (v) a reduced grain N concentration may be of particular value for increasing NUE in feed wheat cultivars; and (vi) for bread-making cultivars, high NUE may be associated with high capacities for uptake and assimilation of N, with high post-anthesis N remobilization efficiency and/or specific grain protein composition.