Diverse concepts of breeding for nitrogen use efficiency

Cropping systems require careful nitrogen (N) management to increase the sustainability of agricultural production. One important route towards enhanced sustainability is to increase nitrogen use efficiency. Improving nitrogen use efficiency encompasses increasing N uptake, N utilization efficiency, and N harvest index, each involving many crop physiological mechanisms and agronomic traits. Here, we review recent developments in cultural practices, cultivar choice, and breeding regarding nitrogen use efficiency. We add a comparative analysis of our own research on designing breeding strategies for nitrogen use efficiency in leafy and non-leafy vegetables, literature on breeding for nitrogen use efficiency in other vegetables (cabbage, cauliflower), and literature on breeding for nitrogen use efficiency in grain crops. We highlight traits that are generic across species, demonstrate how traits contributing to nitrogen use efficiency differ among crops, and show how cultural practice affects the relevance of these traits. Our review indicates that crops harvested in their early or late vegetative phase or reproductive phase differ in traits relevant to improve nitrogen use efficiency. Headforming crops (lettuce, cabbage) depend on the prolonged photosynthesis of outer leaves to provide the carbon sources for continued N supply and growth of the photosynthetically less active, younger inner leaves. Grain crops largely depend on prolonged N availability for uptake and on availability of N in stover for remobilization to the grains. Improving root performance is relevant for all crop types, but especially shortcycle vegetable crops benefit from early below-ground vigor. We conclude that there is sufficient genetic variation available among modern cultivars to further improve nitrogen use efficiency but that it requires integration of agronomy, crop physiology, and efficient selection strategies to make rapid progress in breeding. We also conclude that discriminative traits related to nitrogen use efficiency better express themselves under low input than under high input. However, testing under both low and high input can yield cultivars that are adapted to low-input conditions but also respond to high-input conditions. The benefits of increased nitrogen use efficiency through breeding are potentially large but realizing these benefits is challenged by the huge genotype-byenvironment interaction and the complex behavior of nitrogen in the cropping system.