A profiling approach of the natural variability of foliar N remobilization at the rosette stage gives clues to understand the limiting processes involved in the low N use efficiency of winter oilseed rape

Genotypic variations in Brassica napus reveal four profiles of response to nitrate limitation and that genotypes characterized by efficient N remobilization and N utilization have the highest N use efficiency.Oilseed rape, a crop requiring a high level of nitogen (N) fertilizers, is characterized by low N use efficiency. To identify the limiting factors involved in the N use efficiency of winter oilseed rape, the response to low N supply was investigated at the vegetative stage in 10 genotypes by using long-term pulse-chase N-15 labelling and studying the physiological processes of leaf N remobilization. Analysis of growth and components of N use efficiency allowed four profiles to be defined. Group 1 was characterized by an efficient N remobilization under low and high N conditions but by a decrease of leaf growth under N limitation. Group 2 showed a decrease in leaf growth under low N supply that was associated with a low N remobilization efficiency under both N supplies despite a high remobilization of soluble proteins. In response to N limitation, Group 3 is characterized by an increase in N use efficiency and leaf N remobilization compared with high N that is not sufficient to sustain the leaf biomass production at a similar level to non-limited plants. Genotypes of Group 4 subjected to low nitrate were able to maintain leaf growth to the same level as under high N. The profiling approach indicated that enhancement of amino acid export and soluble protein degradation was crucial for N remobilization improvement. At the whole-plant level, N fluxes revealed that Group 4 showed a high N remobilization in source leaves combined with a better N utilization in young leaves. Consequently, an enhanced N remobilization limits N loss in fallen leaves, but this remobilized N needs to be efficiently utilized in young leaves to improve N use efficiency.