Genotypic variation in nitrogen utilization efficiency in oilseed rape is related to the coordination of leaf senescence and root N uptake during reproductive stage

Due to their low nitrogen utilization efficiency (NUtE) oilseed rape (OSR) crops require large quantities of nitrogen (N) fertilizer to achieve acceptable economic yields. To dissect the underlying mechanisms of oilseed NUtE, we investigated the relationships between NUtE, N uptake, N remobilization and leaf senescence by growing three N-efficient (genotypes with high NUtE at deficient N supplies) and three N-inefficient (ones with low NUtE at deficient N supplies) genotypes in pot experiments. These genotypes showed similar plant N uptake before flowering and similar post-anthesis N remobilization from leaf and stem, but after flowering N-efficient genotypes exhibited higher root N uptake than N-inefficient genotypes. Besides, the pod biomass and leaf N concentration of N-efficient genotypes were higher than those of N-inefficient genotypes at 50 days after flowering, and we observed delayed leaf senescence in N-efficient genotypes. Correspondingly, gene transcripts involved in photosynthesis and carbon metabolism were abundant in green mature leaves of N-efficient genotype, while the expression of genes involved in ubiquitin mediated proteolysis pathway were more abundant in N-inefficient mature leaves. The stay-green phenotype of the N-efficient OSR after flowering provides sufficient photosynthate for root N uptake activity and pod development, and in turn, the prolonged post-anthesis N uptake provides more N to pods, leading to less leaf N remobilization requirement. This work has identified traits important for breeding OSR cultivars with improved NUtE.