Post-flowering Nitrogen Source–Sink Relationship Underlying Mechanisms Explain the Genotypic Variation in Seed N Accumulation of Rapeseed Genotypes

Seed nitrogen (N) in rapeseed genotypes originates from remobilized N before flowering and post-flowering N uptake. In a preliminary study, we found that post-flowering N uptake led to genotypic variation in seed N accumulation of the rapeseed genotypes by subtraction method. However, the contribution of root N to seed N was ignored. In this experiment, a pot experiment and a ¹⁵N label hydroponic culture with N-efficient and the N-inefficient rapeseed genotypes were conducted to characterize the differences in post-flowering N uptake, remobilized N before flowering, the contribution of root, stem, and leaf to seed N before flowering. Results showed there were no significant differences of apparent remobilization efficiency in root, stem, and leaf between the N-efficient and the N-inefficient rapeseed genotypes in the pot experiment. Additionally, no significant differences of the amount of remobilized N before flowering and seed N from remobilized N before flowering were recorded, but the N-efficient rapeseed genotypes exhibited higher post-flowering N uptake, leading to greater seed N accumulation, harvest index, N harvest index, and NUtE. On the other hand, we believed the leaf contributed the most N to seed before flowering under high N level, while the stem acted as the first N sources at the low N treatment among the rapeseed genotypes. Taken together, we used ¹⁵N-labeling technique accurately verified post-flowering N uptake led to genotypic variation in seed N accumulation, and the amount of remobilized N before flowering in leaves (high N) and stems (low N) contributed the most N to seed.