Effects and Mechanism of Nitrogen Regulation on Seed Yield and Quality of Rapeseed (Brassica napus L.)

Appropriate nitrogen is required and important in grain yield formation of crops. To elucidate nitrogen regulation of seed yield and quality of rapeseed (Brassica napus L.), field trials were consecutively conducted in two years with three nitrogen levels of 0, 180, and 240 kg ha−1 (the N0, N180, and N240 treatments). The nitrogen application (N-app) induced increasing trend in the nitrogen accumulation in flowering plants (N-acc), number of siliques per plant (silique-num), number of branches per plant (branch-num), number of seeds per silique (seed-num), and seed yield of rapeseed; there were significant correlational relationships between these indexes (excepting seed-num). The N-app, N-acc, and silique-number showed higher effects on the seed yield. The effect of N-app was mainly achieved through influence on the silique-num, branch-num, and seed-num. When the N-app was increased from 180 to 240 kg ha−1, the nitrogen utilization efficiency (NUE) and the partial productivity of nitrogen fertilizer (PPN) of the rapeseed varieties tested showed a decreasing trend; the NR (nitrate reductase) gene expression level and the NR and GS (glutamine synthetase) activity in leaves was significantly increased under the N180 and N240 treatments compared to the N0 treatment, which achieved peak values at 180 kg ha−1 of N-app. The N-app hardly influenced the seed quality, as well as the gene expression and activity of the enzymes ACCase (acetyl-CoA carboxylase), FAD2 (oleic acid desaturase), and FAD3 (omega-3 fatty acid desaturase) in young seed. In conclusion, N-app induced significant increase in seed yield of rapeseed, the NR gene expression level and the NR and GS activity in leaves was improved; the NUE of rapeseed variety showed decreasing trend with increase in N-app level; while N-app hardly influenced the seed quality.