Effects of Fertilizer Rates on Growth of Brassica napus L.

The edible oil self-sufficiency rate of China is only approximately 30%, with rapeseed as the largest source of edible oil, contributing more than half of the total edible vegetable oil. Therefore, it is very important to achieve high rapeseed yield to ensure supply security. Fertilizer is one of the key factors affecting rapeseed yield and quality, and the appropriate use of fertilizer is essential for ensuring both high yield and high quality. To investigate the effects of fertilizer rates on the growth, yield, and quality of rapeseed, 18 rapeseed varieties were grown under two fertilizer input levels: normal fertilizer (NF, 15-15-15 NPK compound fertilizer at 600 kg·ha−1) and high fertilizer (HF, 15-15-15 NPK compound fertilizer at 1200 kg·ha−1). Agronomic traits, physiological indicators, yield, and quality of these varieties were systematically analyzed in this study. In addition, transcriptomic data from stems collected at the silique stage were used to identify key differentially expressed genes (DEGs), and the correlations between the expression levels of these candidate genes, physiological indicators, agronomic traits, and yield were examined. Compared with the NF treatment, the HF treatment promoted rapeseed growth during the bolting stage (except for root collar diameter). At harvest, 27.8% of the rapeseed varieties showed a significant increase in the number of effective branches per plant, while 33.3% exhibited significant increases in both the number of effective siliques per plant and the yield per plant. Among these varieties, Z01, Z02, Huayouza 50, and Huayouza 652 showed the greatest overall yield increases. The oil content decreased in 88.9% of the varieties, whereas the unsaturated fatty acid content increased in 77.8%, and the protein content increased in 94.4% of the varieties. Under HF treatment, superoxide dismutase activity in leaves at the bolting stage and peroxidase activity in leaves at the initial flowering stage both increased significantly (by 19.07% and 10.41%, respectively). The content of thiobarbituric acid-reactive substances (expressed as malondialdehyde equivalents) in stems at the initial flowering stage increased significantly (by 138.39%). In addition, catalase activity in leaves at the 3- to 4-leaf stage reached its highest value among all treatments (27.31 U·g−1·min−1·FW). The KEGG enrichment analysis indicated that the DEGs were significantly enriched in secondary metabolite biosynthesis, plant hormone signal transduction, plant–pathogen interactions, and metabolic pathways. Through screening against the NCBI database and subsequent validation by RT-qPCR, we ultimately identified four candidate DEGs: BnaC05g02370D, BnaC09g49910D, BnaC03g45470D, and BnaA04g00130D. Among these four varieties, catalase activity and malondialdehyde content in rapeseed stems at the initial flowering stage showed significant positive correlations with yield (r = 0.738 and r = 0.894, respectively). BnaC09g49910D maintained a consistently positive correlation with yield under both the HF and NF treatments (r = 0.977 and r = 0.806, respectively). In contrast, the direction of the association for BnaC05g02370D reversed depending on the fertilizer level (r = −0.858 under HF vs. r = 0.443 under NF). Catalase activity and the content of thiobarbituric acid-reactive substances in stems at the initial flowering stage and the expression levels of BnaC05g02370D and BnaC09g49910D in stems at the silique stage are associated with yield to varying degrees. These indicators can be used as candidate traits for assessing rapeseed yield potential under different fertilizer input levels.