Reducing nitrogen application at high planting density enhances secondary cell wall formation and decreases stem lodging in rapeseed

Reducing the rate of nitrogen fertilizer (N rate) and achieving high plant density is an effective and economically beneficial approach to enhance rapeseed productivity. However, it is unclear how N rate affects stem lodging in rapeseed directly-sown at high densities. In this study, the effect of different N rates on stem lodging at high plant density was examined in field trial and pot experiments using two modern hybrid rapeseed varieties, Fengyou520 (FY520) and Huayouza62 (HYZ62). Increasing N fertilizer from low level to adequate level significantly increased yield by 39% in FY520 and 43% in HYZ62. However, with elevated N yield increases were limited by an increase in stem lodging. At T1 (the first inflorescence branch below the apex has the first flowers open) phloroglucinol-HCl staining for lignin was weaker under high N (N4: 4 g N pot−1) compared with adequate N (N2: 2 g N pot−1), and was correlated with weaker stem strength. Differences in stem lignin were also associated with variations in gene expression as revealed by transcriptomic profiling of middle stems at T1. Differentially expressed genes indicated significant enrichment for cell wall. Several genes encoding known transcriptional regulators of secondary cell walls (SCWs) biosynthesis and deposition were down-regulated with high N compared with adequate N. Many genes putatively involved in lignin biosynthesis, cellulose biosynthesis and assembly and xylan biosynthesis are down-regulated with high N, corresponding to lower accumulation of lignin, cellulose and hemicellulose in these plants. These findings not only provide us with gene candidates potentially affecting stem lodging in direct sown rapeseed, but also reveal that reducing N at high plant density enhances biosynthesis of stem cell walls to alleviate stem lodging.