Surplus boron nutrition enhances accumulation in plant residues, canopy radiation interception and yield of winter direct seeding rapeseed in Southwestern China

Boron (B) plays a critical role in the yield formation of direct-seeded winter rapeseed (Brassica napus L.). However, current management of this nutrient remains suboptimal. We conducted a split-plot field experiment in Bijiang County, Guizhou, China, between 2020 and 2022 to address this issue. The study evaluated how different B application rates (0, 3.75, 7.5, 15, and 30 kg boron ha^-1) influenced yield formation, canopy radiation interception, and B allocation in two cultivars: the hybrid Huayouza 9 and the conventional Zhongshuang 11. Our results demonstrated that an application rate of 7.5-15 kg B ha^-1 led to a 28-39% increase in seed yield. When application rates exceeded 15 kg B ha^-1, 70.5-86.3% of the absorbed B accumulated in the stems and pericarps, which caused a 17% reduction in the B harvest index. The hybrid cultivar HZ9 demonstrated a superior remobilization efficiency that was 12.7-15.5% higher than ZS11 at the same input levels. A strong correlation was observed between canopy radiation interception during flowering and yield, explaining 82% of the variation. While optimal B rates improved interception by 18.3%, excessive B levels delayed senescence, which decreased pod-stage radiation capture by 8.3-11.7%. These findings indicate that returning plant residues to the soil could be an effective strategy to help maintain soil B levels in Southwest China and other B-deficient regions.