Synergistic Effects of Uniconazole and 2, 4-Epibrassinolide on Seed Productivity in High-Density Rapeseed

Currently, effective strategies to enhance yield under high-density conditions in winter rapeseed production are lacking. Uniconazole and 2, 4-epibrassinolide are important plant growth regulators (PGRs) modulating crop development and yield formation. Therefore, flower bud differentiation and non-carbohydrate metabolism were analyzed under two rapeseed planting densities (D3: 45 × 10⁴ plants ha^-1, D5: 75 × 10⁴ plants ha^-1) to evaluate the underlying mechanisms of these PGRs on the yield. Under controlled conditions (double-distilled water), increasing planting density reduced the yield, total revenue, and the net profit of rapeseed plants. The PGR S3307 (uniconazole at the seedling stage), BR (2, 4-epibrassinolide at the flowering stage), and S-BR (combined S3307 and BR treatments) significantly enhanced these parameters under both D3 and D5 conditions versus the control. Notably, the yield increased by 13.8% and 15.6% (D3), 26.2% and 27.8% (D5) in S-BR compared to the control, and increased by 2.9% and 4.7% at D5 than D3 in S-BR, over two growing seasons, respectively. Furthermore, our findings indicated that compared to the control, S3307 accelerated flower bud differentiation by increasing the levels of cytokinin and abscisic acid. Meanwhile, both BR and S-BR significantly enhanced the activities of α-amylase, β-amylase, sucrose phosphate synthetase, and cell wall invertase. These improvements were associated with an enhanced translocation of nonstructural carbohydrates (NSCs), their translocation efficiency, and their contribution to yield. Moreover, the increases in NSC translocation, those enzyme activities, yield, and net profit under D5 were greater than those under D3 in S-BR. Overall, S-BR effectively promoted flower bud differentiation and NSC transport, resulting in increasing yield, numbers of effective branches bearing rapeseed siliques, and siliques per plant under high-density (D5) conditions.