Optimal K+/Na+ ratios under different salinity levels synergistically improve rapeseed (Brassica napus L.) yield by enhancing bud‑flower‑silique conversion via hormonal, carbon‑nitrogen and antioxidant regulation

To investigate the regulatory effects and physiological mechanisms of different K + /Na + ratios on floral bud differentiation, silique formation, and yield in rapeseed ( Brassica napus L.) under salt stress and to identify the optimal K + /Na + ratio under different salt concentrations. Three total salt levels were established: 0.15% (S1), 0.30% (S2), and 0.45% (S3), with different K + /Na + ratios applied at each concentration. Developmental progression, yield components, and leaf physiological parameters were dynamically monitored. (1) Under salt stress, floral bud differentiation exhibited “early initiation yet delayed development,” with moderate and high salinity (S2, S3) prolonging the emergence-to-flowering interval and total growth duration. (2) Salinity elevated leaf Na + and reduced the K + /Na + ratio; optimal K + /Na + ratios restored ionic homeostasis. (3) The optimal K + /Na + ratio upregulated zeatin riboside (ZR) and gibberellins (GAs), downregulated abscisic acid (ABA), increased osmolyte (soluble protein, free amino acid, and soluble sugar) contents and antioxidant enzyme (superoxide dismutase, peroxidase, and catalase) activities, and reduced malondialdehyde (MDA) accumulation. (4) Salt stress reduced yield primarily by decreasing the number of siliques. The optimal K + /Na + ratio improved floral bud-to-open flower and open flower-to-silique conversion efficiency—low and moderate salinity favored the former, and high salinity favored the latter—thereby significantly increasing yield. (5) PLS PM revealed that K + /Na + homeostasis indirectly promoted silique formation and yield by positively regulating carbon–nitrogen metabolism while negatively modulating hormone and antioxidant metabolism. In conclusion, an appropriate K + /Na + ratio enhances conversion efficiency and yield by reestablishing ionic homeostasis, optimizing hormonal balance, and strengthening carbon–nitrogen metabolism and antioxidant capacity. The optimal ratios under low, moderate, and high salinity were 1/7, 1/13, and 1/15, respectively, providing a theoretical basis for zone-specific potassium-mediated salt mitigation in rapeseed cultivation on saline alkali land.