Exogenous gibberellin and spermidine optimize flowering time and ornamental quality by regulating antioxidant capacity, soluble nutrient levels, and endogenous hormone balance of Primula forbesii

Primula forbesii Franch. is a biennial ornamental species increasingly used in flower landscape design, potted displays, and cut flower production. Nevertheless, studies on its flowering regulation remain limited. This study investigated the effects of gibberellin (GA3) and spermidine (Spd) on the flowering performance of P. forbesii, using flowering time, scape morphology, and physiological parameters as key evaluation indices. At the onset of floral bud differentiation, 15 foliar spray treatments were applied. The most effective treatment contained 200 mg·L-1 GA3 and 0.01 mmol·L-1 Spd, which extended flowering duration by 5 days and promoted earlier bud emergence. Notably, this treatment significantly enhanced scape traits compared to the distilled water treatment, increasing internode length between floral whorls, the main scape height, and diameter. The scape number was increased by 361.54%. At the full flowering stage, the combination of 200 mg·L-1 GA3 and 0.01 mmol·L-1 Spd reduced malondialdehyde content and peroxidase activity in petals, while enhancing superoxide dismutase and catalase activities, along with soluble protein and soluble sugar accumulation. Endogenous hormone profiling showed that the treatment significantly raised levels of GA3, indole-3-acetic acid, and zeatin riboside, while reducing abscisic acid content. These results demonstrate that the combined application of 200 mg·L-1 GA3 and 0.01 mmol·L-1 Spd effectively enhances ornamental quality and delays senescence in P. forbesii. The findings provide valuable insights into the hormonal regulation of flowering senescence in ornamental plants and may help guide future strategies for cut‑flower preservation and quality maintenance.