【Objective】This study systematically investigated the regulatory effects of 2, 4-epibrassinolide (EBR) on the postharvest storage quality of apple fruit. The physiological and molecular mechanisms of EBR delaying fruit softening, maintaining flavor quality, and enhancing antioxidant capacity were elucidated so as to offer a theoretical foundation and technical insight for developing novel plant-derived preservatives. 【Method】Using Luli apples as experimental material, fruits were treated by immersion in 3 μmol·L-1 EBR solution for 2 hours, with a distilled water group serving as the control. After treatment, fruits were stored at room temperature for 20 days. Samples were collected at 0, 3, 5, and 10 days to assess textural properties (firmness, brittleness, and flesh homogeneity), ethylene production rate, endogenous BR levels, and the expression of key BR signaling gene MdBZR1, as well as ethylene biosynthesis genes MdACS1 and MdACO1. Changes in cell wall composition (cellulose, protopectin, and soluble pectin) and the activities of related degrading enzymes (α-Gal, β-Gal, PG, PME, and β-GC) were analyzed. The dynamics of flavor compounds were evaluated, including starch, soluble sugars (glucose, fructose, and sucrose) and organic acids (malic acid, and citric acid). Additionally, the activities of antioxidant enzymes (SOD, POD, CAT, and APX), levels of reactive oxygen species ($\mathrm{O}_2^{\bar{.}}$ and H2O2), content of stress-related metabolites (proline, soluble protein, total phenols, and flavonoids), and malondialdehyde (MDA) accumulation were measured.【Result】EBR treatment significantly delayed apple fruit softening. Reductions in firmness, brittleness, and flesh homogeneity in the EBR group were 2.88%, 7.43%, and 4.63% lower than those in the control, respectively. Ethylene production was significantly suppressed during mid-to-late storage, with rates 37.11% and 19.35% lower than the control on days 5 and 10, respectively. Expression of ethylene synthesis-related genes MdACO1 and MdACS1 was notably downregulated. EBR treatment promoted endogenous BR accumulation, resulting in 22.17% higher than the control on day 10, and significantly upregulated MdBZR1 expression. Activities of cell wall-degrading enzymes were generally suppressed, and degradation of cellulose and protopectin was delayed by 29.57% and 16.32%, respectively, while soluble pectin accumulation was reduced by 53.00%. In terms of flavor, EBR treatment significantly increased the content of soluble sugars (glucose, fructose, and sucrose) and organic acids (malic acid, and citric acid), enhancing the fruit’s sweet-sour taste and nutritional quality. Moreover, EBR significantly boosted the activities of SOD, POD, CAT, and APX, reduced $\mathrm{O}_2^{\bar{.}}$ and H2O2 levels, lowered MDA accumulation, and increased the contents of proline, soluble protein, total phenols, and flavonoids, collectively improving the fruit’s antioxidant capacity and delaying senescence.【Conclusion】EBR treatment significantly improves the postharvest storage performance and marketable shelf life of apple fruit. This improvement is achieved through the synergistic action of multiple coordinated pathways, which include activating the BR signaling pathway, suppressing ethylene biosynthesis, retarding cell wall degradation and associated metabolic activities, regulating the balance of sugar and acid metabolism, and enhancing the antioxidant defense system. This study not only revealed the multifaceted physiological roles of EBR in fruit preservation but also provided important theoretical and practical support for the development of hormone-based green preservation technologies.
