Postharvest 2,4-Epibrassinolide Treatment Delays Senescence and Increases Chilling Tolerance in Flat Peach

Chilling injury (CI) frequently occurs in postharvest flat peach fruit during cold storage, leading to quality deterioration and a reduced shelf life. Therefore, investigating the key factors involved in alleviating CI and developing effective preservatives are vital scientific issues for the industry. 2,4-Epibrassinolide (EBR) is a crucial endogenous hormone involved in plant response to both biological and environmental stressors. At present, most studies focus on the mechanisms of mitigating CI using a single concentration of EBR treatment, while few studies focus on the effects varying EBR concentrations have on CI. The purpose of this research is to explore the effects of varying concentrations of EBR on the postharvest quality and cold resistance of peach fruit, thereby establishing a basis for refining a technical framework of environmentally sustainable strategies to mitigate postharvest CI. The results show that EBR treatment effectively inhibits the generation of reactive oxygen species (ROS) and malondialdehyde (MDA) by maintaining the activities of antioxidant enzymes such as superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT), thereby delaying the internal browning process of postharvest peaches. In addition, EBR treatment reduced the consumption of total phenolics by inhibiting the activities of polyphenol oxidase (PPO) and phenylalanine ammonia lyase (PAL). Experimental results identify that 5 μmol L−1 EBR treatment emerged as the most effective concentration for maintaining core postharvest quality attributes. It significantly delayed the decrease in firmness, reduced weight loss, effectively inhibited the production of H2O2 and O2−·, particularly during the early storage period, strongly restrained the activity of PAL, and maintained lower rot rates and internal browning indexes. While the 15 μmol L−1 EBR treatment enhanced antioxidant activity, increased total phenolic content at certain stages, and maintained higher soluble solids and acid content, its effects on key physical quality parameters, like firmness and weight loss, were less pronounced compared to the 5 μmol L−1 treatment.