24-Epibrassinolide alleviates chilling injury in loquat fruit by modulating respiratory metabolism, pyridine nucleotide homeostasis, and cellular energy status

Loquat fruit are highly susceptible to chilling injury (CI) during low-temperature storage. This study investigated the effects of postharvest treatment with 15 μM 24-epibrassinolide (EBR) on CI, respiratory metabolism, pyridine nucleotide homeostasis, and adenylate energy status in ‘Guifei’ loquat fruit during storage at 2 °C for 20 d. Compared with control fruit, fruit treated with 15 μM EBR showed a lower CI index, reduced malondialdehyde (MDA) accumulation and relative electrolyte leakage, and an attenuated cold-induced increase in respiration rate. Fruit treated with 15 μM EBR also exhibited lower activities of phosphoglucose isomerase (PGI), succinate dehydrogenase (SDH), cytochrome c oxidase (CCO), ascorbate oxidase (AAO), but higher polyphenol oxidase (PPO), and alternative oxidase (AOX), but higher combined activity of glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase (G6PDH + 6PGDH) and higher NAD kinase (NADK) activity. These changes were accompanied by lower contents of oxidized nicotinamide adenine dinucleotide (NAD⁺) and NADH but higher contents of oxidized nicotinamide adenine dinucleotide phosphate (NADP⁺) and NADPH, indicating a redistribution of pyridine nucleotide pools toward NADP(H). Compared with control fruit, the 15 μM EBR-treated fruit also displayed higher Ca²⁺-, H⁺-, and Mg²⁺-ATPase activities associated with the plasma, vacuolar, and mitochondrial membranes. The 15 μM EBR-treated fruit retained higher adenosine triphosphate (ATP) and adenosine diphosphate (ADP) contents, restricted adenosine monophosphate (AMP) accumulation, and sustained a higher energy charge. Pearson correlation and Mantel analyses further revealed close associations between CI severity and membrane damage, respiration, pyridine nucleotide homeostasis, and adenylate energy status. Collectively, 15 μM EBR treatment was associated with reduced CI and coordinated changes in respiratory enzyme activities, pyridine nucleotide status, and adenylate energy status. These changes may contribute to the maintenance of membrane homeostasis and lower CI severity during cold storage.