Melatonin alleviates chilling injury in ‘Kim Ju’ guava by maintaining mitochondrial function and suppressing mitochondria-mediated cell death

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Chilling injury (CI) is a major postharvest problem that limits the storage and marketability of ‘Kim Ju’ guava ( Psidium guajava L.) under low temperatures. This study investigated the effect of melatonin treatment on CI, mitochondrial function and cell death regulation during cold storage. Guava fruit were immersed in 0 - 200 µM melatonin for 10 min and stored at 8 ± 1 °C for 14 d, followed by 4 d at 25 ± 1 °C. The 100 µM melatonin treatment markedly reduced CI symptoms, maintained firmness and peel color, and lowered weight loss compared with controls. Melatonin (N-acetyl-5-methoxytryptamine) treatment suppressed the accumulation of superoxide radicals and hydrogen peroxide, while enhancing antioxidant enzyme activities in both cellular (SOD, CAT, APX and GPX activities) and mitochondrial (SOD, APX and GPX activities) fractions. Moreover, melatonin preserved mitochondrial integrity by reducing membrane permeability transition (MPTP) opening, cytochrome c leakage, and the decline of mitochondrial membrane potential (ΔΨm), as well as sustaining the activities of respiratory enzymes (NADH-DH, SDH and COX) and ATP content. The activities of caspase-8- and 9-like enzymes were attenuated, indicating suppression of mitochondria-mediated programmed cell death (PCD). These results demonstrate that melatonin alleviates CI in guava by reinforcing redox homeostasis, maintaining mitochondrial function, and modulating PCD pathways, thereby improving fruit storability and postharvest quality.