The potent role of endogenous melatonin (MEL) in affecting chilling injury (CI) and related chilling tolerance regulatory mechanisms in harvested litchi fruit during refrigeration at 4 °C were investigated in this study. In the first experiment, exogenous MEL exposure exerted an inhibitory impact on the development of CI severity in refrigerated litchi fruit, as indicated by the lower CI index and delayed red color loss of pericarp, whereas treatment with p-chlorophenylalanine (p-CPA), an inhibitor of MEL biosynthesis, aggravated the degree of CI. The suppression of CI afforded by exogenous MEL was abated under the combined application of exogenous MEL and p-CPA. In the second experiment, the effects of p-CPA alone treatment on changes in biochemical parameters during refrigeration were analyzed. The results showed that p-CPA down-regulated the expressions of MEL biosynthesis genes, including LcTDC, LcT5H and LcSNAT, contributing to reduced generation of endogenous serotonin and MEL. p-CPA treatment accelerated oxidative stress-triggered membrane deterioration, as reflected by increases in relative leakage rate (RLR), reactive oxygen species (ROS) production, lipoxygenase (LOX) activity and malondialdehyde (MDA) accumulation. The membrane damage caused by p-CPA further resulted in loss of subcellular compartmentalization and enzymatic oxidation, as demonstrated by increased activities of polyphenol oxidase (PPO) and peroxidase (POD) in parallel with reduced contents of total phenolics and anthocyanin. Compared with control fruit, lower activities of antioxidant enzymes, including superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), monodehydroascorbate reductase (MDHAR), dehydroascorbate reductase (DHAR) and glutathione reductase (GR), were observed in fruit receiving p-CPA. Additionally, p-CPA suppressed the expression levels of genes encoding oxidized protein repair-related enzymes, including LcMsrA1, LcMsrA2, LcMsrB1 and LcMsB2. These results suggest that endogenous MEL might confer a protective role against chilling stress by modulating cellular redox status in harvested litchi fruit during refrigeration.

本研究研究了内源性褪黑激素（MEL）在4°C冷藏期间影响荔枝果实冷害（CI）和相关的耐冷性调控机制的有效作用。在第一个实验中，外源MEL暴露对冷藏荔枝果实CI严重性的发展具有抑制作用，这表明果皮的CI指数较低和果皮的红色消失延迟，而对氯苯丙氨酸（p -CPA）处理可以降低果皮的CI指数， MEL生物合成抑制剂增加了CI的程度。在外源MEL和p -CPA的联合应用下，外源MEL对CI的抑制作用减弱。在第二个实验中，p的影响分析了单独的CPA处理对冷藏期间生化参数变化的影响。结果表明，p -CPA下调了LcTDC，LcT5H和LcSNAT等MEL生物合成基因的表达，有助于减少内源性血清素和MEL的生成。对-CPA处理加速了氧化应激触发的膜变质，反映为相对泄漏率（RLR），活性氧（ROS）产生，脂氧合酶（LOX）活性和丙二醛（MDA）积累的增加。p引起的膜损伤-CPA进一步导致亚细胞区室化和酶促氧化的丧失，如多酚氧化酶（PPO）和过氧化物酶（POD）的活性增加，同时总酚和花色苷含量降低所证明。与对照水果相比，在该地区发现抗氧化剂酶的活性较低，包括超氧化物歧化酶（SOD），过氧化氢酶（CAT），抗坏血酸过氧化物酶（APX），单脱氢抗坏血酸还原酶（MDHAR），脱氢抗坏血酸还原酶（DHAR）和谷胱甘肽还原酶（GR）。水果接受p -CPA。另外，p -CPA抑制了编码氧化蛋白修复相关酶的基因的表达水平，包括LcMsrA1，LcMsrA2，LcMsrB1和LcMsB2。这些结果表明，内源性MEL可能通过调节冷藏过程中收获的荔枝果实中的细胞氧化还原状态来赋予抗寒胁迫的保护作用。