外源褪黑素处理对采后荔枝褐变及活性氧代谢的影响
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为探讨外源褪黑素处理对采后荔枝褐变及活性氧代谢的影响，本研究以“妃子笑”荔枝为材料，分别以清水、不同浓度(50、100、200 μmol/L)的外源褪黑素浸泡处理荔枝，于4℃下贮藏，定期取样测定活性氧代谢相关生理指标。实验数据显示，采后荔枝在贮藏期间，不同处理组果实失重率逐渐升高，而50 μmol/L褪黑素处理组的失重率最低；在贮藏末期，CK组、50、100和200 μmol/L处理组的L*值分别为26.3、27.1、26.9和26.5；在3～12 d内，CK组电导率显著高于处理组(P<0.05)；CK组和处理组的过氧化氢（H2O2）含量都呈现持续上升趋势，但处理组显著低于CK组 (P<0.05)；9 d时，50 μmol/L褪黑素处理组超氧阴离子(O-2)产生速率为14.42 nmol/min•g, 远小于CK组的16.45 nmol/min•g；12 d时，50 μmol/L褪黑素处理组的超氧化物歧化酶（SOD）活性最高（0.812 U/g）, 显著高于其他处理组(P<0.05)，处理组多酚氧化酶（PPO）的活性均显著低于CK组(P<0.05)。结果表明，外源褪黑素处理可降低荔枝的失重率、褐变指数和细胞膜透性，保持果实较好的色度(L*:明亮度)，降低果实中O-2产生速率和PPO活性，减缓丙二醛(MDA)及H2O2的产生和积累，提高SOD活性和维持果实细胞膜的完整性。尤以50 μmol/L的外源褪黑素处理效果最佳。推断褪黑素处理可以通过调节采后荔枝的活性氧代谢而延缓其衰老进程，为褪黑素的应用及荔枝采后保鲜提供理论支撑。
To investigate the effects of exogenous melatonin treatment on browning and reactive oxygen species metabolism of Postharvest litchi, “Feizixiao” litchi was used as the material in this study. Litchi was treated with clear water and exogenous melatonin at different concentrations (50, 100, 200 μmol/L), and the relevant physiological indexes related with active oxygen metabolism were determinated at regular time during storage at 4 ℃. During the storage period of postharvest litchi, the weight loss rate of fruits in different treatment groups gradually increased, and the weight loss rate of fruits in 50 μmol/L melatonin treatment group was the lowest. At the end of storage time (12 d), the L* values of CK, 50, 100 and 50 μmol/L groups were 26.3, 27.1, 26.9 and 26.5, respectively. Within 3-12 days, the conductivity of CK group was significantly higher than that of the treatment group (P<0.05). The content of hydrogen peroxide（H2O2）in CK group and treatment group showed a continuous upward trend, but the H2O2 content in melatonin group was significantly lower than that of CK group (P<0.05). The production rate of superoxide anion in 50 μmol/L melatonin treated group was 14.42, which was much lower than that in CK group (16.45 nmol/min•g). At the 12th day, the superoxide anion (SOD) activity of melatonin treatment group (0.812 U/g) was the highest, which was significantly higher than that of other treatment groups (P<0.05). The activity of polyphenol oxidase (PPO) in treatment group was significantly lower than that in CK group (P<0.05). The results showed that different concentrations of melatonin could effectively reduce the weight loss rate, browning index and cell membrane permeability of litchi, maintain the original color (L*: brightness), reduce the production rate of O-2 and the activity of PPO, slow down the production and accumulation of MDA and H2O2, improve the activity of SOD and maintain the integrity of cell membrane of litchi fruit. In particular, 50 μmol/L melatonin treatment showed the best effect. It could be speculated that melatonin treatment can delay the aging process of postharvest litchi by regulating its reactive oxygen species metabolism, which provides theoretical support for melatonin application and preservation of postharvest Litchi.
果蔬采后品质的下降与活性氧积累产生的氧化胁迫密切相关。正常条件下植物体中的活性氧清除系统(包括防御酶系)可以及时清除植物体产生的活性氧。但是在非生物胁迫条件下，植物细胞中产生的活性氧大于被清除的活性氧，就会引起细胞代谢亲乱。因此，采用合适的保鲜技术和措施对采后果蔬进行一定的保鲜处理，可以减少果实的采后失水活性氧的积累，维持细胞膜的完整性，从而延缓果实的品质下降。
研究结果表明，在4 ℃贮藏条件下，不同浓度的褪黑素处理对采后荔枝果实品质具有一定的保鲜效果，说明褪黑素处理可以有效抑制采后荔枝的褐变和衰老。其主要表现在处理后能有效延缓荔枝失重率的增加，抑制或减少活性氧及丙二醛的产生和积累，促进贮藏初期 SOD 活性的升高，这与褪黑素处理在其他果蔬上的研究结果是一致的。说明褪黑素处理不但能维持果实较高的L*值和较低的褐变指数保证果实具有良好外观品质，还能从一系列生理生化水平上维持果实良好的生理特征和营养品质至于褪黑素处理如何促进保护酶系的活性，抑制超氧阴离子和过氧化氢的的产生，调控果蔬中的抗氧化途径，需要进一步深入研究，而且对于研究褪黑素处理对非跃变型果实的生理影响会有重要参考意义。
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