Effect of Melatonin Treatment on Post-Harvest Life of Broccoli during Storage

Broccoli (Brassica oleracea) belong of the Brassicaceae family, contains high levels of bioactive compounds, such as vitamin C, phenolic compounds, and glucosinolates. These nutrients have antimicrobial, antioxidant and anti-cancer properties. However, broccoli can usually be stored for several days during transport or at the time of sale in the store or after purchase by consumers. During this period, their flowers start to turn yellow and their nutritional content decreases rapidly. In order to delay the aging stage in broccoli, various methods such as changing the atmosphere of the storage place, irradiating red light, the effect of plant hormones and treating chemical compounds can be used to maintain the appearance quality of broccoli and delay the aging stage. Various chemical compounds have been used to delay the aging stage in broccoli, but choosing a compound that has the least harmful effect on human health is one of the main priorities of scientists active in this field. One of these chemical compounds that has the least negative effects on health is melatonin. The purpose of this study is to investigate the effect of melatonin treatment on physiology after harvesting broccoli during storage.

In a primary experiment, broccoli was treated with 10, 25, 50, 100, 250, 400 and 600 μmol/L melatonin. Primary test results showed that 100 μmol/L melatonin had a significant effect on post-harvest appearance quality. Therefore, the 100 μmol/L melatonin treatment was used as a reference for more extensive studies in this paper. In this study a factorial experiment was conducted using a randomized completely design with three replication. Broccoli was divided into control and treatment groups, and after 20 minutes immersion in 100 µmol L-1 as treatment and distilled water as control. Then it was kept at a temperature of 4±1 °C for 4 weeks. In this research, the indicators of weight loss percentage, content of photosynthetic pigments, the activity level of antioxidant enzymes, the content of free radicals, and the amount of phenolic, flavonoid, vitamin C, and glucosinolate in weeks 0, 1, 2, 3 and 4 were evaluated after storage and with three repetitions.

The results of this research showed that broccoli treatment with melatonin was more effective in maintaining fresh weight during storage than the control. In the first to fourth weeks, the percentage of weight loss in the control group was 1.26, 1.57, 2.19 and 2.23 times higher than the melatonin-treated group, respectively. The results indicated that there was a significant difference (p<0.05) in the content of chlorophyll a and b in different weeks of storage between the control and the melatonin-treated groups. The melatonin treatment can increase the content of chlorophyll a, b and total chlorophyll by suppressing the activity of chlorophyllase enzyme. So that, melatonin treatment led to a decrease in chlorophyllase enzyme activity, and this decrease in the fourth week in the melatonin-treated group was about 47.8% less than the control group. The results indicated that there was a significant difference (p<0.05) in the content of vitamin C and glucosinolate between control and the melatonin-treated groups in during storage. Compared with control group, significantly higher total phenolic content was observed in the melatonin treated group in during of storage. Comparison of means showed that the content of flavonoids in broccoli treated with melatonin showed a general increasing trend, so that in the fourth week, the content of flavonoids in the treatment group was 3.67 times that of the control group. Measuring the activity of antioxidant enzymes showed that melatonin treatment was able to reduce the content of superoxide free radicals by increasing the activity of superoxide dismutase enzyme, so that in the group treated with melatonin, SOD activity increased by 35% and 47.6% in the third and fourth weeks, respectively.

In general, the effect of melatonin on broccoli is associated with increasing the activity of antioxidant enzymes and suppressing the activity of chlorophyll-degrading enzymes, which reduced the content of reactive oxygen species and increased the content of chlorophyll. Based on the data obtained in the present study, as a result of the increase in phenolic and flavonoid compounds, antioxidant enzymes and also antioxidant compounds such as vitamin C and glucosinolate, it improves the quality of broccoli. Generally, it seems that postharvest treatment with melatonin can be a useful method to maintain the appearance as well as the quality and nutrient levels of stored broccoli and increase its useful life.