اثر تیمار ملاتونین در بهبود سیستم آنتی اکسیدانی و افزایش عمر پس از برداشت میوه فلفل دلمه ای (Capsicum annuum L.)

Introduction:
Bell pepper (Capsicum annuum L.) belongs to the Solanaceae family, it contains high amounts of bioactive compounds such as phenolic, flavonoid, and antioxidant compounds. These nutrients have anti-cancer properties. However, bell pepper can usually be stored for several days during transport or at the time of sale in the store, or after purchase by consumers. In this period of time, the surface of the fruit loses its appearance and begins to lose its nutritional content. In order to postpone the senescence of bell pepper, various methods can be used, such as the atmosphere changing during the storage period, using chemical compounds, or natural compounds treatment. Researchers are looking for a compound that, while having the least harmful effect on human health, can give pepper the longest lifespan. Melatonin is an organic compound that has been studied in recent years on various biotic and abiotic stresses. The purpose of this study is to investigate the effect of melatonin treatment on the physiology of post-harvesting of bell pepper through its effect on the content of enzymatic and non-enzymatic antioxidant compounds during four weeks of storage time.
Materials and methods
In this research, bell pepper fruit at the stage of physiological maturity was purchased from a local producer located in Tehran and immediately transferred to the research laboratory of Qom University. In the laboratory, the fruits that had damage or defects in appearance were discarded, and the rest were disinfected by immersing in a 1% (v/v) sodium hypochlorite solution for two minutes. In a preliminary experiment, bell peppers were treated with 25, 50, 100, 200, 250 and 300 µmol/L melatonin. Preliminary test results showed that 100 μmol/L melatonin had a significant effect on post-harvest appearance quality (primary test results not shown). Based on an initial experiment and selection of the best effective concentration in maintaining fruit quality, the treatment of 100 micromol/liter melatonin was used as a reference for the studies in this article. Bell peppers were divided into two groups, control and treatment, and after 2 hours immersion in 100 μmol/L as the treatment group and distilled water as the control group. Then it was kept for 4 weeks at a temperature of 4±1 degrees Celsius. In this research, the indicators of weight loss percentage, chilling injury percentage, antioxidant enzyme activity such as CAT and SOD, free radical content such as H2O2 and superoxide free radicals, and total phenol content, flavonoid content, and Phenylalanine amino lyase (PAL) and Tyrosine ammonia lyase (TAL) enzyme activity as enzymes controlling the phenylpropanoid pathway in weeks 0, 1, 2, 3 and 4 were evaluated after storage and with three repetitions.
Results and discussion:
The results of this study showed that the melatonin treatment of bell peppers was more effective in maintaining fresh weight during storage compared to the control group. In the first to the fourth week, the weight loss percentage of the melatonin treatment in the 2nd, 3rd and 4th weeks of storage was reduced by 39, 20 and 26.15%, respectively, compared to the control group. The results showed that there is a significant difference in the percentage of chilling injury in different weeks of storage between the control and the melatonin-treated group. The results showed that there is a significant difference in total phenol and flavonoid content between the control group and the melatonin-treated group during storage, and this increase was in line with the effect on the activity of PAL and Tal enzymes as key enzymes in the propanoid pathway. In both control and treated groups with melatonin, the content of hydrogen peroxide radical and superoxide radical showed a significant increase (p ≥ 0.05) during four weeks of storage at 4 ± 1 °C (Figure 3). However, in the second, third and fourth weeks of storage, the amount of hydrogen peroxide was significantly (p ≥ 0.05) lower in bell peppers treated with melatonin than in the control group. Compared to the control group, in bell peppers treated with melatonin during the second week of storage, the highest percentage decrease in hydrogen peroxide content was observed.The content of H2O2 and superoxide free radicals during the storage period showed a significant decrease compared to the control group, which showed that this result is consistent with the data obtained with the activity of antioxidant enzymes CAT and SOD.
Conclusion:
The results of this research showed that the treatment of bell pepper with melatonin significantly reduced the effect on enzymatic and non-enzymatic antioxidant compounds compared to a control group, and these results confirm the results and the findings of previous studies about exogenous effect of melatonin. Harvested bell peppers during storage are susceptible to freezing stress, which increases the accumulation of reactive oxygen species such as superoxide radicals and hydrogen peroxide, and these compounds induce aging and reduce quality and marketability of agricultural products. The increase in the activity of PAL and TAL enzymes, especially in the first weeks of storage, may be due to the effect of exogenous melatonin on the expression and biosynthesis of genes involved in the phenylpropanoid pathway, and as a result, it leads to an increase in the activity of PAL and TAL enzymes. Increased production of enzymatic and non-enzymatic antioxidant system and activation of key enzymes of phenylpropanoid pathway (PAL and TAL) and increasing the content of total phenol and flavonoid compounds were observed. It seems that the enhanced antioxidant system has maintained the cell stability by reducing the concentration of reactive oxygen species and has increased the tolerance of the fruit to chilling stress. Phenolic and flavonoid compounds make plants resistant to abiotic and biotic stresses. In this study, the decrease in phenolic and flavonoid content due to storage at cold temperature in control group; was significantly increased in the pericarp of pepper treated with melatonin treatment. Based on the results obtained in this research, it can be said that melatonin treatment is beneficial on the post harvesting life of bell peppers and maintaining the appearance as well as the quality and levels of nutrients. Of course, for the final confirmation of this result, we need more studies on this or other fruits.