【Objective】Study the effects of exogenous melatonin on the appearance quality and cellular structure of loquat fruits after sunburn, providing reference for the development of sunburn prevention and control technologies for loquat fruits. 【Method】Using the fruit of Xinbai 8 as the experimental material. Spray exogenous melatonin at concentrations of 50, 100, 150, and 200 μM respectively, and use clean water as a control. 4 hours after spraying, conduct a high temperature stress experiment with an induction temperature set at 40℃. Observe the sun damage of loquat fruits after 0~60 minutes of high temperature stress, statistically analyze the incidence and degree of sun damage, screen for the optimal concentration of exogenous melatonin, and determine the time when exogenous melatonin treatment delays the occurrence of fruit sun damage symptoms; Using paraffin sections and scanning electron microscopy to observe and compare the microstructural and ultrastructural changes of fruit peels under high temperature stress at 0, 10, and 30 minutes after exogenous melatonin treatment.【Result】Under high temperature stress, loquat fruit was severely sunburnt, with a incidence rate of more than 35%. The disease of grade 0 sunburn was only 63.33%, and the disease of grade Ⅰ sunburn was 36.67%,melatonin treatment with a concentration of 50~200 μM can delay the occurrence of fruit sunburn. Among them, melatonin with a concentration of 150 μM showed the latest sunburn, the lightest symptoms, the lowest incidence rate and disease grade.Compared with water treatment, exogenous melatonin treatment significantly reduced the incidence rate by 66.8%.The disease grade 0 fruit treated with 150 μM melatonin was the highest, at 87.80%, and the disease grade Ⅰ fruit was the lowest, both significantly lower than other concentration treatments and water treatment.After being treated at 40℃ for 10 minutes, the fruits treated with water showed small brown spots on the sunny side and showed burn symptoms, while the melatonin treated group did not show any burn symptoms. After 30 minutes, the burn area increased, the lesion became larger, the lesion color became darker, and the melatonin treatment group began to show small sunburn lesions. Microscopic observation results showed that after 10 minutes of high-temperature induction, the thickness of the stratum corneum, epidermis, and subepidermal layer treated with water significantly decreased.Some areas of the stratum corneum begin to wrinkle and thin, some epidermal cell walls rupture, and the epidermis and subepidermal cells deform, resulting in cavities in the lower layer of the epidermis;The stratum corneum treated with melatonin is smooth, thin, and uniform, with visible deformation of the outer layer of subepidermal cells; After 30 minutes of high temperature stress, the stratum corneum in some areas treated with water became thinner, wrinkled and deformed significantly, and the number of cavities formed between the inner side of the stratum corneum and epidermal cells increased. Sub epidermal cells showed significant deformation, forming larger cavities; After melatonin treatment, individual areas of the stratum corneum become thinner, wrinkled and deformed, with intact epidermal cells and a few ruptured cells near the epidermal layer. There are also a few empty spaces between cells in the subepidermal layer; After 10 minutes of treatment with clean water, the change in the aspect ratio of epidermal cells was 2.17 times that of exogenous melatonin treatment, and the change in the aspect ratio of subepidermal cells was 4.72 times that of exogenous melatonin treatment. After 30 minutes, the change in the aspect ratio of epidermal cells was 1.37 times that of exogenous melatonin treatment, and the aspect ratio of subepidermal cells was 8.58 times that of exogenous melatonin treatment. Observation and comparison of ultrastructure revealed that after 10 minutes of high temperature stress, the epidermal cells underwent severe deformation, and the subepidermal layer cells changed from nearly circular or elliptical to elongated or spindle shaped, with a 63.46% increase in cell wall thickness; After 30 minutes of high temperature stress, the stratum corneum became noticeably thinner and deformed, with severe deformation of epidermal cells and irregular formation of subepidermal cells. The thickness of subepidermal cell walls significantly increased by 84.62%; After exogenous melatonin treatment, the cell shape of the epidermal layer and subepidermal layer of loquat remained basically unchanged after 10 minutes of high temperature stress, and the thickness of the subepidermal cell wall increased by 42.60%. After 30 minutes of high temperature stress, the thickness of the subepidermal cell wall of loquat increased by 65.74%.【Conclusion】Under high temperature stress conditions, loquat fruits will suffer from severe sunburn. Exogenous melatonin treatment can reduce the incidence and severity of fruit sunburn. Exogenous melatonin can alleviate the morphological changes of the epidermal stratum corneum, epidermal cells, and subepidermal cells in loquat fruits, and delay and reduce the occurrence of fruit sunburn. 150 μM melatonin treatment was the optimal concentration to alleviate the occurrence of sunburn in loquat fruits.
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