Effects of exogenous melatonin on the sunburn and peel anatomical structure of Xinbai 8 loquat fruit

Abstract: 【Objective】This study examined the effects of exogenous melatonin on the appearance quality and cellular structure of loquat fruit after sunburn in order to provide reference for the development of technology to prevent sunburn in loquat.【Methods】Fruit of Xinbai 8 were used as the experimental material. Exogenous melatonin was sprayed at concentrations of 50, 100, 150, and 200 μmol · L- 1 , and spraying clean water as the control. 4 hours after spraying, a high temperature stress at 40 ℃ was applied. Sun burning symptoms in loquat fruit were recorded after 0- 60 minutes of high temperature stress, and the incidence and degree of sunburn were statistically analyzed to screen for the optimal concentration of exogenous melatonin and to determine the time when exogenous melatonin treatment delayed the occurrence of fruit sunburn symptoms. Paraffin sections and scanning electron microscopy were used to observe and compare the microstructural and ultrastructural changes of fruit peels underhigh temperature stress at 0, 10, and 30 minutes after exogenous melatonin treatments.【Results】Under high temperature stress, loquat fruit was severely sunburnt, with an incidence rate of more than 35%. The rate of grade 0 sunburn was only 63.33%, and the rate of grade Ⅰ sunburn was 36.67%. Melatonin treatment within 50-200 μmol·L- 1 delayed the occurrence of fruit sunburn. Among them, melatonin at 150 μmol · L- 1 showed the latest sunburn occurrence, the lightest symptoms, the lowest incidence rate. Compared with the control, exogenous melatonin treatment significantly reduced the incidence by 66.8% . The rate of grade 0 sunburn fruit treated with 150 μmol · L- 1 melatonin was the highest, at 87.80%, and the rate of grade Ⅰ sunburnt fruit was the lowest, both significantly lower than other concentration treatments and the control. After 10 min of heat stress at 40 ℃, the control showed small brown spots on the sunny side and sunburn symptoms, while the melatonin treated groups did not show any sunburn symptoms. After 30 minutes, the sunburn area increased, the lesion became larger and became darker in the control fruit, and the fruit in melatonin treatment groups began to show small sunburn lesions. Microscopic observation showed that after exposure to high temperature for 10 minutes, the thickness of the stratum corneum, epidermis, and subepidermal layer treated with water significantly decreased. Some areas of the stratum corneum began to wrinkle and became thinner; some epidermal cell walls ruptured; and the epidermis and subepidermal cells deformed, resulting in cavities in the lower layer of the epidermis. The stratum corneum treated with melatonin was 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 in the control fruit became thinner, wrinkled and deformed significantly, and the number of cavities formed between the inner side of the stratum corneum and epidermal cells increased. Subepidermal cells showed significant deformation, forming larger cavities. After melatonin treatment, the stratum corneum occasionally became thinner, wrinkled and deformed, with intact epidermal cells and a few ruptured cells near the epidermal layer. There were also a few empty spaces between cells in the subepidermal layer. After 10 minutes, the change in the ratio of epidermal cells in the control fruit was 2.17 times and the change in the ratio of subepidermal cells was 4.72 times that of exogenous melatonin treatments. After 30 minutes, the change in the ratio of epidermal cells was 1.37 times that of exogenous melatonin treatment, and the 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 fruit 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 fruit 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 fruit, and delay and reduce the occurrence of fruit sunburn. 150 μmol·L- 1 melatonin treatment was the optimal concentration to alleviate the occurrence of sunburn in loquat fruits.