Abstract: 【Objective】The plant epidermal wax serves as the primary barrier for plants against external environmental stresses. It plays an important role in preventing water loss from the plant epidermis, inhibiting pathogen invasion, and extending the storage period of fruits. Currently, many studies have been conducted to regulate and control the synthesis of the plant epidermal wax, with the aim of increasing wax content of plant and subsequently improving the plant's resistance to adverse external environments. Exploring the influence of exogenous reagents on wax content and chemical composition of the mango can provide a reference for the synthesis regulatory mechanism of exogenous reagents on the wax of mango leaf epidermis. 【Methods】 In this study, the sapling of two mango cultivars, 'Chiin Hwang' and 'Tainoung No.1', were used as materials. One-year-old saplings planted in the experimental base were selected, and all saplings were managed according to conventional field measures to ensure consistent growth conditions. After the mango saplings completed sprouting, the upper and lower epidermal surfaces of the light green leaves at growth stage were respectively sprayed with 50 ml of melatonin (100μM MT), palmitic acid (150μM PA, 200μM PA), and their combination reagents (100μM MT + 150μM PA, 100μM MT + 200μM PA), while the mango leaves in the control group were sprayed with the same amount of clear water. During the process of preparing exogenous reagents, melatonin and palmitic acid were separately dissolved in a small amount of absolute ethanol, and an appropriate amount of OP-10 emulsifier was added to assisting dissolution. After thorough mixing, ultrapure water was added in accordance with the molar concentration ratio. Approximately 1mL of absolute ethanol was used for each 1L of exogenous reagent, and approximately 20μL of emulsifier was added. Three mango saplings were included in each treatment, and three replicates were set for each treatment. The exogenous reagents were sprayed every two days for a total of three times. On the seventh day after the exogenous reagent treatment, three leaves of the same position were taken from each plant. The wax of mango leaf epidermis were extracted using the chloroform dissolution, and the leaves surface area were measured using the portable leaf area measuring instrument. The n-tetracosane solution (10mg/mL) was used as an internal standard. The epidermal wax extracts were dried by the termovap sample concentrator to obtain the crude epidermal wax extract and undergo derivatization reactions. The wax content and chemical composition of mango leaf epidermis were analyzed and determined by gas chromatography-mass spectrometry (GC-MS), and statistical data were analyzed to investigate the influence of exogenous reagents on the wax of mango leaf epidermis. 【Results】The GC-MS detection results revealed the epidermal wax of 'Chiin Hwang' and 'Tainoung No.1' mango leaf was mainly composed of aliphatic compounds (alkanes, alcohols, aldehydes, and fatty acids), terpenoids, esters, and other unclassified compounds. The PCA results showed that for 'Chiin Hwang', PC1 and PC2 were 34.8% and 28.6%, respectively, explaining 63.4% of the variation; for 'Tainoung No.1', PC1 and PC2 were 44.1% and 24.8%, respectively, explaining 68.9% of the variation. Different exogenous reagent treatments exerted specific regulatory effects on the wax components and content of mango leaf. The wax of mango leaf epidermis of the control group sprayed with clear water was 9.72 μg/cm2 for 'Chiin Hwang' and 11.76 μg/cm2 for 'Tainoung No.1'. After different exogenous reagent treatments, except for the 'Chiin Hwang' mango leaf treated with 100μM MT + 200μM PA mixed reagent where the wax content slightly decreased, the wax content of mango leaf epidermis in other treatments increased. Among them, after treatment with 100μM MT + 200μM PA, the ester wax compounds of 'Tainoung No.1' mango leaf by 11.95 times, and its proportion in the total wax content increased from 23.59% to 87.88%, with total content increase by 3.2 times, which was the main reason for the total wax increase. The 150μM PA reagent could increase the content of aldehydes with carbon chains of C28, C29, and C32 in the wax of 'Chiin Hwang' mango leaf epidermis, and the terpenoid compounds in 'Tainoung No.1' mango leaf. The effect on increasing the wax content of both mango cultivars was significant, with increase by 1.87 and 2.05 times, respectively. 【Conclusion】Different exogenous reagent treatments can change the carbon chain distribution of aliphatic compounds and the content of each component of terpenoids in mango leaf epidermis, influencing the variation in the total wax content. The 150 μM PA reagent can effectively increase the total wax content of both 'Chiin Hwang' and 'Tainoung No.1' mango cultivars.
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