Effects of melatonin and palmitic acid on the wax content of mango leaf epidermis

【Objective】The 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 plants resistance to adverse external environments. Exploring the influence of exogenous reagents on wax content and chemical composition in mango can provide a reference for the synthesis and regulatory mechanism of exogenous reagents on the wax ofmango leaf epidermis.【Methods】In this study, the saplings of two mango cultivars, Chiin Hwang and Tainong 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 (100 μmol·L-1 MT), palmitic acid (150 μmol·L-1 PA and 200 μmol·L-1 PA), and their combination reagents (100 μmol·L-1 MT + 150 μmol·L-1 PA and 100 μmol·L-1 MT + 200 μmol·L-1 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 assist dissolution. After thorough mixing, ultrapure water was added in accordance with the molar concentration ratio. Approximately 1mL of absolute ethanol was used for each 1 L of exogenous reagent, and approximately 20 μL 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 leaf surface area were measured using the portable leaf area measuring instrument. The n-tetracosane solution (10 mg·mL-1 ) 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 leaf epidermis.【Results】The GC-MS detection results revealed the epidermal wax of Chiin Hwang and Tainong No. 1 leaves 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 Tainong 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 contents of mango leaves. The wax of leaf epidermis of the control group sprayed with clear water was 9.72 μg· cm-2 for Chiin Hwang and 11.76 μg·cm-2 for Tainong No. 1. After different exogenous reagent treatments, except for the Chiin Hwang leaves treated with 100 μmol·L-1 MT + 200 μmol·L-1 PA mixed reagent where the wax content slightly decreased, the wax content of leaf epidermis with other treatments increased. Among them, after treatment with 100 μmol · L- 1 MT + 200 μmol · L- 1 PA, the ester wax compounds of Tainong No. 1 leaves increased 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 150 μmol· L- 1 PA reagent could increase the content of aldehydes with carbon chains of C28, C29 and C32 in the wax of Chiin Hwang leaf epidermis, and the terpenoid compounds in Tainong No. 1 leaves. 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 μmol·L- 1 PA reagent can effectively increase the total wax content of both Chiin Hwang and Tainong No. 1 mango cultivars.