Effects of sunburn on the structure and component changes in pomegranate cuticular wax

Abstract: 【Objective】Cuticular wax acts as a protective barrier that covers the surface cells of leaves, stems, flowers, and fruits, playing a crucial role in resisting heat and UV radiation damage. Pomegranate fruits sensitive to sunburn can easily suffer from sunburn when exposed to intense sunlight and high temperatures from July to September. This study investigated the effects of sunburn on the structure and components of cuticular wax in the Hongyushizi pomegranate variety, known for its susceptibility to sunburn. The aim of this research is to elucidate the physiological mechanisms underlying the response of cuticular wax to sunburn and to establish a theoretical framework for breeding sunburn- resistant pomegranate varieties.【Methods】The sunburn-susceptible variety Hongyushizi was used as the material to study the wax structure of fruits at different sunburn degrees using scanning electron microscopy. The wax content was determined using gas chromatography- mass spectrometry (GC-MS).【Results】 (1) Scanning electron microscopy revealed sunburn caused changes in the wax structure in pomegranate fruit, leading to irregular wax layer, sanding, or flaking depending on the degree of sunburn. (2) A total of 67 wax compounds were detected, mainly including very-long-chain aliphatics (26.36 μg · cm- 2 , accounting for 74.95% of the total) and terpenoid (8.81 μg· cm-2 , accounting for 25.05% of the total). The very-long-chain aliphatics were mainly composed of alkanes (19.70 μg·cm-2 ), fatty acids (2.49 μg·cm-2 ),olefin (0.19 μg · cm- 2 ), alcohols (3.78 μg · cm- 2 ), and esters (0.20 μg · cm- 2 ). Sunburn significantly increased the total wax content (by 6.96%), especially increasing the contents of esters (by 82.78%) and alcohols (by 64.71%). However, it decreased the content of terpenoid by 27.48% compared to the control. (3) A total of 7 terpenoid compounds were detected in the pomegranate pericarp, namely ursolic acid, α-tocopherol, β-sitosterol, β-amyrin, ginkgolide, β-tocopherol and betulin. The total amount of terpenoid in the healthy Hongyushizi pomegranate pericarp was 8.81 μg · cm- 2 , with ursolic acid being the most abundant at 8.66 μg · cm-2 (98.30% of the total). Sunburn significantly reduced the total amount of terpenoids, which was 6.39 μg · cm- 2 in the sunburned Hongyushizi pericarp (SB2) being a decrease of 26.21% compared to the healthy fruit. The content of ursolic acid was significantly reduced (by 29.00%) in the sunburned pericarp. However, sunburn increased the content of α-tocopherol, β-sitosterol, β-amyrin, ginkgolide, β-tocopherol, and betulin, but the differences were not significant. (4) A total of 20 alkanes compounds were detected in the Hongyushizi fruit pericarp. The healthy pericarp had a total amount of 19.70 μg·cm-2 , including C29 (7.32 μg·cm-2 ), C31 (6.70 μg·cm-2 ) and C40 (4.24 μg·cm2 ), which accounted for 92.69% of the total alkanes. Sunburn increased the alkanes content to 22.53 μg·cm-2 . Additionally, 12 alcohol compounds were detected in Hongyushizi fruit pericarp, primarily C30, which accounted for 81.04% of the total alcohol compounds. Sunburn significantly increased the alcohol compound content from 3.78 μg·cm-2 to 6.34 μg·cm-2 , with the C30 content increasing by 49.15%. The fruit pericarp also contained 11 fatty acids, mainly C16 and C18, which accounted for 50.22% and 42.79% of the total fatty acid content, respectively. The content of fatty acids in healthy fruit was 2.49 μg· cm-2 , and decreased to 2.13 μg · cm- 2 under sunburn stress. Furthermore, 14 lipid compounds were detected, mainly C19, C34, C22, C10, and C12, which accounted for 30.36% , 11.33% , 10.11% , 9.64% , and 8.89% of the total lipid content, respectively. Sunburn increased the lipid content from 0.20 μg · cm-2 to 0.37 μg · cm-2 . Lastly, 3 olefin compounds with C17 having the highest content of lipid (0.18 μg · cm-2 ), constituting 95.03% of the total olefins. Sunburn decreased the lipid content, particularly C17 (by 19.20% ), while significantly increased the content of C30.【Conclusion】The cuticular wax of pomegranate pericarp contains alkanes, terpenoids, fatty acids, olefins, alcohols, and lipids, with alkanes and terpenoids being the main compounds. Sunburn results in changes in the structure and total amount of wax in the pomegranate pericarp, with a significant increase in the content of lipids and alcohols, but a significant decrease in the content of terpenoids. Pomegranate pericarps can respond to sunburn stress by altering the wax structure and the contents of its components. The research provides a theoretical basis for further understanding the mechanism of the response of the cuticular wax to sunburn stress.