Abstract:【Objective】 The fruit surface is covered with the cuticular wax layer composed of very long-chain fatty acids (VLCFA) and their derivatives and alicyclic compounds. And fruit cuticular wax can reduce postharvest water loss, and prevent the invasion of microorganisms and pathogens, playing an important role in the growth, development and post-harvest storage of fruit. And avocado (Persea americana Mill.) is one of the typical climacteric fruit, and postharvest water loss and rot of fruit often occur, causing huge economic losses. Cuticular wax can affectfruit storage performance. However, there were few reports on the study of avocado fruit cuticular wax. This study aims to clarify the microstructure and chemical composition of fruit cuticular wax of different avocado varieties, and provide basic data for further research on the cuticular wax functions. 【Methods】 In this work, 5 avocado varieties (Peterson, RKYYL6-2, Pollock, Walter Hole and RKYYL8-13) were used for the analysis of fruit cuticular wax. Firstly, we observed the ultrastructure of avocado fruit cuticular wax using the scanning electron microscope, and then separated and extracted the epicuticular wax and intracuticular wax of avocado fruit. The extraction methods of the epicuticular wax and intracuticular wax were as follows. We first applied gum arabic evenly on the surface of the avocado fruit to separate the epicuticular wax; then, we immersed the fruit with the epicuticular wax removed in chloroform to extract the intracuticular wax of avocado fruit. And chemical composition and content of epicuticular wax and intracuticular wax of avocado fruit were further analyzed by GC-MS and GC-FID. 【Results】 The scanning electron microscope was used to observe the ultrastructure of fruit cuticular wax of 5 avocado varieties, and it was found that the cuticular wax crystals on the avocado fruit surface were mainly plates and granules. At ×30 000 and ×100 000 magnification, Peterson and RKYYL6-2 could clearly show plates wax crystals attached to the fruit surface. The crystal of cuticular wax of Pollock was relatively small, but the plates structure could also be observed at ×100 000 magnification. And the crystal of cuticular wax of Walter Hole was relatively large, and the continuous cuticular wax crystals could be observed at ×5000 magnification, and the crystal structure of cuticular wax of Walter Hole was also plates at ×30 000 and ×100 000 magnification. The crystal structure of cuticular wax of RKYYL8-13 was different from the above-mentioned varieties. At ×30 000 and ×100 000 magnification, the crystal of cuticular wax of RKYYL8-13 appeared granules and closely stacked. Subsequently, we analyzed the content and chemical composition of epicuticular wax and intracuticular wax for 5 avocado varieties. The chemical composition of avocado epicuticular wax consisted of six types of compounds, namely alkanes, aldehydes, fatty acids, alcohols, steroids and monoacylglycerols; besides the above compounds, the chemical composition of avocado intracuticular wax also included tocopherols. In terms of the carbon chain distribution of each component of epicuticular wax, the carbon chain numbers of alkanes included C23, C25, C27, C29 and C31, and the carbon chain numbers of aldehydes were C30 and C32, and the carbon chain numbers of fatty acids were C28, C30 and C32, and the carbon chain number of alcohols were C26, C28, C30, C32 and C34, and steroids included stigmasterol and β-Sitosterol, and monoacylglycerols included 1-Monopalmitin and 1-Monostearin. Besides the above wax components of different carbon chain lengths, the intracuticular wax also contained C26 fatty acid, C34 fatty acid, δ-tocopherol、γ-tocopherol and α-tocopherol. In terms of cuticularwax content, the epicuticular wax content of the 5 avocado varieties ranged from 0.17-0.87 μg · cm-2 , and the intracuticular wax content ranged from 2.00-8.22 μg · cm-2; the content of epicuticular wax and intracuticular wax of RKYYL6-2 was the lowest among the 5 varieties, while Walter Hole was the highest. In most varieties, the alkane had the highest content among the components of epicuticular wax and intracuticular wax. The intracuticular wax embedded in the cutin and the epicuticular wax covering the outermost layer together form the cuticular wax. In terms of the total cuticular wax (the sum of epicuticular wax and intracuticular wax), the variety with the lowest total cuticular wax content was RKYYL6-2, only 2.17 μg·cm-2; Walter Hole had the highest total wax content, up to 9.09 μg · cm-²; and Peterson, Pollock and RKYYL8-13 had similar total cuticular wax content of 4.13, 4.29 and 4.73 μg· cm-², respectively. In terms of the proportion of fruit epicuticular wax and intracuticular wax in the total cuticular wax, among the 5 avocado varieties, the content of the epicuticular wax of avocado only accounted for 8.02%-12.81% of the total wax content, while the content of the intracuticular wax accounted for 87.19%-91.99%. Therefore, the proportion of intracuticular wax in the total cuticular wax was much higher than that of epicuticular wax. 【Conclusion】 The crystal structure of avocado cuticular wax was usually plates or granular, and different avocado varieties clearly differed in the content and chemical composition of epicuticular wax and intracuticular wax. The research results will help further clarify the biological functions of cuticular wax of avocado fruit.
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