Morphological, chemical, and biosynthetic changes in pericarp waxes in response to the browning of litchi fruit during storage

Litchi fruit is susceptible to pericarp browning after harvest, which is regulated by the oxidation of polyphenols and membrane lipids during storage. Cuticular waxes form the outermost lipid layer covering plant surfaces, and their changes in response to pericarp browning in litchi remain unclear. In this study, changes in morphological appearance and chemical composition of cuticular waxes, and expression of their associated biosynthesis regulatory genes in litchi were comprehensively investigated. In terms of pericarp microstructure, cuticular variations, such as wrinkled lines with “tooth-like” cuticular ridges, appeared on the epidermal cell surface. Following pericarp browning, the protruding nano-ridges were damaged, resulting in a disturbed appearance. Parallel to the appearance of morphological changes, we observed a 2.3-fold increase in cuticular waxes in the browned pericarp when compared with fresh fruit. Concerning chemical compositions, slight changes in the cyclic constituents were observed, whereas the synthesis of very-long-chain fatty acids, primary alcohols, aldehydes, and n-alkanes with carbon chains greater than C26 appeared to be stimulated in the browned pericarp. Correspondingly, similar increased trends were detected in the relative gene expression levels of LcLACS1/2, LcCER2, LcKCR1, LcHACD, and LcECR in fatty acid elongation, LcCER1 in n-alkanes, and LcFAR1 in primary alcohols. These results indicated that changes in surface microstructure modifications and the increase in biosynthesis of cuticular waxes, particularly the very-long-chain aliphatic components, occurred in response to the browning of litchi fruit during storage. Accordingly, surface cuticular waxes might also play an important role in regulating the browning of fruits during storage.