Differential changes in cuticular wax affect the susceptibility to fruit decay in pitaya after harvest: A cultivar comparative study

Pitaya (Hylocereus undatus) fruit is susceptible to pathogens that induce fruit decay after harvest. Cuticular waxes covering the outermost plant surfaces are the first site of plant-environment interactions; however, their changes in response to fruit decay are under-researched. Herein, the surface microstructures and chemical composition of wax on pitayas, as well as their changes in response to fruit decay, were comparatively studied. The varieties 'Baishuijing' (BSJ) and 'Hongshuijing' (HSJ) were examined after harvest. The fruit appearance, morphology, and wax composition differed between cultivars. The surface of both pitaya cultivars was smooth, although traces of granules and a thicker lipid layer were observed using histochemical staining in HSJ than in BSJ, as well as a higher cuticular wax content. Fruit decay rate increased rapidly in both cultivars under room storage conditions, and greater sensitivity to pathogens was found in BSJ than HSJ. As well as the occurrence of fruit decay, the total wax coverage, very-long-chain primary alcohols, aldehydes, fatty acids, and triterpenoids decreased in BSJ, while remaining relatively stable in HSJ. Correspondingly, the relative gene expression levels of HuKCS11 (-1, 2, and 3), HuKCS17, and HuKCS19 (for the elongation of very-long-chain fatty acids), HuCER1 (for aldehydes or n-alkanes), HuFAR (-1 and 2) (for primary alcohols), HuBAS1 (-1, 3, and 4) (for beta-amyrin), and HuLUS1 (for lupeol) were considerably modified following fruit senescence during storage. The difference in surface topography and wax accumulation between cultivars might have affected pathogen susceptibility after harvest. These results may help when choosing varieties of pitaya based on surface characteristics and cuticular chemicals. They may also help to optimize strategies for postharvest preservation and storage.