Wax composition and concentration in jujube (Ziziphus jujuba Mill.) cultivars with differential resistance to fruit cracking

Fruit cracking is a key problem restricting the development of the jujube (Ziziphus jujuba) industry, and is closely related to the distribution of the wax layer on the surface of the fruit. Three jujube cultivars with different levels of cracking resistance, namely 'Popozao', 'Banzao', and 'Hupingzao', were selected for comparison. Cracks on the cuticular membrane (CM) of 'Hupingzao' widened and deepened during the coloring period. The wax level of highly cracking-resistant 'Popozao' was significantly higher than that of 'Hupingzao' during the fruit coloring period. The fruit wax composition of the three jujube cultivars were quite similar, consisting mainly of alkanes, triterpenoids, aldehydes, amines, phenols, esters, ketones, fatty acids, primary alcohols, and other, unclassified compounds. Fatty acids, primary alcohols, and alkanes were the predominant fruit wax compounds of the three cultivars. We further analyzed the carbon chain length of aliphatic compounds and found that the concentration of fatty acids in 'Popozao' was significantly lower than that in 'Banzao' and 'Hupingzao' during the coloring period. Moreover, C28-30 were the most abundant primary alcohols during fruit development. Highly cracking-resistant cultivar 'Popozao' contains more very-long-chain alkanes and aldehydes (carbon atom >20) than 'Banzao' and 'Hupingzao' during the coloring period. In addition, we assessed the expression levels of 11 genes involved in fatty acid biosynthesis, elongation, and degradation, and in wax biosynthesis. Gene expression analysis indicated that KCS1, CER1, CYP86B1, and CYP86A play crucial roles in wax formation on jujube fruit. In conclusion, fruit cracking was correlated with whether wax synthesis is coordinated with fruit enlargement and'Popozao' has a stronger ability to synthesize very-long-chain alkanes and aldehydes. Understanding the diff ;erences in the cuticular wax and the activities of the corresponding genes in jujube cultivars with different sensitivities to cracking will provide a specific way to prevent fruit cracking.