Cuticular wax metabolism of lemon (Citrus limon Burm. f. Eureka) fruit in response to ethylene and gibberellic acid treatment

The brightness and color of lemon fruit directly determine its commodity value. Ethylene (ETH) and gibberellic acid (GA3) treatment are widely adopted to achieve postharvest uniform peel color, which may affect fruit surface brightness. Fruit brightness is closely associated with cuticular wax. However, little is known about how ETH and GA3 treatment affect cuticular wax metabolism in lemon fruit. Here, we treated green lemon fruit with ethephon and GA3 solution, and analyzed the changes in wax crystal morphology, chemical composition and expression levels of wax-related genes during storage. ETH increased the density and size of platelet-like wax crystals. Moreover, ETH accelerated cuticular wax accumulation, particularly alkanes and aldehydes, while GA3 inhibited these processes. ETH treatment increased the total wax content by 56% after four-day storage, particularly very-long-chain (VLC) alkanes (by 83%), while GA3 decreased the total wax content by 53% after fourteen-day storage, particularly VLC alkanes (by 56%). The main differentially expressed genes, including those involved in total wax accumulation (KCS19-1 and KCS19-2) and transformation of VLC aldehydes to VLC alkanes (CER1-1 and CER3), were up-regulated by ETH treatment while down-regulated by GA3 treatment. In summary, ETH accelerates and GA3 inhibits the accumulation of total wax, VLC aldehydes and VLC alkanes. I In addition, ETH and GA3 treatment have little effect on the intrinsic quality, but can alter the membrane lipid property and oxidase activity of fruit.