Comparative analysis of surface wax in mature fruits between Satsuma mandarin (Citrus unshiu) and ‘Newhall’ navel orange (Citrus sinensis) from the perspective of crystal morphology, chemical composition and key gene expression Jinqiu Wang a , Haohao Hao a , Runsheng Liu a , Qiaoli Ma a , Juan Xu a , Feng Chen b , Yunjiang Cheng a,⇑ , Xiuxin Deng a a Key Laboratory of Horticultural Plant Biology(Ministry of Education), National Key Laboratory of Crop Genetic Improvement, College of Horticulture and Forestry, Huazhong Agricultural University, Wuhan 430070, People’s Republic of China b Department of Food, Nutrition and Packaging Sciences, Clemson University, Clemson, SC 29634, United States article info Article history: Received 6 July 2013 Received in revised form 28 October 2013 Accepted 4 December 2013 Available online 11 December 2013 Keywords: Wax Citrus fruits GC–MS Gene expression abstract Surface wax of mature Satsuma mandarin (Citrus unshiu) and ‘Newhall’ navel orange (Citrus sinensis) was analysed by crystal morphology, chemical composition, and gene expression levels. The epicuticular and total waxes of both citrus cultivars were mostly composed of aldehydes, alkanes, fatty acids and primary alcohols. The epicuticular wax accounted for 80% of the total wax in the Newhall fruits and was higher than that in the Satsuma fruits. Scanning electron microscopy showed that larger and more wax platelets were deposited on the surface of Newhall fruits than on the Satsuma fruits. Moreover, the expression lev- els of genes involved in the wax formation were consistent with the biochemical and crystal morpholog- ical analyses. These diversities of fruit wax between the two cultivars may contribute to the differences of fruit postharvest storage properties, which can provide important information for the production of syn- thetic wax for citrus fruits. Ó 2013 Elsevier Ltd. All rights reserved. Introduction Cuticular wax represents the first barrier of self-defence of plants. Not only can it prevent non-stomatal water loss, but it can also keep plant surfaces clean (Barthlott & Neinhuis, 1997) to protect the plants from invasions of pathogens and insects (Eigenb- rode & Espelie, 1995). Plant wax consists mostly of very long chain fatty acids (VLCFAs) and their derivatives, besides small amounts of other components, such as terpenoids, flavonoids, tocopherols, which might protect the plant against biotic stress (Kunst & Sam- uels, 2003). Plant wax embedded in the cutin polymer matrix is called ‘intracuticular wax’, whereas that on the outer surface of the cutin polymer is called ‘epicuticular wax’ (Koch & Ensikat, 2008). Self-assembly processes of epicuticular wax result in several crystal types (Barthlott et al., 1998), among which platelet- and plate-shaped crystals are the predominant types. Very long chain (VLC) aliphatics in wax are synthesised by a series of enzyme-cat- alysed reactions in the endoplasmic reticulum of epidermal cells. For example, fatty acid elongases (FAEs) catalyse the conversion of 16C and 18C fatty acids into various VLCFAs of different chain lengths. VLCFAs are then converted to primary alcohols by acyl reduction pathways. In addition, the decarbonylation pathway re- sults in the production of aldehydes, alkanes, secondary alcohols and ketones (Kunst & Samuels, 2003). Citrus fruits are economically important. They are usually sold in the commercial market after some required postharvest pro- cesses (e.g., washing and waxing). However, washing reduces the wax content on the surface of citrus fruits, resulting in an increased water loss and respiration. Therefore, citrus fruits need to be waxed to alleviate possible quality deterioration. Synthetic (i.e., commercial) wax is likely to partially or completely block stomatal pores and make the fruit more resistant to exchange of ethylene, O 2 and CO 2 gases, causing off-flavours, reduced shelf-lives and poor fruit quality (Ben-Yehoshua, Burg, & Young, 1985). By contrast, the natural surface wax usually neither restricts gas exchange nor neg- atively affects the shelf-life and/or quality of fruits. Such a differ- ence between the synthetic and natural waxes was attributed to their chemical compositional difference. Therefore, it is useful to study natural surface waxes of citrus fruits to develop better syn- thetic waxes. According to previous studies, a wide variety of com- pounds exists in the cuticular wax of citrus, and this depends on its cultivars. Aldehydes are the most predominant compounds in the 0308-8146/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.foodchem.2013.12.021 ⇑ Corresponding author. Tel.: +86 2787281796; fax: +86 2787280626. E-mail address: yjcheng@mail.hzau.edu.cn (Y. Cheng). Food Chemistry 153 (2014) 177–185 Contents lists available at ScienceDirect Food Chemistry journal homepage: www.elsevier.com/locate/foodchem