Turkish Journal of Botany Turk J Bot (2018) 42: 135-149 http://journals.tubitak.gov.tr/botany/ © TÜBİTAK Review Article doi:10.3906/bot-1702-25 Properties, variations, roles, and potential applications of epicuticular wax: a review 1 1 2 1, Priyal SHARMA , Shankar Lal KOTHARI , Mahender Singh RATHORE , Vinod Singh GOUR * 1 Amity Institute of Biotechnology, Amity University, Jaipur, Rajasthan, India 2 Basic Tasar Silkworm Seed Organization, Central Silk Board, Bilaspur, India Received: 16.02.2017 Accepted/Published Online: 16.07.2017 Final Version: 20.03.2018 Abstract: The cuticular wax layer covers the aerial surface of plants and acts as a barrier between plants and the environment. The cuticle plays a key role in the protection of plants from pathogens, UV light, and transpiration. Variation in the wax quality and quantity is influenced by factors like the solvent used for extraction, species, ontogeny, and season. Compounds isolated from the cuticle layer have been studied by various methods and were found to play an important role from the ecological and physiological points of view. These compounds include esters, alcohols, ether, alkane, and aldehydes. Nonpolar compounds help reduce water loss in plants. The wax can be explored for its potential applications in developing sustainable green packaging material. This review article will facilitate biologists and nonbiologists to get comprehensive and updated knowledge about various aspects of cuticular wax including its chemical composition and variations among different species and seasons. Further studies of the wax composition will pave the way for classification of plant species and an understanding of plant protection from biotic and abiotic stresses. Key words: Cuticle, protection, lotus effect, chemical composition 1. Introduction low chemical reactivity, and hydrophobicity (Domínguez The aerial plant surface is usually covered with a and Heredia, 1998). Hydrophobicity of the epicuticular hydrophobic material called epicuticular wax. This wax depends upon the chemical composition of the wax. waxy layer is an active interface between a plant and The presence of higher amounts of compounds having the environment. It is the first line of defense for the functional groups like –OH, -COOH, -NO2, or -CO- aerial plant body. Epicuticular wax protects the plant provides hydrophilicity to the surface. from several abiotic and biotic stresses. Epicuticular According to Barnes et al. (2009), 10% of total wax protects the plant from infection by insect, fungal, municipal waste consists of plastic. Plastic waste and bacterial pathogens and helps plants conserve water contaminates terrestrial, freshwater, and marine habitats. content by reducing the rate of transpiration (Taíz and Plastic material and its fragments cause soil pollution Zeiger, 1991; Rhee et al., 1998; Znidarcic et al., 2008; Dutta (Zubris and Richards, 2005; Brinton, 2005). The need for and Laskar, 2009). biodegradable packaging material becomes important in Epicuticular wax is a mixture of various classes of the contemporary scenario. compounds, including n-alkanes (chain-length C21– Here, epicuticular wax offers a promising option in C35), primary alcohols (C22–C40), fatty acids (C20–C24), developing hydrophobic packaging material. Paper may aldehydes (C24–C36), secondary alcohols (C21–C35) with be coated with epicuticular wax to obtain hydrophobic a tendency for midchain hydroxylation, ketones (C21– packaging material. In this aspect, recently Yadav et al. C35), diketones (C22–C36), and n-alkyl esters (C36–C60) (2014) reported the use of epicuticular wax derived from in combination with long-chain primary alcohols and fatty Calotropis procera to increase hydrophobicity of paper. acids (Baker, 1982; Walton, 1990). In this review, various features of surface wax have The composition of epicuticular wax is influenced by been discussed in detail, like its morphology, extraction plant genotype, the side and age of the leaf, and seasonal procedures, characterization, and factors affecting the and climatic conditions. According to Walton (1990) composition and yield of wax along with the applications of the quantity of the wax is influenced by environmental epicuticular and cuticular wax in various sectors. Emphasis factors but chemical composition remains conserved. has been given on composition, yield, and application of Plant cuticular wax shows a high degree of crystallinity, surface wax. This paper will help readers gain knowledge * Correspondence: [email protected] 135 SHARMA et al. / Turk J Bot and will aid in the goal of isolating an ecofriendly plant- alcohols, ketones, and alkanes with chain length from C20 derived material to develop hydrophobic material to be to C36 (Jeffree, 2006). Alkyl esters ranging from C38 to used in packaging and other industries. C70 are present in cuticular wax. Some cyclic compounds like triterpenoids, tocopherols, and aromatic compounds 2. Cuticle as a source of wax are present but their quantities vary. It is possible to extract The cuticle comprises the outermost layers of aerial plant the epicuticular and intracuticular wax separately with parts (Jetter et al., 2006). It consists of cutin and wax. the help of solvents (Jetter et al., 2000; Jetter and Schaffer, Based on the location of the wax, it can be divided into two 2001). The epicuticular wax of Prunus laurocerasus consists layers: inside or outside the surface (Jeffree, 1986). In most of aliphatic compounds and the intracuticular wax consists plant species, epicuticular wax forms a smooth layer on the of high percentages of two cyclic triterpenoids (Jetter et surface, whereas wax crystals form a rough surface in some al., 2000). Some compounds like triterpenoids are also species (Jeffree, 2006). The internal structure and design of reported in wax crystals of Ricinus communis (Guhling et cuticular wax can be studied easily by the extraction of the al., 2006) and Macaranga spp. (Markstadter et al., 2000). wax layer with the help of organic solvents (Jetter et al., There is wide diversity in the composition of cuticular wax 2006) because the solvent molecules enter the cuticle and among plant species (Vogg et al., 2004). provide a combination of epicuticular and intracuticular Hydroxy fatty acids are present as a major compound in wax (Jetter et al., 2000). Gas chromatography and scanning the cuticular layer of all vascular plants (Kolattukudy, 1970, electron microscopy have been used for separate analysis 1980; Martin and Juniper, 1970). Aliphatic compounds of both intracuticular and epicuticular wax layers (Jetter et present in the epicuticular layer are n-alkanes, n-alkanals, al., 2000; Jetter and Schaffer, 2001). n-alkanols, n-alkanoic acids, and wax esters (Kolattukudy, Cuticle and epicuticular layers play important roles 1970; Simoneit and Mazurek, 1982; Simoneit, 1989). in the deposition of pesticides, growth regulators, and Plant species show a very common composition other agricultural chemicals (Martin and Juniper, 1970). of cuticular wax having very-long-chain aliphatic The wax layer present on the outer surface of the plant components, namely fatty acids, aldehydes, primary and is not only important for fruit development but also has secondary alcohols, ketones, and alkanes of chain lengths implications on the economic aspects of viticultural C20–C36, as well as C38–C70 alkyl esters (Bianchi, 1995; commodities. It also scatters the light and gives a shiny Jetter et al., 2006). appearance and this attracts the consumers of table grapes. Abas and Simoneit (1998) reported hydrocarbons like The wax biosynthesis in plants is genetically governed carboxylic acid, ketone, and alcohol in the epicuticular by crosstalk, so it can also be used in the classification wax of ten plants, namely Calophyllum inophyllum, of plants or in establishing the relationship among them Cerbera odollarn, Eugenia grandis, Fagraea jragrans, (Lemieux, 1996). Hevea brasiliensis, Melaleuca leucadendron, Mirnusops 2.1. Morphology of surface wax elengi, Mesua ferrea, Lagerstroemia indica, and Pterocarpus The cuticular wax layer consists of dendrites, filaments, indicus. Mesua ferrea showed the highest amount of wax plates, and tubes when viewed through a scanning electron (35.4 mg/g dry weight), whereas the lowest amount (1.2 microscope (Rashotte and Feldmann, 1998). According mg/g dry weight) was present in Eugenia grandis. Analysis to Vogg et al. (2004) the cuticle is a thin, hydrophobic, revealed that out of the n-alkenes present in the wax, the and flexible membrane (0.1–10 µm). In some species, maximum amount was found to be of hentriacontane the epicuticular wax exists as a smooth layer that gives (C31), followed by tritriacontane (C33) and finally the surface a shiny appearance and sometimes it forms nonacosane (C29). A strong even-to-odd carbon number wax crystals, which result in the textured structure of the predominance is observed for both the n-alkanoic acids surface (Jeffree, 2006). and alkanols. Epicuticular wax has several compounds 2.2. Composition like p-sitosterol, triterpenoids, triterpenes with α- and Cuticular wax is very specific in nature. It is organ- and β-boswellic acids, α- and β-amyrin, α- and β-amyrones, tissue-specific in terms of composition. Plant epicuticular friedelin, friedelanol, friedelane, olean-12-ene, taraxerene, wax is a mixture of primarily long-chain aliphatic squalene, dihydronyctanthic acid, dihydroroburic acid, and compounds. Primary alcohols, aldehydes, fatty acids, and dihydrocanaric