Epicuticular wax

In botany, the plant cuticle is covered by epicuticular wax mainly consisting of straight-chain aliphatic hydrocarbons with a variety of substituted groups. Common examples are paraffins in leaves of peas and cabbages, alkyl esters in leaves of Carnauba palm and banana, the asymmetrical secondary alcohol 10-nonacosanol in most conifers such as Ginkgo biloba and Sitka spruce, many of the Ranunculaceae, Papaveraceae and Rosaceae and some mosses, symmetrical secondary alcohols in Brassicaceae including Arabidopsis thaliana, primary alcohols (mostly octacosan-1-ol) in most grasses Poaceae, Eucalyptus and legumes among many other plant groups, β-diketones in many grasses, Eucalyptus, box Buxus and the Ericaceae, aldehydes in young beech leaves, sugarcane culms and [lemon] fruit and triterpenes in fruit waxes of apple, plum and grape (Baker 1982; Holloway and Jeffree 2005).

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  These compounds are mostly soluble in organic solvents such as chloroform and hexane, making them accessible for chemical analysis, but in some species esterification of acids and alcohols into estolides or polymerization of aldehydes may give rise to insoluble compounds. Solvent extracts of cuticle waxes contain both epicuticular and cuticular waxes, often contaminated with cell membrane lipids of underlying cells. Epicuticular wax can now also be isolated by mechanical methods (Ensikat, Neinhuis and Barthlott, 2000) which distinguish the epicuticular wax outside the Plant Cuticle from the cuticular wax embedded in the cuticle polymer. These two are consequently now known to be chemically distinct (Jetter, Schäffer, and Riederer 2000), although the mechanism which segregates the molecular species into the two layers is unknown.

Epicuticular wax crystals

Epicuticular wax forms crystalline projections from the plant surface, which enhance their water repellency (Holloway, 1969; see Lotus effect Barthlott and Neinhuis 1997) and reflect UV radiation. The shapes of the crystals are dependent on the wax compounds present in them. Asymmetrical secondary alcohols and β-diketones form hollow wax nanotubes, while primary alcohols and symmetrical secondary alcohols form flat plates (Hallam, 1967; Jeffree, Baker and Holloway 1975). Although these have been observed using the Transmission Electron Microscope (Juniper and Bradley 1958; Hallam 1967) and Scanning Electron Microscope (Jeffree 2006 and references therein) the process of growth of the crystals had never been observed directly until Koch and coworkers (2004, 2005) studied growing wax crystals on leaves of snowdrop (Galanthus nivalis) and other species using the Atomic force microscope. These studies show that the crystals grow by extension from their tips, raising interesting questions about the mechanism of transport of the molecules.

References

• Baker, E. A. (1982) Chemistry and morphology of plant epicuticular waxes, in The Plant Cuticle, (eds D. J. Cutler, K. L. Alvin, and C. E. Price), Academic Press, London, pp. 139-165.
• Hallam, N.D. (1967) An electron microscope study of the leaf waxes of the genus Eucalyptus L'Heritier, PhD thesis, University of Melbourne.
• Holloway, P. J. and Jeffree, C. E. (2005) Epicuticular waxes, Encyclopedia of Applied Plant Sciences, 3, pp. 1190-1204.
• Riederer, M. & Müller, C., eds (2006) Biology of the Plant Cuticle. Blackwell Publishing. [1]
• Barthlott, W. and Neinhuis, C. (1997) Purity of the sacred lotus, or escape from contamination in biological surfaces. Planta 202, 1–8.
• Eigenbrode, S.D. (1996) Plant surface waxes and insect behaviour, in Plant Cuticles: an integrated functional approach, (ed G. Kerstiens), Bios Scientific Publishers, Oxford, pp. 201-221.
• Ensikat, H. J., Neinhuis, C., and Barthlott, W. (2000) Direct access to plant epicuticular wax crystals by a new mechanical isolation method, International Journal of Plant Sciences, 161, 143-148.
• Holloway, P. J. (1969) The effects of superficial wax on leaf wettability, Annals of Applied Biology, 63, 145-153.
• Jeffree, C.E. (2006)The fine structure of the Plant Cuticle. Chapter 2 In: Riederer, M. & Müller, C., eds (2006) Biology of the Plant Cuticle. Blackwell Publishing. pps 11-125.
• Jeffree, C. E., Baker, E. A., and Holloway, P. J. (1975) Ultrastructure and recrystallisation of plant epicuticular waxes, New Phytologist, 75, 539-549.
• Jetter, R., Schäffer, S., and Riederer, M. (2000) Leaf cuticular waxes are arranged in chemically and mechanically distinct layers: evidence from Prunus laurocerasus L., Plant, Cell and Environment, 23, 619-628.
• Juniper, B. E. and Bradley, D. E. (1958) The carbon replica technique in the study of the ultrastructure of leaf surfaces, Journal of Ultrastructure Research, 2, 16-27.
• Koch, K., Neinhuis, C., Ensikat, H. J., and Barthlott, W. (2004) Self assembly of epicuticular waxes on living plant surfaces imaged by atomic force microscopy (AFM), Journal of Experimental Botany, 55, 711-718.
• Koch, K., Barthlott, W., Koch, S., Hommes, A., Wandelt, K., Mamdouh, H., De-Feyter, S. and Broekmann P. Structural analysis of wheat wax (Triticum aestivum, c.v. 'Naturastar' L.): from the molecular level to three dimensional crystals, Planta, 223, 258-270