- Epicuticular wax
In
botany , theplant cuticle is covered by epicuticular wax mainly consistingof straight-chainaliphatic hydrocarbon s with a variety of substituted groups. Commonexamples areparaffin s in leaves ofpea s andcabbage s, alkylester s in leaves ofCarnauba palm andbanana , the asymmetricalsecondary alcohol 10-nonacosanol in mostconifer s such as "Ginkgo biloba " andSitka spruce , many of theRanunculaceae ,Papaveraceae andRosaceae and somemosses , symmetricalsecondary alcohol s inBrassicaceae including "Arabidopsis thaliana ",primary alcohol s (mostly octacosan-1-ol) in most grassesPoaceae , "Eucalyptus " andlegume s among many other plant groups, β-diketone s in many grasses, "Eucalyptus ", box "Buxus " and theEricaceae ,aldehyde s in youngbeech leaves,sugarcane culms and [lemon] fruit andtriterpene s in fruit waxes ofapple ,plum andgrape Baker, EA (1982) Chemistry and morphology of plant epicuticular waxes. In: "The Plant Cuticle"(eds DJ Cutler, KL Alvin, and CE Price), Academic Press, London, pp. 139-165] Holloway, PJ and Jeffree, CE (2005) Epicuticular waxes, Encyclopedia of Applied Plant Sciences, 3, pp. 1190-1204]These compounds are mostly soluble in organic solvents such as
chloroform andhexane , 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 withcell membrane lipids of underlying cells. Epicuticular wax can now also be isolated by mechanical methodsEnsikat, HJ, Neinhuis, C, & Barthlott, W. (2000) Direct access to plant epicuticular wax crystals by a new mechanical isolation method. International Journal of Plant Sciences, 161, 143-148] which distinguish the epicuticular wax outside thePlant Cuticle from thecuticular wax embedded in the cuticle polymer. These two are consequently now known to be chemically distinctJetter, 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] , 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 repellencyHolloway, PJ (1969) The effects of superficial wax on leaf wettability, Annals of Applied Biology, 63, 145-153] , create a self-cleaning property known as the
Lotus effect Barthlott, W & Neinhuis, C (1997) Purity of the sacred lotus, or escape from contamination in biological surfaces. Planta 202, 1-8] and reflectUV radiation. The shapes of the crystals are dependent on the wax compounds present in them. Asymmetricalsecondary alcohol s and β-diketones form hollow waxnanotubes , whileprimary alcohol s and symmetricalsecondary alcohol s form flat platesJeffree, CE, Baker, EA, and Holloway, PJ (1975) Ultrastructure and recrystallisation of plant epicuticular waxes. New Phytologist, 75, 539-549.] Although these have been observed using theTransmission Electron Microscope Juniper, BE & Bradley, DE (1958) The carbon replica technique in the study of the ultrastructure of leaf surfaces, Journal of Ultrastructure Research, 2, 16-27] Hallam, ND (1967) An electron microscope study of the leaf waxes of the genus Eucalyptus L'Heritier, PhD thesis, University of Melbourne] andScanning Electron Microscope Jeffree, CE (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.] Riederer, M & Müller, C, eds. (2006) "Biology of the Plant Cuticle". Blackwell Publishing] [http://www.blackwellpublishing.com/book.asp?ref=140513268X&site=1] the process of growth of the crystals had never been observed directly until Koch and coworkersKoch, K, Neinhuis, C, Ensikat, HJ, 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 (2005) "Sructural analysis of wheat wax (Triticum aestivum, c.v. 'Naturastar' L.): from the molecular level to three dimensional crystals" Planta, 223, 258-270] studied growing wax crystals on leaves ofsnowdrop ("Galanthus nivalis") and other species using theAtomic 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
*"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."
Wikimedia Foundation. 2010.