{{Short description|Wax coating on the plant cuticle}} [[File:Dudleya Brittonii.jpg|thumb|The epicuticular wax produced by ''[[Dudleya brittonii]]'' has the highest [[ultraviolet light]] (UV) [[reflectivity]] of any known naturally occurring biological substance.]] '''Epicuticular wax''' is a [[wax]]y coating which covers the outer surface of the [[plant cuticle]] in [[land plants]]. It may form a whitish film or bloom on leaves, fruits and other plant organs. Chemically, it consists of hydrophobic organic compounds, mainly straight-chain [[aliphatic]] [[hydrocarbon]]s with or without a variety of substituted [[functional group]]s. The main functions of the epicuticular wax are to decrease surface wetting and moisture loss. Other functions include reflection of ultraviolet light, assisting in the formation of an ultra-hydrophobic and self-cleaning surface and acting as an anti-climb surface.
==Chemical composition== Common constituents of epicuticular wax are predominantly straight-chain [[aliphatic]] [[hydrocarbon]]s that may be saturated or unsaturated and contain a variety of functional groups, such as -[[Hydroxy group|hydroxyl]], carboxyl, and -[[Ketone|ketoyl]] at the terminal position. This broadens the spectrum of wax composition to [[Fatty acid|fatty acids]], [[Primary alcohol|primary alcohols]], and [[Aldehyde|aldehydes]]; if the substitution occurs at the mid-chain, it will result in ''β''-[[Dicarbonyl|diketones]] and [[secondary alcohols]].<ref name="Baker-1982" /> Other major components of epicuticular waxes are long-chain [[Carboxylic acid|''n''-alkanoic]] acids such as C<sub>24</sub>, C<sub>26</sub>, and C<sub>28</sub>.<ref>{{Cite book |last1=Peters |first1=K. E. |title=The Biomarker Guide |last2=Walters |first2=C. C. |last3=Moldowan |first3=J. M. |publisher=[[Cambridge University Press]] |year=2005 |isbn=0521781582 |edition=2nd |volume=1 |pages=47}}</ref> [[File:Wax morphologies.png|thumb|Wax morphologies visualized with SEM: wax tubules dominated by nonacosan-10-ol on a ''Thalictrum flavum glaucum'' L. (Desf.) leaf in (a), tubules dominated by ''β''-diketones of a ''Eucalyptus gunnii'' Hook leaf in (b), leaf wax of ''Triticum aestivum'' 'Naturastar' in (c), and rodlets demonstrating terminal dendritic branching composed of a complex mixture of several compounds on a ''Brassica oleracea'' L. leaf in (d).<ref name="Koch-2006" />]] These waxes can be composed of a variety of compounds which differ between plant species. Wax [[Tubule|tubules]] and wax platelets often have chemical as well as morphological differences. Tubules can be separated into two groups; the first primarily containing secondary alcohols, and the second containing ''β''-diketones. Platelets are either dominated by [[Triterpene|triterpenoids]], [[Alkane|alkanes]], aldehydes, [[Ester|esters]], secondary alcohols, or [[Flavonoid|flavonoids]]. However, chemical composition is not diagnostic of a tubule or platelet, as this does not determine the morphology.<ref name="Koch-2006">{{Cite journal |last1=Koch |first1=Kerstin |last2=Barthlott |first2=Wilhelm |date=2006 |title=Plant Epicuticular Waxes: Chemistry, Form, Self-Assembly and Function |journal=Natural Product Communications |language=en |volume=1 |issue=11 |pages=1934578X0600101 |doi=10.1177/1934578X0600101123 |issn=1934-578X |doi-access=free }}</ref>
[[Alkane|Paraffins]] occur in leaves of [[pea]]s and [[cabbage]]s, for example. Leaves of [[carnauba]] palm and [[banana]] feature alkyl esters. The asymmetrical secondary alcohol 10-nonacosanol appears in most [[gymnosperms]] such as ''[[Ginkgo biloba]]'' and [[Sitka spruce]] as well as many of the [[Ranunculaceae]], [[Papaveraceae]] and [[Rosaceae]] and some [[mosses]]. Symmetrical secondary alcohols are found in [[Brassicaceae]] including ''[[Arabidopsis thaliana]].'' [[Primary alcohol]]s (most commonly [[octacosan-1-ol]]) occur in ''[[Eucalyptus]],'' [[legume]]s, and most [[Poaceae]] grasses. Grasses may also feature β-diketones, as do ''[[Eucalyptus]]'', box ''[[Buxus]]'' and the [[Ericaceae]]. Young [[beech]] leaves, [[sugarcane]] culms and [[lemon]] fruit exhibit aldehydes. [[Triterpene|Triterpenes]] are the primary component in fruit waxes of [[apple]], [[plum]] and [[grape]].<ref name="Baker-1982">{{cite book |last=Baker |first=E. A. |year=1982 |chapter=Chemistry and morphology of plant epicuticular waxes |title=The Plant Cuticle |editor-first=D. J. |editor-last=Cutler |editor2-first=K. L. |editor2-last=Alvin |editor3-first=C. E. |editor3-last=Price |publisher=Academic Press |location=London |pages=139–165 |isbn=0-12-199920-3}}</ref><ref name="Holloway-2005">{{cite journal |last1=Holloway |first1=P.J. |last2=Jeffree |first2=C.E. |date=2005 |title=Epicuticular waxes |journal=Encyclopedia of Applied Plant Sciences |volume=3 |pages=1190–1204}}</ref> Cyclic constituents are often recorded in epicuticular waxes, as in ''Nicotiana'' but are generally minor constituents. They may include [[phytosterol]]s such as [[β-sitosterol]] and pentacyclic triterpenoids such as [[ursolic acid]] and [[oleanolic acid]] and their respective precursors, [[Amyrin|α-amyrin]] and β-amyrin.<ref name="Baker-1982" />
==Farina== Many species of the genus ''[[Primula]]'' and ferns, such as ''[[Cheilanthes]]'', ''[[Pityrogramma]]'' and ''[[Notholaena]]'', as well as many genera of [[Crassulaceae]] succulent plants, produce a mealy, whitish to pale-yellow glandular secretion known as farina that is not an epicuticular wax, but consists largely of crystals of a different class of [[polyphenol]]ic compounds known as flavonoids.<ref name="Blasdale-1945">{{cite journal |author=Walter C. Blasdale |date=1945 |title=The composition of the solid secretion produced by ''[[Primula denticulata]]'' |journal=Journal of the American Chemical Society |volume=67 |issue=3 |pages=491–493 |doi=10.1021/ja01219a036|bibcode=1945JAChS..67..491B }}</ref> Unlike epicuticular wax, farina is secreted by specialised [[Trichome#Plant trichomes|glandular hairs]], rather than by the cuticle of the entire epidermis.<ref name="Blasdale-1945"/>
==Physical properties== [[Image:Rosa epicuticular wax stoma.jpg|thumb|right|250px|Epicuticular wax crystals surrounding a stomatal aperture on the lower surface of a rose leaf]] Epicuticular waxes are mostly solids at ambient temperature, with melting points above about {{convert|40|C|F|-1|abbr=on}}. They are 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 the 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 that distinguish the epicuticular wax outside the [[plant cuticle]] from the [[cuticular wax]] embedded in the cuticle polymer.<ref name="Ensikat-2000">{{cite journal |last1=Ensikat |first1=H. J. |last2=Neinhuis |first2=C. |last3=Barthlott |first3=W. |display-authors=1 |year=2000 |title=Direct access to plant epicuticular wax crystals by a new mechanical isolation method |journal=International Journal of Plant Sciences |volume=161 |issue=1 |pages=143–148 |doi=10.1086/314234|pmid=10648204 |bibcode=2000IJPlS.161..143E |s2cid=92392 }}</ref> As a consequence, these two are now known to be chemically distinct,<ref name="Jetter-2000">{{cite journal |last1=Jetter |first1=R. |last2=Schäffer |first2=S. |last3=Riederer |first3=M. |display-authors=1 |year=2000 |title=Leaf cuticular waxes are arranged in chemically and mechanically distinct layers: evidence from ''Prunus laurocerasus'' L. Plant |journal=Cell and Environment |volume=23 |issue=6 |pages=619–628 |doi=10.1046/j.1365-3040.2000.00581.x |doi-access=free}}</ref> although the mechanism that segregates the molecular species into the two layers is unknown. Recent [[scanning electron microscopy]] (SEM), [[atomic force microscopy]] (AFM) and [[neutron reflectometry]] studies on reconstituted wax films have found wheat epicuticular waxes;<ref>{{cite journal |last1=Pambou |first1=E. |last2=Li |first2=Z. |last3=Campana |first3=M. |last4=Hughes |first4=A. |last5=Clifton |first5=L. |last6=Gutfreund |first6=P. |last7=Foundling |first7=J. |last8=Bell |first8=G. |last9=Lu |first9=J. R. |display-authors=1 |year=2016 |title=Structural features of reconstituted wheat wax films |journal=J. R. Soc. Interface |volume=13 |issue=120 |at=20160396 |doi=10.1098/rsif.2016.0396 |pmid=27466439 |doi-access=free|pmc=4971226 }}</ref> made up of surface epicuticular crystals and an underlying, porous background film layer to undergo swelling when in contact with water, indicating the background film is permeable and susceptible to the transport of water.
Epicuticular wax can reflect UV light, such as the white, chalky, wax coating of ''[[Dudleya brittonii]]'', which has the highest [[ultraviolet light]] (UV) [[reflectivity]] of any known naturally occurring biological substance.<ref>{{cite journal |last=Mulroy |first=Thomas W. |title=Spectral properties of heavily glaucous and non-glaucous leaves of a succulent rosette-plant |journal=Oecologia |date=1979 |volume=38 |issue=3| pages=349–357 |doi=10.1007/BF00345193 |pmid=28309493 |bibcode=1979Oecol..38..349M |s2cid=23753011}}</ref>
The term 'glaucous' is used to refer to any foliage, such as that of the family [[Crassulaceae]], which appears whitish because of the waxy covering. Coatings of epicuticular flavonoids may be referred to as 'farina', the plants themselves being described as 'farinose' or '[[Glossary of botanical terms#farinaceous|farinaceous]]'.<ref name="Beentje-2016">{{cite book | author=Henk Beentje |date=2016 |title=The Kew plant glossary |edition=2 |publisher=Kew Publishing |location=Richmond, Surrey |isbn=978-1-84246-604-9}}</ref>{{rp|51}}
==Epicuticular wax crystals== Epicuticular wax forms crystalline projections from the plant surface, which enhance their water repellency,<ref name="Holloway-1969">{{cite journal |last=Holloway |first=P. J. |year=1969 |title=The effects of superficial wax on leaf wettability |journal=Annals of Applied Biology |volume=63 |issue=1 |pages=145–153 |doi=10.1111/j.1744-7348.1969.tb05475.x}}</ref> create a self-cleaning property known as the [[lotus effect]]<ref name="Barthlott-1997">{{cite journal |last1=Barthlott |first1=W. |last2=Neinhuis |first2=C. |year=1997 |title=Purity of the sacred lotus, or escape from contamination in biological surfaces |journal=Planta |volume=202 |issue= 1|pages=1–8 |doi=10.1007/s004250050096|bibcode=1997Plant.202....1B |s2cid=37872229 }}</ref> 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 [[Carbon nanotube|nanotubes]], while primary alcohols and symmetrical secondary alcohols form flat plates<ref name="Hallam-1967"/><ref name="Jeffree-1975">{{cite journal |last1=Jeffree |first1=C. E. |last2=Baker |first2=E. A. |last3=Holloway |first3=P. J. |year=1975 |title=Ultrastructure and recrystallisation of plant epicuticular waxes |journal=New Phytologist |volume=75 |issue= 3|pages=539–549 |doi=10.1111/j.1469-8137.1975.tb01417.x |doi-access=free|bibcode=1975NewPh..75..539J }}</ref> Although these have been observed using the [[transmission electron microscope]]<ref name="Hallam-1967">{{cite thesis |last=Hallam |first=N. D. |year=1967 |title=An electron microscope study of the leaf waxes of the genus Eucalyptus L'Heritier |type=PhD thesis |publisher=University of Melbourne |oclc=225630715}}</ref><ref name="Juniper-1958">{{cite journal |last1=Juniper |first1=B. E. |last2=Bradley |first2=D. E. |year=1958 |title=The carbon replica technique in the study of the ultrastructure of leaf surfaces |journal=Journal of Ultrastructure Research |volume=2 |issue= |pages=16–27 |doi=10.1016/S0022-5320(58)90045-5}}</ref> and [[scanning electron microscope]]<ref name="Jeffree-2006">{{cite book |last=Jeffree |first=C. E. |year=2006 |chapter=The fine structure of the Plant Cuticle |editor-last=Riederer |editor-first=M. |editor2-last=Müller |editor2-first=C. |archive-url=https://web.archive.org/web/20070406172655/http://www.blackwellpublishing.com/book.asp?ref=140513268X&site=1 |archive-date=April 6, 2007 |url=http://www.blackwellpublishing.com/book.asp?ref=140513268X&site=1 |title=Biology of the Plant Cuticle |publisher=Blackwell Publishing |pages=11–125}}</ref> the process of growth of the crystals had never been observed directly until Koch and coworkers<ref name="Koch-2004">{{cite journal |last1=Koch |first1=K. |last2=Neinhuis |first2=C. |last3=Ensikat |first3=H. J. |last4=Barthlott |first4=W. |display-authors=1 |year=2004 |title=Self assembly of epicuticular waxes on living plant surfaces imaged by atomic force microscopy (AFM) |journal=Journal of Experimental Botany |volume=55 |issue= 397|pages=711–718 |doi=10.1093/jxb/erh077 |pmid=14966216 |doi-access=free}}</ref><ref name="Koch-2005">{{cite journal |last1=Koch |first1=K. |last2=Barthlott |first2=W. |last3=Koch |first3=S. |last4=Hommes |first4=A. |last5=Wandelt |first5=K. |last6=Mamdouh |first6=H. |last7=De-Feyter |first7=S. |last8=Broekmann |first8=P. |display-authors=1 |year=2005 |title=Structural analysis of wheat wax (Triticum aestivum, c.v. 'Naturastar' L.): from the molecular level to three dimensional crystals |journal=Planta |volume=223 |issue= 2|pages=258–270 |doi=10.1007/s00425-005-0081-3|pmid=16133211 |s2cid=20775168 }}</ref> 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.
== Measurement techniques == Epicuticular waxes are recovered from terrestrial, marine, and lake environments, allowing for solvent extraction of biomarkers and then qualitative and quantitative profiling through [[gas chromatography–mass spectrometry]] (GC-MS) and [[Flame ionization detector|GC flame ionization detection]] (GC-FID). GC-MS and GC-FID are preferential for identifying and quantifying ''n''-alkanes and ''n''-alkanoic acids. [[Isotopic signature|Isotope ratio analysis]] (GC-IRMS) measures relative abundance of carbon, hydrogen, and other isotopes with high precision. The carbon isotopic ratio is expressed between [[carbon-13]] and [[carbon-12]] as [[Δ13C|δ<sup>13</sup>C]] relative to the international standard. The hydrogen isotopic ratio between [[deuterium]] and [[Hydrogen atom|protium]] is expressed as [[Δ-Decalactone|δD]] relative to the international standard.<ref name="Patalano-2021" />
== Use as a biomarker == [[File:N-alkanoic GC-MS.png|thumb|GC-MS trace of ''n''-alkanoic even-over-odd and ''n''-alkane odd-over-even (top and bottom, respectively), carbons, particularly the long chains produced by terrestrial plants for C-13 analysis.<ref name="Patalano-2021" />]] <ref name="Patalano-2021" /> Epicuticular wax has been used as a [[biomarker]] to observe human evolution patterns. The lipids of these plant waxes have been analyzed when extracted from [[ocean]] and [[lake]] cores, paleo-lake drilling projects, [[Archaeology|archeological]] and [[Geology|geological]] [[Outcrop|outcrops]], [[cave]] [[Deposition (geology)|deposits]], and human-bearing [[Sediment|sediments]]. This data provides insight into past plant [[ecology]] and [[Stress (biology)|environmental stresses]], particularly by reconstructing landscapes at a high [[Taxonomy|taxonomic]] resolution.{{citation needed|date=January 2025}}
Epicuticular wax δ<sup>13</sup>C is a favorable biomarker due to its benefits: it is not biased towards feeding like [[tooth enamel]] biomarkers, and are more widespread than [[paleosol]] carbonates that are biased based on rainfall amount. This marker can also identify [[C3 carbon fixation|C<sub>3</sub>]] and [[C4 carbon fixation|C<sub>4</sub>]] photosynthetic pathways. Biosynthesis of these lipids result in further [[fractionation]] that results in lighter the bulk δ<sup>13</sup>C. [[Isotope]] stability studies that characterize [[Diagenesis|diagenetic]] process can identify carbon and hydrogen alteration through chemical and microbial activity, but these studies often have mixed results. The state of plant wax preservation in soils and sediments is still unknown due to complex interactions in the depositional environments, including [[pH]], [[Microbial population biology|microbial communities]], [[alkalinity]], [[temperature]], and oxygen/moisture content.
δ<sup>13</sup>C of higher order plants has been used at [[Holocene]] and [[Pleistocene]] archeological sites. Diverse environments in modern [[Africa]] have been analyzed through the interpretation of epicuticular wax proxies, from wooded [[grassland]] vegetation (where the C<sub>31</sub> homolog is most abundant) to [[arid]] and semi-arid regions of southern Africa (characterized by an abundance of C<sub>29</sub>). [[Turkana people|Turkana]] paleo-lake sediments from the East (3.45–3.4 Ma Wargolo Formation) and the West (1.9–1.4 Ma Nachukui Formation) suggest precession-controlled summer insolation is the primary driver of [[Pliocene]] and Pleistocene [[hydrology]] in the Basin. Variance of δD and δ<sup>13</sup>C at certain dates coincide with changes in variables such as [[orbital eccentricity]] and hominid [[Tool|tools]].<ref name="Patalano-2021">{{Cite journal |last1=Patalano |first1=Robert |last2=Roberts |first2=Patrick |last3=Boivin |first3=Nicole |last4=Petraglia |first4=Michael D. |last5=Mercader |first5=Julio |date=2021 |title=Plant wax biomarkers in human evolutionary studies |url=https://onlinelibrary.wiley.com/doi/10.1002/evan.21921 |journal=Evolutionary Anthropology: Issues, News, and Reviews |language=en |volume=30 |issue=6 |pages=385–398 |doi=10.1002/evan.21921 |pmid=34369041 |issn=1060-1538 |via=Wiley Online Library|hdl=10072/409183 |s2cid=236960097 |hdl-access=free }}</ref> [[File:N-alkyl compounds.png|thumb|Chemical compounds of the four most abundant ''n''-alkyl compounds in epicuticular waxes of terrestrial waxes, with I being n-alkane, II n-alkanol, III n-alkanoic acid, and IV wax ester.<ref name="Pancost-2004">{{Cite journal |last1=Pancost |first1=Richard D. |last2=Boot |first2=Christopher S. |date=2004-12-01 |title=The palaeoclimatic utility of terrestrial biomarkers in marine sediments |url=https://www.sciencedirect.com/science/article/pii/S0304420304002099 |journal=Marine Chemistry |series=New Approaches in Marine Organic Biogeochemistry: A Tribute to the Life and Science of John I. Hedges |language=en |volume=92 |issue=1 |pages=239–261 |doi=10.1016/j.marchem.2004.06.029 |bibcode=2004MarCh..92..239P |issn=0304-4203|url-access=subscription }}</ref>]] Epicuticular wax and its successor aliphatic compounds are also used as biomarkers for higher plants. Long-chain ''n''-alkyl compounds from [[Vascular plant|vascular plants]] leaves are major components of epicuticular waxes that are resistant to degradation and thus effective biomarkers for higher plants. These terrestrial biomarkers can also be present in marine sediments. Due to the lack of higher plant material in aqueous settings, the presence of higher plant biomarkers in these ecosystems infer that these biomarkers were transported from their original terrestrial environment. Carbon isotopic compositions, specifically, their δ<sup>13</sup>C value, reflect their metabolism and environment, as <sup>13</sup>C is discriminated against during photosynthesis.<ref name="Pancost-2004" />
== See also == *[[Wax]] *[[Plant cuticle]] *[[Glaucous]]
== References == {{Reflist}}
== Bibliography == {{Scholia|topic}} {{refbegin}} *{{cite book |last=Eigenbrode |first=S. D. |year=1996 |chapter=Plant surface waxes and insect behaviour |title=Plant Cuticles: an integrated functional approach |editor-first=G. |editor-last=Kerstiens |publisher=Bios Scientific Publishers |location=Oxford |pages=201–221 |isbn=1-85996-130-4 }} {{refend}}
{{Botany}}
{{DEFAULTSORT:Epicuticular Wax}} [[Category:Plant anatomy]] [[Category:Plant physiology]]