Epicuticular wax is awaxy coating which covers the outer surface of theplant cuticle inland 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-chainaliphatichydrocarbons with or without a variety of substitutedfunctional groups. 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.
Common constituents of epicuticular wax are predominantly straight-chainaliphatichydrocarbons that may be saturated or unsaturated and contain a variety of functional groups, such as -hydroxyl, carboxyl, and -ketoyl at the terminal position. This broadens the spectrum of wax composition tofatty acids,primary alcohols, andaldehydes; if the substitution occurs at the mid-chain, it will result inβ-diketones andsecondary alcohols.[1] Other major components of epicuticular waxes are long-chainn-alkanoic acids such as C24, C26, and C28.[2]
These waxes can be composed of a variety of compounds which differ between plant species. Waxtubules 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 containingβ-diketones. Platelets are either dominated bytriterpenoids,alkanes, aldehydes,esters, secondary alcohols, orflavonoids. However, chemical composition is not diagnostic of a tubule or platelet, as this does not determine the morphology.[3]
Paraffins occur in leaves ofpeas andcabbages, for example. Leaves ofcarnauba palm andbanana feature alkyl esters. The asymmetrical secondary alcohol 10-nonacosanol appears in mostgymnosperms such asGinkgo biloba andSitka spruce as well as many of theRanunculaceae,Papaveraceae andRosaceae and somemosses. Symmetrical secondary alcohols are found inBrassicaceae includingArabidopsis thaliana.Primary alcohols (most commonlyoctacosan-1-ol) occur inEucalyptus,legumes, and mostPoaceae grasses. Grasses may also feature β-diketones, as doEucalyptus, boxBuxus and theEricaceae. Youngbeech leaves,sugarcane culms andlemon fruit exhibit aldehydes.Triterpenes are the primary component in fruit waxes ofapple,plum andgrape.[1][4] Cyclic constituents are often recorded in epicuticular waxes, as inNicotiana but are generally minor constituents. They may includephytosterols such asβ-sitosterol and pentacyclic triterpenoids such asursolic acid andoleanolic acid and their respective precursors,α-amyrin and β-amyrin.[1]
Many species of the genusPrimula and ferns, such asCheilanthes,Pityrogramma andNotholaena, as well as many genera ofCrassulaceae 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 ofpolyphenolic compounds known as flavonoids.[5] Unlike epicuticular wax, farina is secreted by specialisedglandular hairs, rather than by the cuticle of the entire epidermis.[5]
Epicuticular waxes are mostly solids at ambient temperature, with melting points above about 40 °C (100 °F). They are soluble in organic solvents such aschloroform andhexane, 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 withcell membrane lipids of underlying cells. Epicuticular wax can now also be isolated by mechanical methods that distinguish the epicuticular wax outside theplant cuticle from thecuticular wax embedded in the cuticle polymer.[6] As a consequence, these two are now known to be chemically distinct,[7] although the mechanism that segregates the molecular species into the two layers is unknown. Recentscanning electron microscopy (SEM),atomic force microscopy (AFM) andneutron reflectometry studies on reconstituted wax films have found wheat epicuticular waxes;[8] 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 ofDudleya brittonii, which has the highestultraviolet light (UV)reflectivity of any known naturally occurring biological substance.[9]
The term 'glaucous' is used to refer to any foliage, such as that of the familyCrassulaceae, 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 'farinaceous'.[10]: 51
Epicuticular wax forms crystalline projections from the plant surface, which enhance their water repellency,[11] create a self-cleaning property known as thelotus effect[12] and reflectUV radiation. The shapes of the crystals are dependent on the wax compounds present in them. Asymmetrical secondary alcohols and β-diketones form hollow waxnanotubes, while primary alcohols and symmetrical secondary alcohols form flat plates[13][14] Although these have been observed using thetransmission electron microscope[13][15] andscanning electron microscope[16] the process of growth of the crystals had never been observed directly until Koch and coworkers[17][18] 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.
Epicuticular waxes are recovered from terrestrial, marine, and lake environments, allowing for solvent extraction of biomarkers and then qualitative and quantitative profiling throughgas chromatography–mass spectrometry (GC-MS) andGC flame ionization detection (GC-FID). GC-MS and GC-FID are preferential for identifying and quantifyingn-alkanes andn-alkanoic acids.Isotope ratio analysis (GC-IRMS) measures relative abundance of carbon, hydrogen, and other isotopes with high precision. The carbon isotopic ratio is expressed betweencarbon-13 andcarbon-12 asδ13C relative to the international standard. The hydrogen isotopic ratio betweendeuterium andprotium is expressed asδD relative to the international standard.[19]
[19]Epicuticular wax has been used as abiomarker to observe human evolution patterns. The lipids of these plant waxes have been analyzed when extracted fromocean andlake cores, paleo-lake drilling projects,archeological andgeologicaloutcrops,cavedeposits, and human-bearingsediments. This data provides insight into past plantecology andenvironmental stresses, particularly by reconstructing landscapes at a hightaxonomic resolution.[citation needed]
Epicuticular wax δ13C is a favorable biomarker due to its benefits: it is not biased towards feeding liketooth enamel biomarkers, and are more widespread thanpaleosol carbonates that are biased based on rainfall amount. This marker can also identifyC3 andC4 photosynthetic pathways. Biosynthesis of these lipids result in furtherfractionation that results in lighter the bulk δ13C.Isotope stability studies that characterizediagenetic 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, includingpH,microbial communities,alkalinity,temperature, and oxygen/moisture content.
δ13C of higher order plants has been used atHolocene andPleistocene archeological sites. Diverse environments in modernAfrica have been analyzed through the interpretation of epicuticular wax proxies, from woodedgrassland vegetation (where the C31 homolog is most abundant) toarid and semi-arid regions of southern Africa (characterized by an abundance of C29).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 ofPliocene and Pleistocenehydrology in the Basin. Variance of δD and δ13C at certain dates coincide with changes in variables such asorbital eccentricity and hominidtools.[19]
Epicuticular wax and its successor aliphatic compounds are also used as biomarkers for higher plants. Long-chainn-alkyl compounds fromvascular 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 δ13C value, reflect their metabolism and environment, as13C is discriminated against during photosynthesis.[20]
| Subdisciplines | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| Plant groups | |||||||||||
| Plant anatomy |
| ||||||||||
| Plant physiology Materials | |||||||||||
| Plant growth and habit | |||||||||||
| Reproduction | |||||||||||
| Plant taxonomy | |||||||||||
| Practice | |||||||||||
| |||||||||||
