Epicuticular wax

Chemically, it consists of hydrophobic organic compounds, mainly straight-chain aliphatic hydrocarbons with or without a variety of substituted functional groups.

Common constituents of epicuticular wax are predominantly straight-chain aliphatic hydrocarbons that may be saturated or unsaturated and contain a variety of functional groups, such as -hydroxyl, carboxyl, and -ketoyl at the terminal position.

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.

[1] 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 polyphenolic compounds known as flavonoids.

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.

[10]: 51 Epicuticular wax forms crystalline projections from the plant surface, which enhance their water repellency,[11] create a self-cleaning property known as the lotus effect[12] and reflect UV radiation.

Asymmetrical secondary alcohols and β-diketones form hollow wax nanotubes, while primary alcohols and symmetrical secondary alcohols form flat plates[13][14] Although these have been observed using the transmission electron microscope[13][15] and scanning 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 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.

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 GC Flame Ionization Detection (GC-FID).

The lipids of these plant waxes have been analyzed when extracted from ocean and lake cores, paleo-lake drilling projects, archeological and geological outcrops, cave deposits, and human-bearing sediments.

This data provides insight into past plant ecology and environmental stresses, particularly by reconstructing landscapes at a high taxonomic resolution.

The state of plant wax preservation in soils and sediments is still unknown due to complex interactions in the depositional environments, including pH, microbial communities, alkalinity, temperature, and oxygen/moisture content.

The epicuticular wax produced by Dudleya brittonii has the highest ultraviolet light (UV) reflectivity of any known naturally occurring biological substance
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). [ 3 ]
Epicuticular wax crystals surrounding a stomatal aperture on the lower surface of a rose leaf.
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. [ 19 ]
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. [ 20 ]