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Concept
Epicuticular wax
Summary
Epicuticular wax is a waxy 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 hydrocarbons with or without a variety of substituted functional 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-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. This broadens the spectrum of wax composition to fatty acids, primary alcohols, and aldehydes; if the substitution occurs at the mid-chain, it will result in β-diketones and secondary alcohols. Other major components of epicuticular waxes are long-chain n-alkanoic acids such as C24, C26, and C28.
These waxes can be composed of a variety of compounds which differ between plant species. Wax 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 triterpenoids, alkanes, aldehydes, esters, secondary alcohols, or flavonoids. However, chemical composition is not diagnostic of a tubule or platelet, as this does not determine the morphology.
Paraffins occur in leaves of peas and cabbages, 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.
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