Plant cuticles are a protective waxy covering produced only by the
epidermal cellsThe epidermis is a single-layered group of cells that covers plants leaves, flowers, roots and stems. It forms a boundary between the plant and the external world. The epidermis serves several functions, it protects against water loss, regulates gas exchange, secretes metabolic compounds, and ...
of
leavesIn botany, a leaf is an above-ground plant organ specialized for photosynthesis. For this purpose, a leaf is typically flat and thin. There is continued debate about whether the flatness of leaves evolved to expose the chloroplasts to more light or to increase the absorption of carbon dioxide. In...
, young shoots and all other aerial plant organs without periderm. The cuticle tends to be thicker on the top of the leaf, but is not always thicker in
xerophyticA xerophyte or xerophytic organism is a plant which is able to survive in an environment with little available water or moisture, usually in environments where potential evapotranspiration exceeds precipitation for all or part of the growing season...
plants living in dry climates than in
mesophyticMesophytes are terrestrial plants which are adapted to neither a particularly dry nor particularly wet environment. An example of a mesophytic habitat would be a rural temperate meadow, which might contain Goldenrod, Clover, Oxeye Daisy, and Rosa multiflora.Mesophytes make up the largest ecological...
plants from wetter climates, despite a persistent myth to that effect.
The cuticle is composed of an insoluble cuticular membrane impregnated by and covered with soluble
waxWax refers to beeswax or another substance with similar properties. The traditional meaning, beeswax, refers to a substance secreted by bees and used by them in constructing their honeycombs...
es.
CutinCutin is one of two waxy polymers that are the main components of the plant cuticle which covers all aerial surfaces of plants. The other major cuticle polymer, which is much more readily preserved in the fossil record, is cutan...
, a
polyesterPolyester is a category of polymers which contain the ester functional group in their main chain. Although there are many polyesters, the term "polyester" as a specific material most commonly refers to polyethylene terephthalate...
polymerA polymer is a large molecule composed of repeating structural units typically connected by covalent chemical bonds. While polymer in popular usage suggests plastic, the term actually refers to a large class of natural and synthetic materials with a variety of properties.Due to the extraordinary...
composed of inter-esterified
omega hydroxy acidOmega hydroxy acids are a class of naturally-occurring straight-chain aliphatic organic acids n carbon atoms long with a carboxyl group at position 1 and a hydroxyl at position n. The C16 and C18 omega hydroxy acids 16-hydroxy palmitic acid and 18-hydroxy stearic acid are key monomers of cutin in...
s which are cross-linked by
esterEsters are chemical compounds derived by reacting an oxoacid with a hydroxyl compound such as an alcohol or phenol. Esters are usually derived from an inorganic acid or organic acid in which at least one -OH group is replaced by an -O-alkyl group, and most commonly from carboxylic acids and...
and
epoxideAn epoxide is a cyclic ether with three ring atoms. This ring approximately defines an equilateral triangle, which makes it highly strained. The strained ring makes epoxides more reactive than other ethers. Simple epoxides are named from the parent compound ethylene oxide or oxirane, such as in...
bonds, is the best-known structural component of the cuticular membrane. The cuticle can also contain a non-saponifiable
hydrocarbonIn organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. With relation to chemical terminology, aromatic hydrocarbons or arenes, alkanes, alkenes and alkyne-based compounds composed entirely of carbon and hydrogen are referred to as "pure"...
polymer known as
CutanCutan is one of two polymers which occur in the cuticle of some plants. The other and better-known polymer is Cutin. Cutan is believed to be a hydrocarbon polymer, whereas cutin is a polyester, but the structure and synthesis of cutan are not yet fully understood...
.
[Tegelaar, EW, et al. (1989) Scope and limitations of several pyrolysis methods in the structural elucidation of a macromolecular plant constituent in the leaf cuticle of Agave americana L., Journal of Analytical and Applied Pyrolysis, 15, 29-54] The cuticular membrane is impregnated with cuticular waxes and covered with
epicuticular waxIn botany, the plant cuticle is covered by epicuticular wax mainly consistingof straight-chain aliphatic hydrocarbons with a variety of substituted groups...
es, which are mixtures of hydrophobic
aliphatic compoundIn organic chemistry, compounds composed of carbon and hydrogen are divided into two classes: aromatic compounds, which contain benzene rings or similar rings of atoms, and aliphatic compounds , which do not contain aromatic rings.
Aliphatic compounds can be cyclic, like cyclohexane, or...
s, hydrocarbons with chain lengths typically in the range C16 to C36.
The plant cuticle is one of a series of innovations, together with
stomaIn botany, a stoma is a pore, found in the leaf and stem epidermis that is used forgas exchange. The pore is formed by a pair of specialized parenchyma cells known as guard cells which are responsible for regulating the size of the opening...
ta,
xylemIn vascular plants, xylem is one of the two types of transport tissue, phloem being the other. The word "xylem" is derived from classical Greek ξυλον , "wood", and indeed the best-known xylem tissue is wood, though it is found throughout the plant...
and
phloemIn vascular plants, phloem is the living tissue that carries organic nutrients , particularly sucrose, a sugar, to all parts of the plant where needed. In trees, the phloem is the innermost layer of the bark, hence the name, derived from the Greek word "bark"...
and intercellular spaces in
stemA stem is one of two main structural axes of a vascular plant. The stem is normally divided into nodes and internodes, the nodes hold buds which grow into one or more leaves, inflorescence , cones or other stems etc. The internodes act as spaces that distance one node from another...
and later
leafIn botany, a leaf is an above-ground plant organ specialized for photosynthesis. For this purpose, a leaf is typically flat and thin. There is continued debate about whether the flatness of leaves evolved to expose the chloroplasts to more light or to increase the absorption of carbon dioxide. In...
mesophyll tissue, that plants evolved more than 450 million years ago during the transition between life in water and life on land. Together, these features enabled plant shoots exploring aerial environments to conserve water by internalising the gas exchange surfaces, enclosing them in a waterproof membrane and providing a variable-aperture control mechanism, the
stomaIn botany, a stoma is a pore, found in the leaf and stem epidermis that is used forgas exchange. The pore is formed by a pair of specialized parenchyma cells known as guard cells which are responsible for regulating the size of the opening...
tal guard cells, which regulate the rates of transpiration and CO
2 exchange.
In addition to its function as a permeability barrier for water and other molecules, the micro and nano-structure of the cuticle confer specialised surface properties that prevent contamination of plant tissues with external water, dirt and microorganisms. Many plants, such as the leaves of the sacred lotus (
Nelumbo nuciferaNelumbo is a genus of aquatic plants with large, showy, water lily-like flowers commonly known as lotus or sacred lotus. The generic name is derived from the Sinhalese word Nelum. There are two species in the genus, the better known of which, N...
) exhibit ultra-hydrophobic and self-cleaning properties that have been described by Barthlott and Neinhuis (1997). The
lotus effectThe Lotus effect refers to the very high water repellency exhibited by the leaves of the lotus flower ....
has potential uses in biomimetic technical materials.
"The waxy sheet of cuticle also functions in defense, forming a physical barrier that resists penetration by virus particles, bacterial cells, and the spores or growing filaments of fungi".