Barks suberin layers

Suberin is a composite of polymeric phenylpropanoids and ester-linked long chain fatty acids and alcohols and consists of a hydrophobic layer attached to the cell walls of roots, bark and the vascular system (8,10). The phenylpropanoid portion of suberin purportedly has a lignin-like structure to which both aliphatic domains and hydroxycinnamic acids are esterified. 

Suberins. The cork cells in the outer bark contain polyestolides or su-berins. The suberin content in the outer layer of the cork oak bark (cork) is especially high and amounts to 20-40% in the periderm of birch bark. Polyestolides are complicated polymers composed of co-hydroxy monobasic acids which are linked together by ester bonds. In addition, they contain a,/3-dibasic acids esterified with bifunctional alcohols (diols) as well as ferulic and sinapic acid moieties. The chain lengths vary but suberins are enriched with molecules having 16 and 18 carbon atoms. There are also double bonds and hydroxyl groups through which ester and ether cross-links are possible. The outer layer of the epidermis contains so-called cutin, which is heavily branched and has a structure similar to suberin. 

Sediments of higher plant remains like seeds, pollen, cuticle, bark, and wood remains, indicated that their fossilization was comprised of highly resistant biopolymers like cutan, suberin, and lignin. In most cases only the seed coats are found in the geological record, because the outer layers of seeds contain resistant compounds to protect the genetic material against physical and chemical processes such as temperature and humidity changes and bacterial... 

Cutin. Structural component of the outer lipophilic protective layer (cuticle) of the aerial parts of plants, especially leaves. Suberin serves similar functions in roots and bark. C. is a natural polyester, formed enzymatically from hydroxyfatty acids with 16 and 18 C atoms. o+Hydroxy- and dihydroxyfatty acids, e.g., 10,16-dihydroxypalmitic acid, as well as epoxy- and oxofatty acids, and a,o>-dicarboxylic acids are the main components of cutin. Cutinases (C.-cleaving enzymes) occur especially in pollen and in plant-pathogenic fungi, e.g., Fusarium solani (while rot in potatoes). 

Suberin impregnates the walls of cork cells and in layers covers bark, tubers, roots, wound peridorm and bundle sheaths of monocotyledons to provide a protective mantle impervious to liquids and gases. The associated waxes are not as well studied as those from cutin but do exhibit certain periodicities. The hydrocarbons have a broader distribution of chain lengths than the cuticular material, a predominance of shorter carbon chains and a higher proportion of even-length chains. In addition, no single alkane predominates usually several are present in similar proportions " . 

Cuticular waxes are removed from surface layers of intact tissues by washing with organic solvents such as hexane or chloroform. It is very important to use redistilled solvents particularly since compounds such as hydrocarbons are natural components of most waxes. Internal waxes are extracted from tissues following homogenization by the usual methods (Section 6.3.1). Waxes have also been extracted from suberin-rich barks (Martin and Juniper, 1970). 

Waxes are usually isolated by extracting the tissue with a nonpolar solvent such as chloroform or hexane. Cuticular waxes can be extracted by a quick dip into the solvent at room temperature, but suberin-associated waxes are more difficult to remove because they are embedded in the suberized cell wall (232). Wax from the suberized cells of bark has to be isolated by Soxhlet extraction of dried and powdered tissue to assure its complete removal. Isolated wax can be subjected to GC/MS analysis either after separation into various classes by thin-layer or column chromatography or directly after derivitization of the functional groups (232, 253, 459, 460). 

Free fatty alcohols are perhaps the most common components of cuticular waxes and often are also a major constituent of suberin-associated waxes (232). ThQr comprised between 10% and 45% of the waxes from the periderm of underground storage organs (116). The dominant chain lengths of free fatty alcohols of cuticular waxes are C26 and C28 (232, 253, 292, 460), but the free fatty alcohols of suberin-associated waxes usually have a shorter chain length. For example, the dominant alcohols in the leaf cuticular wax of Agave americana were (36% of the alcohols) and C28 (62%), whereas the dominant component in the fatty alcohols of the suberin-associated wax from the periderm layer surrounding the crystal idioblasts within the same leaf was C22 (88%) (117). The most common fatty alcohols reported as components of bark wax are C24 and C22. 

Bark is composed of three layers an inner bark or phloem, a periderm layer, and an outer bark or rhytidome (110). Each of these layers can vary widely in appearance and thickness depending upon the tree, its age, and environmental conditions. The periderm layer consists of three cell types. The phellogen is the meristem that produces the phelloderm, a parenchymatous layer of cells produced internally, and the phellem, which is formed on the external side. The phellem cells are the cork cells that lay down a layer of suberin over the primary cell wall and seal off the cell, leading to eventual cell death. The anatomy of cork cells in bark has been reviewed by several authors (71, 73, 195, 295, 301, 312, 315, 320, 339, 344, 386, 432, 489, 490), and many of the woody plants whose periderm lay-... 

Recently periderm layers from the bark of several species of trees were ultra-structurally characterized. Suberin deposition appeared to immediately precede the deposition of a tertiary carbohydrate layer during the formation of cork cells in the bark of Acacia Senegal (477). A tertiary carbohydrate layer overlaying the suberin lamellae was also seen in the periderm of Acorus calamus, but not in the suberized periderms of Larix decidua or Picea abies (478, 482). It is possible that... 

Suberin also plays a role in cold stress. The periderm layers in the bark serve as insulation for the plant (110). Thus suberin deposition in the bark of Vitis vinifera was shown to protect the plant from freezing damage (303, 351). The leaves of Secale cereale were shown by both ultrastructural and chemical analysis to deposit increased levels of aliphatic components in the epidermis and in the mestome sheath in response to growth at low temperatures (146). 

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