Cork cells

Removal of the aliphatic materials by hydrogenolysis leaves a residue that contains low amounts of polymethylenic components, suggesting that the suberized material contains some aliphatic components not susceptible to cleavage by such methods [3]. On the other hand, removal of suberin from cork cell wall preparations was examined by CPMAS and the results showed that the aliphatic components were nearly completely removed from this suberin preparation as the spectra showed that the residual material was virtually devoid of methyl... 

In contrast to the exterior localization of cutin, suberin can be deposited in both external and internal tissues. External deposition occurs in the periderm of secondary roots and stems and on cotton fibers, whereas internal deposition occurs in the root endodermis and the bundle sheath of monocots. The Casparian strip of the root en-dodermis contains suberin, which produces a barrier isolating the apoplast of the root cortex from the central vascular cylinder. Suberin also produces a gas-impermeable barrier between the bundle sheath and mesophyll cells in C4 plants. The bark of trees contains periderm-derived cork cells that have a high suberin content. 

Cork cells Tabular with all walls suberized occur in thick layers on the outer surfaces of older stems and roots Secrete a fatty substance, suberin, into the walls, suberin renders cork cells waterproof and helps protect the tissues beneath... 

Peridermal tissue of stems Cork cells and cork cambium Mechanical protection... 

In addition to the walls of the parenchyma cells, the walls of the periderm (skin) cork cells form part of the total intake of dietary fiber and a waste product of potato processing for food as well as for starch. Although much is known about the suberin present in these cell walls (Bernards, 2002 Franke and Schreiber, 2007 Grafos and Santos, 2007), little is known about their polysaccharides (Harris et al., 1991). Nonetheless, because of the presence of suberin, these cell walls are able to adsorb hydrophobic dietary carcinogens and their intake may be important in the prevention of colorectal cancer (Harris et al., 1991 Ferguson and Harris, 1998, 2001). 

Cytology (science ol cells) had its beginnings when Robert Hooke. English physicist, described the nature of cork cells. 

Suberins lipid polyester polymers in cell-wall components of cork cells. 

The outer bark, which consists mainly of periderm or cork layers, protects the wood tissues against mechanical damage and preserves it from temperature and humidity variations. In most woody plants a periderm replaces the epidermis within the first year of growth. The first periderm in stems usually arises from the cork cambium in the outer surface of bark, either in the subepidermal layer or in the epidermis. The following periderms are then formed in successively deeper layers of the bark or in the bast tissue. Cork tissue is predominantly formed in the outward direction, but some division also occurs inward resulting in so-called phelloderm tissue resembling parenchyma cells. Owing to this sequence the final rhytidome usually occurs as scaly bark and, in addition to the cork cells, contains the same cells as those present in the bast. 

Bark can roughly be divided into the following fractions fibers, cork cells, and fine substance including the parenchyma cells. The fiber fraction is chemically similar to that of the wood fibers and consists of cellulose, hemicelluloses, and lignin. The other two fractions contain large amounts of extractives. The walls of the cork cells are impregnated with suberin, whereas the polyphenols are concentrated in the fine fraction. 

The lipophilic fraction, extractable with nonpolar solvents (ethyl ether, dichloromethane, etc.) consists mainly of fats, waxes, terpenes and terpenoids, and higher aliphatic alcohols (cf. Sections 5.3.1 and 5.3.2). Terpenes, resin acids, and sterols are located in the resin canals present in the bark and also occur in the cork cells and in the pathological exudate (oleore-sin) of wounded bark. Triterpenoids are abundant in bark /3-sitosterol occurs in waxes, as an alcohol component, and the cork cells in the outer bark (periderm) of birch contain large amounts of betulinol (cf. Fig. 5-6). 

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. 

We report here the first results obtained in high-pressure extractions of cork with dioxane, C02 and dioxane - C02 mixtures seeking to obtain more information on the structure and interaction of cork cell wall components. 

The cork oak (Q. suber) is another commercially valuable species found throughout the Mediterranean region. The thick bark composed mostly of cork cells can be harvested every 10 years in early summer to provide sheets of soft, smooth cork useful in many ways. The cork cells capture air inside as they dry, making the material extremely resilient and buoyant. Cork has been used to manufacture floats, handles, stoppers, and as insulation, since it is a poor conductor of heat and sound. 

Robert Hooke observed cork cells under a microscope. 

Cork or suberous tissue is composed of cells of tabular shape, whose walls possess suberized layers. Its cells are mostly filled with air containing a yellow or brownish substance. It is derived from the phellogen or cork cambium which cuts off cork cells outwardly. Cork tissue is devoid of intercellular-air-spaces. It forms a protective covering to the roots of secondary growth, stems (after the first season) of Dicotyledons and Gymnosperms, and wounds of stems and branches. Living cork cells contain protoplasm and cell sap while dead cork cells are filled with air. 

The walls of cork cells resist the action of concentrated sulphuric acid. They are colored green, when in contact with alcoholic extract of chlorophyll for several days in the dark. 

Cork, or outer bark, composed of several layers of rectangular cork cells. The most external layers are dead and appear black because they are filled with air. The inner layers of this region are living and contain brownish contents. 

Senescence in plants is seen to occur in several ways. When a plant ages, its tissues cannot heal as rapidly and it is much less likely to defend itself against disease. Xylem vessel elements and cork cells age and die before they can assume their intended functions (Campbell et al., 1999). Eventually, this decline leads to death. Some plants die after one reproductive cycle (annuals), others after many cycles (perennials). 

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 " . 

Figure 14.1 Electron microscopy of cork cell. (Reprinted with permission. Copyright 2002 by APCOR.)...

Gil A.M., Lopes M., Rocha J., Neto C.P., A C-13 solid state nuclear magnetic resonance spectroscopic study of cork cell wall structure The effect of suberin removal, Int. J. Biol. MacromoL, 20(4), 1997,293-305. 

Lopes M.H., Sarychev A., Neto C.P., GHl A.M., Spectral editing of C-13 CP/MAS NMR spectra of complex systems Application to the structural characterisation of cork cell walls. Solid State Nuc. Magn. Reson., 16(3), 2000,109-121. 

Cutin, the lipid that covers the outside of epidermal cells, and suberin, which is associated with cork cells in plants, are largely comprised of cross-linked hydroxy fatty acids. The composition of each type of compound varies from plant to plant. Generally, cutin is distinct from suberin in the type of monomeric units present (Harwood, 1980). 

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-... 

Pinus sylvestris SEM Thin-walled cork cells 121... 

Quercus alba LM Layers of cork cells alternate with layers of lignified cells 195, 196... 

Quercus suber LM, TEM, SEM Lamellar suberin in cork cell walls 205, 276, 345, 417-420... 

Fouquieria splendens LM Thick-walled cork cells 322... 

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... 

Litvay J D, Krahmer R L 1976 The presence of callose in cork cells. Wood Fiber 8 146-151... 

Rainbow A, White D J B 1972 Preliminary observations on the ultrastructure of maturing cork-cells from tubers of Solanum tuberosum L. New Phytol 71 899-902... 

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