Casparian strip

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. 

Suberized lamellae in casparian strips can be located using the berbe-rine-aniline blue fluorescence staining method, which works equally... 

Casparian strip In plants, an impermeable waxy layer between the cells of the endodermis that stops water and solutes from entering the xylem, except by passing through the cytoplasm of adjacent cells. 

In the roots of many species a siibepidennal layer or layers of hypodermis occur. Radial walls of hypodennal cells can also be blocked with a waxy material analogous to the Casparian strip in the endodermis, in which case the layers are often termed an exodermis. 

If a plant is decapitated at the ground level a large part of the proton flux to the soil is cut off, but that due to the sink tissues of the root system continues to flow. As the casparian strip blocks the apoplasmic pathway from the stele to the cortex, the counterflux must occur via the... 

Dupont E M, Leonard R T 1977 The use of lanthanum to study the functional development of the Casparian strip in corn roots. Protoplasma 91 315-323... 

Nagahashi G, Thomson W W, Leonard R T 1974 The Casparian strip as a barrier to the movement of lanthanum in corn roots. Science 183 670-671... 

Work using La as a marker for water movement demonstrated that La " was largely distributed in the inner cellulosic walls (Oparka and Gates 1981). The primary endodermis of the spruce root Picea abies), however, has been demonstrated to prevent the apoplastic transport of lanthanum into the stele (Jorns 1987). This indicates the presence of a casparian strip which is impermeable to La. In Yucca flaccida, however, La " penetrated to the phloem cells, demonstrating clearly that it must have crossed the cell membrane at least at the endodermis (Wolterbeek and Van Die 1980). 

Transport across the casparian strip may be dependent on the extent of development of the casparian strip (Maier-Maercker 1980), as well as by soil REE levels and pH (Lawton et al. 1981, Gross and Thomson 1982). At a pH of 7.8, small and infrequent deposits of La " were found in cap cell walls of the desert plant Cynodon while at pH 7.2, La was found throughout the walls (Gross and Thomson 1982). At a lower pH it would seem that no barrier exists to La movement, lanthanum precipitates being found throughout apoplastic and symplastic spaces. 

However, at the endodermis, which is the cylindrical sheath of cells that surround the vascular tissues, the apoplastic pathway is blocked by the Casparian strip. If compounds are to enter the stele and reach the xylem vessels, they must pass through the plasmalemma membrane of the endo-dermal cells and so enter the symplasm. The transpiration stream concentration factor (TSCF) describes the efficiency of this process for compounds relative to that for water " ... 

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