Internal Structures Of The Plant Involved In Internalization

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The morphology of the surface pores and interconnected intercellular spaces has a direct influence on how readily particulate matter moves through the plant surface as well as the size of particles admitted. Intercellular spaces are delimited by the walls of the surrounding cells. The spaces form when cells dissolve (lysigenous), tear (rhexigenous), or separate (schizogenous) [18]. Cell walls are primarily composed of cellulose existing as microfibers bound to hemicellulose, specialized structural proteins, and pectins [1,16-18]. The walls of adjacent cells are initially cemented together by pectic compounds that compose a middle lamella. As these cells mature, they assume a more rounded as compared with an initial square or rectangular shape. The rounding splits the middle lamella apart at cell-to-cell contact points leaving a pectic sheath on the exposed walls [1]. Micropores, sometimes called micro-capillaries, exist in the lattice of microfibers and associated carbohydrates. These pores may be partially filled with pectic compounds or other wall material. Additionally, the microcapillaries contain water in a pectin gel or as free water, such that the relative humidity in the intercellular spaces ranges from 98 to 100%. Sakurai [16] suggested that a plant's symplast is surrounded by a liquid medium.

The precise environment within apertures and intercellular spaces is unclear. Internalized, nonplant pathogenic microbes are not likely to be in direct contact with plant cell membranes due to the thickness and structure of the plant cell walls, which was referred to as a matrix by Sattelmacher et al. [17]. Apoplastic fluid containing an array of solutes exists in the wall matrix. However, the fluid's solute concentration and pH is not likely to equal those reported to make plant tissues a favorable nutritional environment for growth of bacteria [35]. The pH of the apoplastic fluid, which varies with the location in the plant, the nutrition of the plant, and even the time of day [17], would seldom be as low as that reported for macerated plant tissues, where cell vacuoles have been ruptured. Xylem sap generally has a pH between 5 and 7, whereas the average pH of all apoplastic fluid ranges from 4.5 to 7.0. In ripening fruit, the apoplast pH is reduced due to leakage of organic acids through the plasmalemma and exposure of carboxyl groups from the hydrolysis of pectin [16]. However, the contents of cell vacuoles normally have a much lower pH than does the xylem sap or apoplastic fluid [21]. The pH of vacuoles in lemon fruit was measured down to 2.4, whereas a pH of 0.9 was reported for fluid in the cell vacuoles of a species of begonia.

Certain reports conclude that the cell walls bounding intercellular spaces have a coating of water, whereas others have suggested the exposed wall is actually hydrophobic due to an incrustation of cutin [17]. The plant cuticle has been observed to cover the guard cells and pore of a stoma and to extend partially into the substomatal chamber [1,3,15]. Schonherr and Bukovac [22] noted that the chemical characteristics of the surfaces of the cuticle on the plant surface were similar to those within the substomatal chamber. The cuticular complex, however, contains both polar carbohydrates and relatively nonpolar cuticular components [15]. Cutin has been described as a polyester with polar properties and an affinity for water [1]. The thickness of cutin on the plant surface increases with light intensity and exposure to moisture stress, which seem related to a restriction in water loss [3]. The thickness of an internal cuticle in the succulent tissues of fruits and vegetables could be quite different from that in leaf tissues where water loss through transpiration can be a major stress on the plant. Thus, an incrustation of cutin might not make cell walls hydrophobic, particularly in fruits and vegetables.

A combination of waxes and epidermal hairs help keep stomata from being clogged with water as a result of dew formation or rainfall [2]. Such a plug of water might substantially impair gas exchange [20]. The waxes on the stomata surfaces repel water, whereas the stomatal pore contains a bubble of air [1]. Thus, during dew formation, water droplets would bead up over the surface waxes and air bubble associated with the aperture. Goodman et al. [1] suggest, however, that changes in temperature or leaf movement could create pressure differentials that would draw surface water into stomata. By contrast, a wind and rainstorm during daylight hours would produce substantial water soaking of leaves through open stomata [36].

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