Water vapor cuticle

The two main pathways for the uptake of toxic substances by plants are through their root systems and across their leaf cuticles. Stomata, the specialized openings in plant leaves that allow carbon dioxide required for photosynthesis to enter the leaves and oxygen and water vapor to exit, are also routes by which toxic substances may enter plants. The mechanisms by which plants take up systemic pesticides and herbicides, which become distributed within the plant, have been studied very intensvively. 

The resistances and the conductances that we will discuss in this section are those encountered by water vapor as it diffuses from the pores in the cell walls of mesophyll cells or from other sites of water evaporation into the turbulent air surrounding a leaf We will define these quantities for the intercellular air spaces, the stomata, the cuticle (see Fig. 1-2 for leaf anatomy), and the boundary layer next to a leaf (Fig. 7-6). As considered later in this chapter, CO2 diffuses across the same gaseous phase resistances or conductances as does water vapor and in addition across a number of other components in the liquid phases of mesophyll cells. 

Water vapor that evaporates from cell walls of mesophyll cells or the inner side of leaf epidermal cells (Fig. 1-2) diffuses through the intercellular air spaces to the stomata and then into the outside air. We have already introduced the four components involved—two are strictly anatomical (intercellular air spaces and cuticle), one depends on anatomy and yet responds to metabolic as well as environmental factors (stomata), and one depends on leaf morphology and wind speed (boundary layer). Figure 8-5 summarizes the symbols and arranges them into an electrical circuit. We will analyze resistances and conductances for these components, some of which occur in series (i.e., in a sequence) and some in parallel (i.e., as alternatives). 

Foliar Uptake Adsorption and Absorption The leaf surface provides an extensive surface area that can interact with chemicals either in the vapor phase or through wet or dry deposition. The cuticle limits water loss and protects against infection by plant pathogens. Detailed descriptions of the plant cuticle are available " and a brief synopsis has been provided in the discussion of the sorption of chemicals by foliage (see Sorption, Chapter 3). While the hydrophobic cuticle... 

The uptake of organic contaminants by plants from biosolids-amended soil depends on the physicochemical properties of organic compoimds and the physiology of plants [92-95]. Plant uptake of organic chemicals and their distribution within plants have been shown to be affected by (1) the organic chemicals physicochemical properties, including solubility, vapor pressure, octanol-water partition coefficient Kow)> and Henry s law constants (iC ), (2) environmental conditions such as temperature, air disturbance and soil organic matter content, and (3) plant characteristics, for example the shape of the leaves, type of root system, and lipid and cuticle characteristics and contents [94]. 

Owing to the method of contact-angle or surface-energy measurement, the surface of wool necessarily includes the region between cuticle cells in addition to the cuticle itself Horr has further suggested that vapor adsorption due to capillary condensation may occur at the fiber cuticle scale edges, and that the phenomenon may contribute to the above interpretation that the wool surface is not entirely methyl. Horr also found that the possible composition of the wool fiber surface may even vary depending on the liquid with which it is in contact (e.g., water or methylene iodide). 

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