Resistance cuticle

In fine wool such as that obtained from merino sheep, the cuticle is normally one cell thick (20 x 30 x 0.5 mm, approximate dimensions) and usually constitutes about 10% by weight of the total fiber. Sections of cuticle cells show an internal series of laminations (Figs. 1 and 2) comprising outer sulfur-rich bands known as the exocuticle and inner regions of lower sulfur content called the endocuticle (13). On the exposed surface of cuticle cells, a membrane-like proteinaceous band (epicuticle) and a unique hpid component form a hydrophobic resistant barrier (14). These hpid and protein components are the functional moieties of the fiber surface and are important in fiber protection and textile processing (15). 

The aliphatic components of SOM, derived from various sources, tend to persist in soil (Almendros et al. 1998 Lichtfouse et al. 1998a Lichtfouse et al. 1998b Mosle et al. 1999 Poirier et al. 2000). The principal source of aliphatic materials in soil is plant cuticular materials, especially cutin, an insoluble polyester of cross-linked hydroxy-fatty acids and hydroxy epoxy-fatty acids (Kolattukudy 2001). Some plant cuticles also contain an acid and base hydrolysis-resistant biopolymer, comprised of aliphatic chains attached to aromatic cores known as cutan (Tegelaar et al. 1989 McKinney et al. 1996 Chefetz 2003 Sachleben et al. 2004). 

Cuticular diterpenes-duvanes and labdanes. Cutler have found that the cuticular diterpenes of green tobacco have both allelopathic and insect-deterrent effects (38). Present in the cuticle are duvane and/or labdane diterpenes (Figure 3) The levels of these specific cuticular components are believed to be responsible for the observed resistance of some types of tobacco to green peach aphids Myzus persicae (Sulzer), tobacco budworm Heliothis virescens (F.), and tobacco hornworm Manduca sexta (L.) (39). 

Figure 2.21 schematically depicts the dry deposition of a pollutant to a typical surface in the form of resistances (Lovett, 1994 Wesely and Hicks, 1999). In this case, the surface resistance rsurf has been broken down even further into a combination of parallel and series resistances (rs, rm, rct, rsoil, rwa(cl, etc.). Since leaves may absorb pollutants either through stomata or through the cuticles, the absorption into the leaf is represented by two parallel resistances, rcl for the cuticular resistance and rs for the stomatal resistance, which is in series with a mesophyllic resistance rm. Also shown are resistances for uptake into the lower part of the plant canopy and into water, soil, or other surfaces. 

Several other varietal properties influence susceptibility to noble rot. Very thin cuticles and the compact grape clusters favor gray rot, whereas thick cuticles resist Botrytis attack (Ribereau-Gayon et ah, 2000). High stomatal number, which is variety- or even clone-dependent, favors infection by Botrytis (Pucheu-Plante and Leclair, 1990 Pulcheu-Plante and Mercier, 1983). 

We are just beginning to evaluate why cuticular hydrocarbons are especially suited to regulate reproduction. It could be that the specificity of biosynthesis and transport to ovaries, eggs, and cuticle is important here (Smith et al., 2009). Alterations in the profile may increase water permeability, making hydrocarbon profiles a signal that is reliable because of the associated costs of potentially lower desiccation resistance (Hefetz, 2007). On the other hand reproductive individuals usually stay in nest areas with high humidity. 

The persistence of PCDD/Fs in plants has not been extensively investigated to date. There is little reason to expect that biodegradation is relevant, given that the PCDD/Fs are found primarily in the non-viable cuticle and that they are very resistant to microbial degradation. 

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). 

How is this achieved The aerial organs of terrestrial plants have epidermal cells that are covered by a more or less thick cuticle, which consists of waxes, alkanes, and other lipophilic natural products (4,7). This cuticle layer is water repellent and chemically rather inert, and it thus constitutes an important penetration barrier for most bacteria and fungi. In perennial plants and in roots we find another variation of this principle in that plants often form resistant bark tissues. 

It is clear from the preceding discussion that insect cuticle can be considered a two-phase, lipophilic-hydrophilic system. The outermost phase is waxy and hence hydrophobic (i.e., lipophilic). Because most insecticides are nonpolar, this first barrier is advantageous to their contact action. Therefore, in insects, the contact toxicity of an insecticide is similar to the oral toxicity. In contrast, the acute oral toxicity is much higher in mammals than the contact toxicity because mammalian skin is relatively resistant to the entry of insecticides. 

Reynolds, S.E. and Kotze, A.C., Cyromazine—An insecticide that affects the cuticle, in Mechanism of action and resistance, Otto, D. and Weber, B., Eds., Andover Intercept Ltd., 1992, p. 135. 

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