Cuticular resistance

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. 

When the stomata are open, the cuticular resistance (r ) is usually much larger than the resistance in parallel with it, rj s 4- In that case, the leaf resistance as given by Equation 8.11 is approximately equal to 4- r, . 

Fig. 4.22 Resistance model of dry deposition, — aerodynamic resistance, r — quasi-lami-nar resistance, — soil resistance, - soil resistance (water), - soil resistance (other ground), r f - fohar resistance (weighted by leaf area index), r s — stomatal resistance, r c cuticular resistance, mesophyhc resistance.

The foliar resistance consists again of two parallel resistances the cuticular resistance and the stomatal resistance which is in series with the mesophyll resistance r . 

In addition, the resistance accounted for by the nonpored surface of the leaf can be included. This resistance, termed cuticular resistance, is in parallel with Vs and can not be added directly. Since it is usually large and constant, for most purposes it can be ignored because it will not significantly alter the sum ofr + r. ... 

There have been few measurements of cuticular resistance to gas transfer in succulent plants. By estimating water loss for cacti with stomata closed, Johnson estimated 619, 624, and 1022 s cm" for Opuntia basilaris, 0. acanthocarpa, and 0. bigelovii respectively (Ting and Szarek, 1975). These extremely high resistances to water loss must not only be due to the hermetically sealed stomata, but also to the heavy deposition of wax on the stoma-bearing surface (Fig. 5.24). 

Enzymes can be used to modify the surface of wool fibres in order to improve lustre, softness, smoothness or warmth of the fabric. Since such processes involve attack on the cuticular scales of the fibre, there is clearly a resemblance to shrink-resist treatments and similar methods are used [116] ... 

Whilst elimination (by oxidation) or masking (by polymer deposition on the cuticular scales) are the accepted mechanisms by which shrink resistance is achieved, there is evidence that other factors need to be considered, particularly as it is possible to obtain a shrink-resist effect without degradation or masking of the scales. A review is available [310] of the mechanism of chlorine-based shrink-resist processes. 

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). 

Waxes are biosynthesized by plants (e.g., leaf cuticular coatings) and insects (e.g., beeswax). Their chemical constituents vary with plant or animal type, but are mainly esters made from long-chain alcohols (C22-C34) and fatty acids with even carbon numbers dominant (Fig. 7.11). They may also contain alkanes, secondary alcohols, and ketones. The majority of wax components are fully saturated. The ester in waxes is more resistant to hydrolysis than the ester in triacylglycerols, which makes waxes less vulnerable to degradation, and therefore more likely to survive archaeologically. 

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 3. Cuticular Diterpenoids—Duvanes and Labdanes important in host-plant resistance to insects.

Rouault J., Marican C., Wicker-Thomas C. and Jallon J. M. (2003) Relations between cuticular hydrocarbon polymorphism, resistance against dessication and breeding temperature a model for their evolution in D. melanogaster and D. simulans. Genetica (in press). 

Figure 6.6 Within- and between-population variation in desiccation resistance (A) and cuticular lipid quantity (B) as a function of body melanization in northern and southern Indian populations of Zaprionus indianus and Drosophila melanogaster. In both species, populations from drier (northern) habitats are more desiccation resistant. The desiccation-resistant population of D. melanogaster is melanic, whereas desiccation resistance in Z. indianus is correlated with higher amounts of surface lipids. From Parkash et al. (2008a) reproduced with permission.

Parkash, R., Kalra, B. and Sharma, V. (2008a). Changes in cuticular lipids, water loss and desiccation resistance in a tropical drosophilid - analysis of within population variation. Fly, 2,187-197. 

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. 

We next examine a simplified expression for the total water vapor resistance that often adequately describes diffusion of water vapor from the sites of evaporation in cell walls to the turbulent air surrounding a leaf and is useful for considering diffusion processes in general. We will consider the case in which nearly all of the water vapor moves out across the lower epidermis and when cuticular transpiration is negligible. By Equations 8.11 and 8.12, the total resistance then is... 

Patil, V. and Guthrie, F.E., Cuticular lipids of two resistant and a susceptible strain of house flies, Pestic. Sci., 10, 399,1979. 

---