Transpiration cuticular

Consideration of the waterproofing function of cuticular lipids first requires an assessment of cuticular transpiration relative to the overall water budget. The fact that organismal water-loss rates increase greatly when surface lipids are removed does not necessarily mean that increased cuticular permeability is responsible. Insects can lose water by transpiration through the cuticle, by evaporation from the tracheal system through open spiracles, and by... 

We have, to some extent, set up a straw man in this section, but it is important to recognize that most studies have not distinguished cuticular transpiration from other components of the overall water budget. This is probably not a serious problem for work with inactive insects it may be in other cases. The permeability of the cuticle to water is clearly an important aspect of insect water balance, but rigorous analysis requires a good quantitative understanding of cuticular and other routes for water-loss. Below, we first discuss the role... 

Figure 6.1 Discontinuous gas exchange in a grasshopper, Trimerotropis pallidipennis. Note that total water-loss more than doubles when the spiracles open to allow gas exchange. In this case, cuticular transpiration is easily estimated by measuring water-loss when C02 release is negligible.

Figure 6.2 Correlation between cuticular water-loss and total water-loss in female ants, Pogonomyrmex barbatus. Differences in cuticular transpiration account for 97% of the variation in total water-loss. Different symbols indicate differences in mating status. From Johnson and Gibbs (2004) reproduced with permission.

Transpiration through the cuticle involves more than just the single step of diffusion through the epicuticular lipid layer. Molecules of water must leave the tissues adjacent to the cuticle, diffuse through the cuticle itself, enter the lipid layer, diffuse across the lipids, and enter the gas phase outside the animal. Each step is likely to be affected by temperature to a different extent. Lipid composition and physical properties can also differ from one region of the cuticle to the next, so that the biophysical details of cuticular transpiration may not be homogeneous across the entire animal. Thus, transpiration at the organismal level involves multiple steps, and parallel routes for water flux. 

Finally, even when HC composition and cuticular transpiration are correlated, causation cannot be assumed. For example, higher cuticular water-loss rates in the desert ant, Pogonomyrmex barbatus, are correlated with a decrease in abundance of an n-alkane and an increase in a methylalkane (Figure 6.2 Johnson and Gibbs, 2004). This is exactly what one would expect if lipid melting points affect cuticular permeability, but this increase is also accompanied by a change in mating status. Mated, de-alate queens that have founded... 

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... 

The rate of water vapor diffusion per unit leaf area, Jw> equals the difference in water vapor concentration multiplied by the conductance across which Acm occurs (// = g/Ac - Eq. 8.2). In the steady state (Chapter 3, Section 3.2B), when the flux density of water vapor and the conductance of each component are constant with time, this relation holds both for the overall pathway and for any individual segment of it. Because some water evaporates from the cell walls of mesophyll cells along the pathway within the leaf, is actually not spatially constant in the intercellular airspaces. For simplicity, however, we generally assume that Jm, is unchanging from the mesophyll cell walls out to the turbulent air outside a leaf. When water vapor moves out only across the lower epidermis of the leaf and when cuticular transpiration is negligible, we obtain the following relations in the... 

Figure 8-7. Representative values of quantities influencing the diffusion of water vapor out of an actively transpiring leaf. Conductances are given for the indicated parts of the pathway assuming that water moves out only through the lower leaf surface and ignoring cuticular transpiration.

G. Under the conditions of E and F, what is the drop in water vapor concentration and mole fraction along the stomatal pores (ignore cuticular transpiration) ... 

Cuticular waxes certainly minimize cuticular transpiration and thus play an important role in preserving a favorable water balance for the plant (Mar-... 

Provided that the magnitude of cuticular transpiration is insignificant (which is true in the majority of CAM plants), the transpiration rate is a convenient indicator of the diffusive capacity of stomata. Hence, it is reasonable to consider the properties of transpiration as observed in CAM plants in terms of stomatal movements, and vice versa. In this chapter, we will therefore mainly detail the current knowledge of stomata in CAM plants. The ecological implications of water vapor exchange of CAM plants will be discussed in detail in Chapter 6. 

The loss of water by transpiration can take place through the cuticle and the stomata. Correspondingly, one speaks of cuticular and stomatal transpiration. Cuticular transpiration assumes some importance only in plants with a very thin cuticle. Usually it amounts to less than 10% of the total transpiration. Thus, stomatal transpiration is unquestionably the more important process. This is true in the first place in regard to quantity, as already mentioned. Owing to the edge effect (Fig. 215), the surface of a leaf, which is equipped with stomata, is capable of exuding very much more water than one would expect. In many cases this is an amount of water equal to the weight of the plant and more per day. 

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