Water and Solute Permeability of Plant Cuticles: Measurement and ...

152 6 Diffusion of Non-Electrolytes
6.1.6.3 Polar Solutes with Extremely High Water Solubility
Solutes having a very high water solubility (i.e., urea, glucose and sucrose) dissolve
in extremely small amounts in octanol, cuticles or cutin. These polar compounds
are highly soluble in water, and the concentration in the aqueous phase will
hardly decrease during equilibration even with very large mass cuticle/water ratios.
Amounts sorbed in the CM must be measured by separating cuticles from the aqueous
phase and determining the amounts of polar solutes associated with the cuticle.
Small droplets sticking to the waxy surface of the CM, or aqueous films spread over
the inner surface, present huge problems and prevent the obtaining of valid partition
coefficients.
This problem can be illustrated by a simple model calculation. At an external
aqueous solution of the polar compound of 1µg g −1 , and a partition coefficient
of 0.01, the solute concentration in the CM calculated from (2.12) is 0.01µg g −1 .
This amount of solute is contained in 10 mg of solution. With an average weight of
300µg cm −2 typical for many leaf CM (Table 4.8), one gram of cuticle has a total
area of 0.33m 2 . Let us assume that after equilibration and blotting, a thin water film
of 0.1µm thickness remains on the inner side of the CM and the waxy outer surface
is dry and free of water. This water film would have a mass of 33 mg water,
which contains 0.033µg solute. Since only 0.01µg solute are sorbed in the CM, the
amount contained in the water film would be 3.3 times higher. The total amount of
solute associated with 1 g CM is 0.043µg, and dividing this by concentration of the
solution (1µg g −1 ) we obtain a partition coefficient of 0.043 rather than the real one
of only 0.01. This example clearly shows that it is practically impossible to accurately
determine partition coefficients of very polar solutes. Apart from the tedium
involved in isolating 1 g of thin cuticles, the assumption of a very thin water film
of only 0.1µm is very optimistic. In our experience, wet cuticles contained 30–50
weight percent water after thorough blotting with tissue paper, while the amount of
sorbed water at 100% humidity is less than 10% by weight (Chamel et al. 1991).
Popp et al. (2005) published partition coefficients measured for various sugars
and ivy CM. Predicted log Kow values ranged from −1.52 (erythrose) to −7.36
(maltotriose). Sugar concentration in water was 50g kg −1 (Popp, personal communication),
and experimental Kmxw value for erythrose was 99 and the values for
glucose, maltose and maltotriose were between zero and 10. They were not related
to predicted log Kow. The authors suggested that the sugars were not sorbed in cutin
but in aqueous pores traversing the CM and MX membranes. This appears reasonable,
since cuticles are heterogeneous membranes composed of lipids and polar
polymers (Chaps. 1 and 4). Cutin and polar polymers sorb up to 80g kg −1 water
when exposed to 100% humidity (Chamel et al. 1991). Only about 20g kg −1 are
sorbed in cutin. Water content in aqueous pores would amount to not more than
60g kg −1 (Fig. 4.6). Popp et al. (2005) measured a total water content of ivy cuticles
of 30g kg −1 . If it is assumed that these aqueous pores can be accessed by the
sugars, 1.5–3 g sugar would be in 1 kg cuticle and the partition coefficient would be
0.03 or 0.06. These estimates are maximum values, and they are about two orders
of magnitude smaller than the values published by Popp et al. (2005). Liquid films