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

4.4 Water Vapour Sorption and Permeability as Affected by pH, Cations and Vapour Pressure 77
in Citrus MX the reticulum contains carboxyl groups and is continuous across the
entire polymer matrix, including the cuticle proper. This is convincing evidence,
even though such a reticulum is rarely seen in TEM (Sect. 1.4). The aqueous pores
across the polymer matrix formed by the reticulum are further characterised in
Sect. 4.5. Comparable data for MX from other plant species, including those having
a lamellated cuticle proper, are not available.
Polar polymers sorb much more water than hydrophobic ones, and permeance
increases with increasing partial pressure (Fig. 4.5) because of increased sorption
of water (Barrie 1968). Sorption in the polymer matrix was measured with carboxyl
groups in the hydrogen form, that is, in absence of inorganic cations (Chamel
et al. 1991). Sorption isotherms are not linear (Fig. 4.7) and resemble B.E.T. type
II isotherms. Water vapour sorption in tomato and Citrus MX was similar to that in
EC. For the MX, sorption close to 100% humidity is not available, but extrapolation
leads to figures somewhere between 70 and 80gkg −1 , which is 7–8% by weight.
The plateau seen in Fig. 4.6 for permeance of MX is strikingly absent in sorption
data (Fig. 4.7). This may be due to the fact that sorption in MX was measured with
carboxyl groups in the hydrogen form and in the absence of inorganic counter ions.
COOH and OH groups probably sorb similar amounts of water, because characteristic
dipole moments for COOH and OH groups are similar and amount to 1.7 and
1.65 Debye respectively (Israelachvili 1991).
Sorption in tomato fruit cutin was considerably lower, and the isotherm was linear.
Sorption at 100% humidity was 19gkg −1 , which is only about 25% of the
amount sorbed in the tomato MX. Cutin was generated by acid hydrolysis of tomato
fruit MX (6 N HCl, 110 ◦ C, 12 h). This hydrolysis eliminates polysaccharides and
polypeptides, and probably also liberates phenolic compounds bound covalently to
the MX (Schönherr and Bukovac 1973). The bulk (75%) of the water in the MX is
sorbed by dipoles contributed by these compounds.
Polyethylene also has a linear sorption isotherm, and maximum sorption at 100%
humidity is 0.65kgm −3 (Table 4.1). With a specific gravity of 950kgm −3 maximum
sorption is 0.68gkg −1 . Hence, cutin sorbed 28 times more water than PE. Most of
this water is probably bound to free hydroxyl groups of cutin acids. When 19 g water
is sorbed in 1 kg cutin, this amounts to 1.06molkg −1 . Tomato fruit cutin is made up
mainly of C16-dihydroxyfatty acids (Baker et al. 1982), which have a molecular
weight around 300gmol −1 . This yields a concentration of 3.33 mol hydroxy fatty
acids per kg of cutin. Hence, only a third of the mid-chain hydroxyl groups sorbed
a water molecule at 100% humidity. Those hydroxyl groups that did not sorb water
were probably engaged in intermolecular hydrogen bonds, and this indicates that
they were not distributed at random but are arranged close to each other.
At a partial pressure of 0.22, permeance was smaller by a factor of 0.5 than
at p/p0 = 1. Sorption in MX at p/p0 = 0.22 is about 10gkg −1 , while at p/p0 = 1
sorption amounted to about 70–80gkg −1 . With ethyl cellulose the difference is even
larger (Fig. 4.7). It follows that permeance and water content of the membranes
were not proportional. Apparently, not all water sorbed in the MX participated in
transport.