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

96 4 Water Permeability
only permeances have been measured, but not diffusion and partition coefficients in
the transport-limiting barrier.
Water permeability of CM has been studied using different approaches. With the
cup method (Schönherr and Lendzian 1981) the inner surface of the CM is in contact
with water, and the outer surface is exposed to dry air of nearly 0% humidity
(Sect. 9.7). The amperometric method (Becker et al. 1986) used nitrogen gas saturated
with water vapour as donor at the morphological inner surface of the CM, and
dry nitrogen served as receiver facing the morphological outer surface of the CM.
Permeances obtained with these two methods are minimum permeances, because
the outer surface of the CM is not or only weakly hydrated. Other experiments
had been conducted with partial vapour pressures between 0.02 and 1.0 or with
liquid water in contact with the outer surface of the CM while the inner surface
was wet by water (Niederl et al. 1998; Schönherr 1976b; Schönherr and Schmidt
1979; Schreiber 2001). These experiments show if permeance increases with partial
vapour pressure. With 100% humidity or liquid water on the receiver side, maximum
permeances are measured.
4.6.2.1 Chemical Composition of Wax and Its Relationship
to Water Permeability
Since neither thickness of the CM nor amounts of wax provide a satisfactory
explanation for the large variability in water permeability of CM from different
plant species, some workers in the field have postulated that permeability might be
related to wax composition. Qualitative and quantitative wax composition can vary
considerably with leaf development and with growing conditions. Wax amounts
and composition can be measured with high accuracy using modern capillary gas
chromatography/mass spectrometry. Each plant species has its own specific wax
composition, with homologues varying in chain lengths distribution and substance
class composition. This might account for differences in cuticular transpiration
observed with different species. By analysing wax composition and measuring water
permeability in parallel, this hypothesis can be tested. Suitable data sets are available
for astomatous isolated CM from Citrus aurantium (Geyer and Schönherr 1990;
Riederer and Schneider 1990) and Hedera helix (Hauke and Schreiber 1998).
Wax composition of Citrus aurantium leaves varied with leaf age and depended
on growing conditions (Riederer and Schneider 1990). Increased day temperatures
(15–35 ◦ C) resulted in smaller wax amounts (alkanes, alcohols, acids and esters)
by factors of 2–3, whereas increased night temperatures had the opposite effect.
Humidity (50% or 90%) had no effect on wax composition. Water permeability
determined using the cup method (Sect. 9.7) was measured at 25 ◦ C with subsets of
CM from identical isolations. Even though wax coverage and composition depended
on growing conditions, water permeability of CM was not affected. Permeability
of all sets of CM decreased during storage of CM at 8 ◦ C. During about 50 days
of storage, permeance decreased to about 40% of the initial Pw. This reduction in
permeance was attributed to a change in wax structure during storage. Possibly the