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

14 1 Chemistry and Structure of Cuticles as Related to Water and Solute Permeability
The contributions of epi- and intracuticular waxes to permeability of cuticles is
not known. Such research requires that epicuticular waxes can be removed quantitatively
without disturbing intracuticular waxes. As solvents very rapidly penetrate
into the cuticle, the only method available today is mild stripping of waxes. Permeability
would have to be determined prior to and after stripping. This has not
been done so far, or at least not published. As already indicated, it is not certain
that all waxes are removed by stripping, including thin continuous wax layers on
the surface of cutin. In this context, significant progress has been made by a technique
which first lifts off epicuticular wax from the surface before chemical analysis
(Jetter et al. 2000). Prunus laurocerasus leaves were used. The wax of this species
is dominated by triterpenoids, and it was shown that linear long-chain aliphatics
were mainly deposited on the outer surface of the MX, whereas triterpenoids could
rarely be lifted off. Triterpenoids were nearly exclusively found in the chloroform
extracts of the stripped CM, showing that they were located within the polymer.
This is good evidence that linear long-chain aliphatics and triterpenoids were spatially
segregated. Effects of stripping on water permeability unfortunately have not
been reported. Further evidence for a layered structure of waxes has been provided
by atomic force microscopy, offering a resolution on the molecular level (Koch et
al. 2004). After epicuticular waxes were lifted off using an epoxide glue, the surface
of the CM appeared smooth. Within 80 min, a new film of 3–5 nm thickness was
regenerated on the surface of the cuticle. The chemical identity of these regenerated
films and the effects of stripping with epoxy glue were not investigated. These
two approaches (Jetter et al. 2000; Koch et al. 2004) immediately raise a series of
important questions: (1) how do these observations relate to barrier properties of
CM? (2) do mainly linear long-chain aliphatics contribute to the formation of the
transport barrier, or are triterpenoids also important? (3) where is the waxy cuticular
transport barrier located? and (4) how is the waxy barrier structured on the molecular
level? These are some of the questions which we will address in the following
chapters.
Extraction of wax has been shown to increase water permeability of CM 50- to
1,000-fold (see Chap. 4). Anyone trying to relate water and solute permeability to
wax amounts, location and composition should have reliable data. Permeability and
wax composition should be studied using identical or at least comparable samples.
This requires reliable and reproducible methods of wax analysis. To our knowledge,
only three studies comparing water permeability of Citrus leaf wax with wax composition
using the same population of isolated cuticles have been published (Haas
and Schönherr 1979; Geyer and Schönherr 1990; Riederer and Schneider 1990).
Results and conclusions are presented in Sect. 4.6.
1.4 Fine Structure of Cuticles
In his recent review, Jeffree (2006) has summarised the literature on fine structure
of cuticles and epicuticular waxes, and there is no need to repeat this. Instead,
we shall evaluate the literature for hints concerning a correlation between fine