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

6 1 Chemistry and Structure of Cuticles as Related to Water and Solute Permeability
are guesswork, and are as good as the underlying assumptions (Kolattukudy 2004).
A new approach is non-destructive NMR spectroscopy (Fang et al. 2001), directly
mapping the intact polymer and the intermolecular cross-linking of the monomers
without prior degradation of the MX. This approach allows the identification of
ester linkages in cutin in vivo, and it shows that sugar moieties can be linked to
cutin monomers, although the exact type of bond has not yet been identified. In
spite of these numerous attempts, we must admit that we are still far away from a
complete picture of the molecular architecture of cutin or the MX.
Another complication often overlooked is the fact that cuticles containing epoxyfatty
acids (Holloway et al. 1981) are only partially degraded by transesterification,
and the chemical analysis of these MX is incomplete. A significant if not major fraction
of the MX resists degradation, indicating the existence of bonds other than ester
linkages in the cutin polymer. This “non-ester cutin” is also called cutan, whereas
that fraction of the MX cross linked by ester bonds is called cutin. There is evidence
that intermolecular cross-linking in cutan is mainly by ether bonds (Villena
et al. 1999). This non-degradable fraction of the cuticle is still poorly characterised,
because of major methodological limitations.
Clivia miniata plants are monocots, and leaves grow at their base. The age of
leaf segments increases with distance from leaf base. Adaxial cuticles have been
isolated from leaf strips, and the MX has been fractionated into ester cutin and cutan
(Riederer and Schönherr 1988). Fractionation of total cutin into ester cutin and nonester
cutin (cutan) was possible only starting with position 3–4 cm from base. Most
of the young cuticle is made up of ester cutin (Fig. 1.2), which increases rapidly with
age, and in the study above its amount doubled at position 5–6 cm when epidermal
Mass of fraction (µg/cm 2 )
350
300
250
200
150
100
50
cutan
total cutin
ester cutin
0
0 5 10 15 20
Distance from leaf base (cm)
Fig. 1.2 Fractionation of total cutin of Clivia miniata leaves as a function of position. Total cutin
was obtained by acid hydrolysis of MX leaf strips. The resulting total cutin was subjected to transesterification
using BF3–MeOH. The polymer-resisting transesterification is cutan. The amount of
ester cutin was obtained by difference. (Redrawn from Riederer and Schönherr 1988)