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

24 1 Chemistry and Structure of Cuticles as Related to Water and Solute Permeability
The chemical nature of the CP is a matter of debate (Jeffree 2006). Some workers
believe that electron-lucent lamellae are waxes, while the more dense lamellae are
made of cutin. This is pure speculation, because it has not been established that
typical ester cutin is present in the CP at all. The mass of the CP is very small relative
to the total mass of the CM, and constituents that occur only in traces might be lost
during processing. After transesterification of cutin from positions 2–3 cm where
the CP contributes most of the mass of the CM (Fig. 1.8), only nine n-fatty acid
(C12, C14, C16 and C18) homologues have been identified which amounted to 52%
of the total mass of cutin. The most frequent cutin acid (11%) was 9,10-epoxy-18hydroxyoctadecanoic
acid (Riederer and Schönherr 1988). It is difficult to envision
a polymer composed of 50% of simple fatty acids.
The CP of Clivia cuticle survived extraction of CM with chloroform (Fig. 1.9)
and exhibited heavy contrast. Electron-lucent lamellae were preserved, and this is
unlikely to happen if they were made of waxes. The CL was differentiated into an
external (ECL) and internal (ICL) layer, with large differences in contrast. Since
the specimen was extracted, penetration by KMnO4 was not hindered by waxes,
and failure of the ECL to develop contrast indicates that reactive groups had been
eliminated during cutan formation.
Fig. 1.9 Transverse section of a polymer matrix membrane obtained from the adaxial surface of
a young Clivia miniata leaf. The MX was stained with KMnO4 prior to embedding, and sections
were stained with uranyl acetate and lead citrate. (Taken from Schmidt and Schönherr 1982)