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

28 1 Chemistry and Structure of Cuticles as Related to Water and Solute Permeability
2. In textbooks it is usually stated that cutin is a polymer composed of hydroxylated
fatty acids cross linked via ester bonds. Is this statement correct, partially correct
or wrong?
3. What is the average amount of cuticular wax?
4. What is the chemical composition of cuticular waxes?
5. What are the major problems often occurring in wax analysis?
6. Is there an easy way to separate epicuticular waxes from intracuticular waxes?
7. Before looking at cuticles with the TEM, they are often treated with OsO4 and
KMnO4. What is the reason for this treatment and how do these chemicals react
with cuticles?
8. Why are cuticles heterogeneous membranes? What is the difference between
transversal and lateral heterogeneity?
Solutions
1. Most cuticles are about 2–3µm thick (Citrus aurantium, Hedera helix, Prunus
laurocerasus); however, depending on the species thickness of cuticles can vary
between 30 nm (leaf cuticle of Arabidopsis) and 30µm (fruit cuticle of apple).
2. This statement is partially correct. Plant cuticles are composed only to a certain
degree of hydroxylated fatty acids which are cross-linked via ester bonds. Many
cuticles contain cutan, which is less well characterised and cross-linked by other
bonds (ether bonds and probably direct C–C-bonds) than ester bonds. Therefore,
it cannot be degraded by transesterification reactions. Furthermore, there
are polar compounds (carbohydrates, proteins and phenols) forming a small but
important fraction of the cuticle mass.
3. On average, the cuticle contains about 100µgcm −2 . However, depending on
the species, wax coverage can vary tremendously between 10 (leaf cuticle of
Citrus aurantium) and 400 (leaf cuticle of Nerium oleander (oleander)) to
3,000µgcm −2 (fruit cuticle of Malus domestica).
4. Cuticular waxes are in most cases composed of linear long-chain aliphatic compounds
and cyclic terpenoids. Linear long-chain aliphatics are composed of
different substance classes (e.g., acids, aldehydes, alcohols, alkanes, secondary
alcohols and esters) with chain length ranging from C20 to C36. Esters composed
of primary fatty acids (C16–C36) and alcohols (C20–C36) have chain lengths
between C36 and C70.
5. A major problem often encountered in wax analysis is the fact that gravimetrically
determined amounts are often higher than wax amounts determined by
gas chromatography. The reasons for this observation are variable and not fully
understood. Gravimetric determination of wax amounts could lead to an overestimation,
whereas determination by GC could lead to an underestimation of wax
amounts.
6. Different approaches (e.g., varying extraction times and mechanical removal of
waxes) have been suggested, but most of these procedures can be questioned.