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

10 1 Chemistry and Structure of Cuticles as Related to Water and Solute Permeability
Total amount of waxes were obtained by Soxhlet extraction of CM from 21
species. The average mass of wax was about 100µgcm −2 (Schreiber and Riederer
1996a). This amount was determined gravimetrically by subtracting the mass of the
MX from that of the CM. Wax coverage of the leaf CM from single species varied
40-fold between 10 (Citrus aurantium) and 400µgcm −2 (Nerium oleander). With
Malus domestica fruit, wax coverage of more than 3,000µgcm −2 was measured.
1.3.2 Chemistry of Waxes
In most species, waxes are composed of two major substance classes: (1) linear longchain
aliphatic compounds and (2) cyclic terpenoids. Linear long-chain aliphatics
can be divided into different substance classes. Compounds with chain length of
C20 and higher most frequently belong to alkanes, primary alcohols, aldehydes, and
primary fatty acids (Table 1.3). Some of the acids and alcohols found in waxes are
also released by depolymerisation of cutin (see Sect. 1.1). Secondary alcohols and
ketones with functional groups attached to carbon numbers between C4 and C16
have also been identified. Covalent binding between primary fatty acids (C16–C36)
and alcohols (C20–C36) results in long chain esters with chain lengths between C36
and C70 (Table 1.3).
Biosynthesis of these long-chain aliphatic compounds is localised in epidermal
cells, and it starts from C16 to C18 fatty acids. The elongation process leading to
very long chain fatty acids with the chain length between C20 and C34 (Kunst
and Samuels 2003) is based on the step-by-step condensation of C2-units to the
substrate. Consequently, elongated fatty acids predominantly have even-numbered
carbon chains. Oxidation leads to aldehydes and alcohols, also with even-numbered
chain-length. Since esters are the condensation products of long-chain alcohols and
acids, they are also characterised by even-numbered chain lengths. Alkane synthesis
involves a decarbonylation step, and thus they are characterised by odd-numbered
chain lengths. Secondary alcohols are synthesised from alkanes, and thus they are
also odd-numbered.
Table 1.3 Most common substance classes of cuticular waxes identified by gas chromatography
and mass spectrometry
Substance Chemical formula Range of Major
class chain lengths homologues
Acids CH3–(CH2)n–COOH C16–C32 C24, C26, C28
Aldehydes CH3–(CH2)n–CHO C22–C32 C26, C28, C30
Alcohols CH3–(CH2)n–CH2OH C22–C32 C26, C28, C30
Alkanes CH3–(CH2)n–CH3 C21–C35 C29, C31
Secondary alcohols CH3–(CH2)n–CHOH–(CH2)n–CH3 C23–C33 C29, C31
Esters CH3–(CH2)n–COO–(CH2)n–CH3 C36–C70 C40, C42, C44