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

26 1 Chemistry and Structure of Cuticles as Related to Water and Solute Permeability
waxes increased permeance by factors of 438 and 216 in young and mature CM
respectively. This demonstrates that water permeance of MX decreased during leaf
development, when MX mass increased from 0.4 to 0.7mgcm −2 . The lower permeance
of MX from mature leaves indicates that cutan has a lower permeability than
cutin (Figs. 1.8, 1.9 and 1.10).
1.4.3 Cuticle Synthesis
Lendzian and Schönherr (1983) studied cutin synthesis in adaxial cuticles of Clivia
miniata leaves. Aqueous solutions of 3 H-hexadecanoic acid buffered at pH5 were
applied as 10µl droplets to the surface of detached leaves. The leaves were incubated
in the dark at 100% humidity for 24 h. After incubation, the leaves were exhaustively
extracted with methanol/chloroform (1:1) in a Soxhlet apparatus to remove
3 H-hexadecanoic acid. After drying, an autoradiograph was made (Fig. 1.11). It
shows black spots at the positions of the droplets. Their intensity decreased from
the base to the tip of the leaves, except that little blackening was obtained at the
leaf base. Greatest intensity can be seen at the position 1–5 cm from leaf base, and
the droplets are not circular but oblong. At higher positions most spots are circular,
but some are irregular because not all droplets spread evenly and were segments of
spheres.
The 3 H-radioactivity was obviously immobilised in the cuticle, and was called
3 H-cutin. When depolymerised with BF3–MeOH, the radio-TLC showed at least
four peaks which were not identified, but the methyl ester of hexadecanoic acid
was not detected as depolymerisation product. The most likely sequence of events
is as follows: 3 H-hexadecanoic acid administered to the cuticle surface penetrated
into epidermal cells and was hydroxylated. These cutin acids somehow reached the
cuticle and were attached to cutin, probably catalyzed by cutinases.
Maximum rates of 3 H-cutin synthesis coincide with regions of maximum
expansion of epidermal cells (Fig. 1.7). At a hexadecanoic acid concentration of
0.023gl −1 as donor, the authors calculated maximum rates of 3 H-cutin synthesis of
around 0.3µgcm −2 h −1 . The 3 H-label was attached to carbon atoms number 9 and
Fig. 1.11 Autoradiograph of the adaxial surface of young Clivia miniata leaf of 17 cm length.
Droplets containing 3 H-hexadecanoic acid were placed on the cuticle, and incubated for 24 h in the
dark at 100% humidity. Before applying the X-ray film, the leaf was extracted exhaustively with
chloroform/methanol to remove all soluble radioactivity. (Taken from Lendzian and Schönherr
1983)