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

Problems 27
10. One of them is eliminated during hydroxylation in synthesis of cutin acids of
the C16-family. Thus, the specific radioactivity of the cutin acids was only half as
high as that of 3 H-hexadecanoic acid, and the maximum rate of 3 H-cutin synthesis
should be twice as high, that is 0.6µgcm −2 h −1 . At this rate, 14.4µgcm −2 of
3 H-cutin could be synthesised in 24 h. Between positions 3 and 5, the ester cutin
fraction increased by about 50µg (Fig. 1.2). We do not know the growth rate of
leaves and epidermis cells. If they need a day for 1 cm, about one third of the cutin
synthesised would belong to the C16-family.
About 2% of the weight of the CM were C16-cutin acids at position 2–3 cm
(Riederer and Schönherr 1988). At 5–6 cm, C16-cutin acids amounted to about 25%.
This shows that C16-cutin acids occur in Clivia CM, but it is also clear that ester
cutin is made up primarily by C18-cutin acids. Some of the 3 H-hexadecanoic acid
molecules may have been elongated and turned into C18-cutin acids. Unfortunately,
the radioactive cutin acids were not identified chemically.
These studies demonstrate that Clivia cuticles are very dynamic structures that
greatly change during development. The CP appears first and is maintained, but
thickness of the CL and its chemical composition undergo major changes. Cutin synthesis
occurred even at the tip of the leaf. After cell expansion is complete, non-ester
cutin occurs in large amounts because ester cutin is converted into non-ester cutin
(Fig. 1.10). There are no comparable studies of epidermis and cuticle development
in other plant species.
1.4.4 Lateral Heterogeneity
The cuticle over ordinary epidermal cells (pavement cells) covers the surfaces of
stems, leaves, flowers and fruits. Many mechanistic studies into permeability of cuticles
were carried out using isolated cuticular membranes which lack trichomes and
stomata. Much less is known about the involvement of specialised epidermal cells in
water and solute permeation. However, trichomes and stomata occur on most leaves
and stems (Glover and Martin 2000: Bird and Gray 2003), and it is well-established
that permeability of cuticles over these special structures differs from that over pavement
cells (Strugger 1939; Bauer 1953; Franke 1960; 1967; Meier-Maercker 1979).
The cuticle over guard cells and trichomes is often traversed by aqueous pores, and
they play a decisive role in foliar penetration of ionic solutes (Schönherr 2006). This
aspect of foliar penetration is treated comprehensively in Chap. 5.
Problems
1. What is the average thickness of a cuticle, and what are the upper and lower
values of very thick and very thin cuticles?