Structural changes and permeability of ivy (Hedera helix L.) leaf cuticles in relation to leaf development and after selective chemical treatments

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Abstract
Summary: The fine structure of ivy leaf cuticles was investigated as a function of leaf development before and after cuticle isolation as well as after selective extraction of cuticle components. Cuticle mass increased with increasing age from 234°3 to 539.1 μg cm−2 Waxes increased from 12.3 to 18.6%, of cuticle mass from young to old leaves. However, percentages of cutin and non‐lipid constituents did not vary significantly with leafage. They represented approx. 58 and 26%, of the cuticle mass, respectively. Cuticle thickness determined from electron and light microscopy increased 12–fold during leaf growth to reach 4.25 μm for mature leaves. Transmission electron microscopy of transverse sections of non‐isolated and isolated cuticles showed an outer lamellate zone gradually merging from an inner reticulate zone the thickness of which increased with leaf growth. The lamellate zone appeared very early (in vitro unexpanded leaf) and was characterized by a constant thickness (0.2 μm). Cuticle lamellation seemed to disappear after extraction of soluble cuticular wax, which confirmed intracuticular wax localization in lamellae. Electron‐dense fibrillae observed in the reticulate zone disappeared after acid hydrolysis showing that they are made of non‐lipid components. X‐ray diffraction confirmed the presence of crystalline structures and revealed a cuticle disorganization alter wax extraction. Permeability measurements showed an increase rather than a decrease in glyphosate and isoproturon diffusion through cuticles with increasing leafage, suggesting that the main barrier to diffusion is the outer lamellate zone. A considerable increase in isoproturon penetration through cuticles resulted from the wax extraction, demonstrating the major role of wax in the control of cuticular permeability to this herbicide.