The cuticle on the surface of leaves create a smart barrier by having selective permeability to both hydrophobic and hydrophillic molecules.
One might assume the wax coating on the upper surface of plant leaves serves as a simple barrier to keep leaves from losing water during dry conditions and from becoming waterlogged during wet conditions. But nature rarely makes single-function materials and these cuticular coatings are no exception.
How a leaf cuticle selectively allows chemical entities to pass. a: water, hydrated calcium and other ions b: carbohydrates and other hydrophillic compounds, and c: hydrophobic compounds.
While the main function of the cuticular coating is to protect leaves from gaining or losing too much water, it is also a smart membrane, allowing two-way transport of select molecules. While made up predominantly of water-proof waxes, the coating contains about one-fifth hydrophilic compounds such as . Microfibrils of cellulose or other carbohydrates are thought to form tortuous, branched pathways through the cuticular coating that allow limited transport of water and small water-soluble constituents, such as mineral salts, to and from leaf tissues when a droplet of water sits on the leaf surface. Molecular-scale imperfections or cracks form across the cuticular coating that can fill with water allowing water and water-soluble compounds to pass based on size, shape, electrical charge, and other physical/chemical attributes. Diffusion of fat-soluble compounds across the membrane occurs through molecular-size holes that form temporarily by the movement of cuticular wax and cutin molecules.
