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.
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 cellulose. 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.
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. Artist: Emily Harrington. Copyright: All rights reserved. See gallery for details.
“The plant cuticle constitutes the interface between the primary parts of higher plants and the atmosphere. The cuticular membrane is composed of the biopolymer cutin…Cuticular waxes have been identified as the main transport barrier of cuticular membranes that reduce uncontrolled water loss, minimize leaching of plant metabolites from the leaf interior, and limit the entrance of xenobiotics such as…environmental pollutants…Although cuticular membranes are mainly considered as a lipid barrier, hydrophilic structures are also present. Cutin contains non-esterified hydroxyl- and carboxyl-groups. In addition, polysaccharides, such as pectin and cellulose, have been identified as components of cuticular membranes in considerable amounts and water sorption to cuticular membranes can be attributed to a polysaccharide.” (Popp 2005:2797)
“For lipophilic compounds, it was shown that cuticular waxes form the transport-limiting barrier of cuticular membranes” (Popp 2005:2801)
“Cuticular permeation can be described in the simplest way as a diffusion process from an aqueous donor compartment across the cuticular membrane into an aqueous receiver compartment. This implies that the permeating compound has to be dissolved in the corresponding pathway of the cuticular membrane…diffusion in the lipophilic pathway takes place in holes which temporarily emerge by the segmental motion of the aliphatic alkyl chains of the amorphous cutin and wax fractions…the hydrophilic pathway probably consists of a reticulum of polysaccharide microfibrils ramifying and stretching through the cuticular membrane.” (Popp 2005:2802)
“[P]ore size distribution limits the permeation of the large hydrophilic compounds.” (Popp 2005:2803)