Characterization and Distribution of Water-repellent, Self-cleaning Plant Surfaces

During the last 20 years, a wealth of data dealing with scanning electron microscopy of plant surfaces has been published. The ultrastructure of epidermal surfaces has been investigated with respect to taxonomic, as well as functional aspects. Within the latter, water-repellency has received much attention and has been well documented. Water-repellency is based on surface roughness caused by different microstructures (trichomes, cuticular folds and wax crystals), together with the hydrophobic properties of the epicuticular wax. In addition, contaminating particles are carried away by water droplets, resulting in a cleaned surface (Lotus-effect). Therefore, rough, waxy leaves are not only water-repellent but anti-adhesive with respect to particulate contamination. Based on 200 water-repellent plant species, the present paper surveys micromorphological characteristics of anti-adhesive plant surfaces. Leaves that are permanently water-repellent can be differentiated by distictively convex to papillose epidermal cells and a very dense layer of epicuticular waxes. Leaves that are water-repellent for only a limited period of time have only slightly convex epidermal cells and often have a less dense wax layer. Finally, an overview is given on the occurrence of water-repellency among different life forms and within different habitats. Water-repellency is concentrated in herbaceous species, while it is rare in trees. Among different habitats, subtropical regions, wetlands and disturbed areas appear to have more species with water-repellent leaves. The importance of roughness and water-repellency, respectively, as the basis of an anti-adhesive, self-cleaning surface, in comparison to other functions of microstructures, is discussed.