The Wetting of Leaf Surfaces and Its Ecological Significances

Abstract

Leaf wettability, indicating the affinity for water on leaf surfaces, is a common phe‐ nomenon for plants in a wide variety of habitats. The contact angle (θ) of water on leaves measured at the gas, solid and liquid interface is an index of surface wettabili‐ ty. Leaves are termed as “super-hydrophilic” if θ < 40°, “highly wettable” if θ < 90°, and “wettable” if θ < 110°. If θ > 110°, the leaves are classified as being non-wettable, while θ > 130° for highly non-wettable and θ > 150° for super-hydrophobic. Both in‐ ternal and external factors can influence leaf wettability. The chemical composition and structure of leaf surfaces are internal causes, but the external environment can al‐ so influence wettability by affecting the structure and composition of the surface. The main internal factors that affecting leaf wettability include the content and microstruc‐ ture of the epidermal wax, the number, size and pattern of trichomes, stomatal densi‐ ty, the shape of epidermal cells, and leaf water status. The leaf contact angles increased with the increasing of leaf wax content. However, studies have shown that the contact angles were more dependent on the complexity of wax structure than on the absolute amount. For trichomes, there are three types of interaction between tri‐ chomes and water droplets, including (1) low trichomes density: no apparent influ‐ ence of trichomes on the location of surface moisture, droplet formation and retention ; (2) medium trichomes density: trichomes appear to circle surface moisture into patches; (3) high trichomes density: trichomes appear to hold water droplets above the trichomes. In som