10 - Plant cuticles: Multifunctional interfaces between plant and environment

Abstract

This chapter discusses the complex processes at the interface between plant and environment along with the multifunctional hydrophobic coverage of land plants. The plant cuticle covers all primary parts of vascular plants (except roots) and many bryophytes as a thin extracellular membrane. The cuticle represents a natural composite that consists mainly of two hydrophobic components, the insoluble biopolyester cutin and soluble lipids. One of the major functions of the cuticle is the limitation of uncontrolled water loss via evaporation. Thus, selective pressure acted strongly on plants to develop an outer hydrophobic envelope functioning as a compromise between the contrary demands of desiccation avoidance and free gas exchange. The multiple constraints on the evolution of such an outer envelope (plant cuticle), found on fossil and recent plant material, can be summarized by transport phenomena across the cuticle, interaction with biotic and abiotic factors, and biomechanical requirements. In addition to the hydrophobic nature of the cuticle, the presence of epicuticular wax crystals often leads to water repellency instead of wetting. This is because of their hydrophobic nature and microroughness. The plant cuticle can also be seen as the first mechanical barrier against microorganisms and herbivores. From a mechanical point of view, the location at the outer perimeter of plant organs and its rigid appearance, at least in succulents, indicates that the cuticle may function as external structural element that possibly adds mechanical support for tissue integrity and impacts on morphogenetic processes.