Epicuticular Wax Ultrastructure

Abstract

Over the past decades, systematically relevant information on structure and composition of the plant cuticle based on SEM studies have been published (survey in Barthlott 1990). The cuticles of the majority of higher plants are covered with epicuticular wax secretions. They often cause a glaucous appearance. Epicuticular ``waxes'' occur throughout bryophytes, pteridophytes, gymnosperms, and angiosperms. They are chemically multicomponent mixtures, in which a particular compound or class(es) of compounds prevails, such as lipophilic substances like long-chain fatty acids (true waxes), as well as alkanes, ketones, exters, and cyclic compounds such as pentacyclic triterpenes, phytosterols, and flavonoids (Barthlott and Wollenweber 1981; surveys by Baker 1982; Jeffree 1986; Bianchi 1995). These substances occur usually as local projections of crystalline nature (Jeffree et al. 1975) and exhibit a high ultrastructural diversity. The genetic control of their biosynthesis has been studied by Wettstein-Knowles (1979, 1995). The dominating constituent(s) are responsible for the particular structure of the crystalloids. The correlation between micromorphology and chemistry has been proven by recrystallization experiments (Jeffree et al. 1975; e.g., Jeffree 1986; Jetter and Riederer 1994, 1995; Meusel 1997). Hence, the ultrastructure of epicuticular waxes manifests a visualized chemotaxonomy. Yet in cases of ultrastructural similarity and simultaneously uncertain relationships, it is necessary to analyze the chemical composition of the waxes in order to differentiate between ultrastructural homologies and convergencies.