3Essential oil chemistry of the genus Thymus - a global view

Abstract

The subject of plant chemistry has developed enormously in the last four decades and this has been due to the increasingly successful identification of organic molecules in minor quantities by means of sophisticated chemical techniques. It has also been due to the awareness that secondary metabolites have a significant role in the complex interaction occurring between plants and animals or plants and plants in their exposition to the environment. Economic and medicinal interests as well as taxonomical studies, all three in quest of new natural products, have always been the strongest stimulants for research in plant chemistry. Concerning the genus Thymus, we can state that its chemistry is fairly well known at least concerning the two main classes of secondary products, the volatile essential oils on the one hand and the polyphenols, especially the flavonoids, on the other hand. Both, essential oils and flavonoids, are mainly responsible for the pharmacological activities of Thymus plants (Simeon de Bouchberg et al., 1976, Van den Broucke, 1983). Traditionally essential oils have been regarded as the relatively toxic waste products of plant metabolic processes with no practical value to the plant. Nowadays it is thought that they possess properties that assist the plant in repelling leaf-eating insects and in preventing microbial attack. There is also evidence that terpenes leached from the leaves contribute to the allelopathic effects on the ground inhibiting the germination and growth of competitors. It has been suggested although not proven, that oil vapours near the leaf surface may reduce water loss, and the oils in the flowers might release odours attractive to pollinating agents. In Lamiaceae, essential oils are widespread (Hegnauer, 1966; Richardson, 1992) and many species are used as aromatic herbs for flavouring foods. The essential oils themselves are products of great demand in the manufacture of perfumes and cosmetics, and they are also used for medicinal purposes. This fact also holds good for the genus Thymus. Indeed all the Thymus species produce essential oils, and several representatives are important herbs and spices used in all parts of the world. As will be shown the oils of Thymus species have been studied extensively. In Lamiaceae, essential oils are stored in glandular peltate trichomes. They are situated on the epidermal surface on both sides of the leaves and show a very typical anatomy (Figure 3.1). Bruni and Modenesi (1983) intensively studied the trichomes and their development in Thymus vulgaris by conventional, fluorescent and electron scanning microscopy. The glandular peltate trichomes are composed of one basal stalk cell, an