ABSTRACT
References ........................................................................................................................... 77
Essential oils are lipophilic substances produced by specialized secreting tissues called glan-
dular trichomes. According to Fahn (1979, 1988a, 1988b), glandular trichomes secreting
essential oils are the base for the economic importance of several plant families, including
the Lamiaceae. In peppermint, the site of terpene biosynthesis has been localized to the
secretory cells of the glandular trichomes (Gershenzon et al., 1989; McCaskill et al., 1992),
mainly located on the leaf and stem surfaces (Amelunxen, 1965; Fahn, 1979). Two types of
glandular trichomes occur on peppermint: small, capitate glandular trichomes, with a single
secretory head cell; and peltate glandular trichomes, with an eight-celled apical cluster of
secretory cells, subtended by a stalk and a basal cell, the latter embedded between the
ordinary epidermal cells (Fahn, 1979; Maffei et al., 1989, Werker, 1993). Peltate trichomes
are enveloped by a large cuticular sheath, which accumulates the essential oil in the under-
lying storage space (Figure 2.1). Capitate glandular trichomes have only limited storage
capacity, and their secretion, which is extruded to the outside through a porous cuticle,
appears to consist mainly of a complex mixture of carbohydrates, lipids, and proteins
(Werker et al., 1985; Ascensa˜o and Pais, 1998). Whatever the exact nature of the capitate
gland secretory products, it is clear that the bulk of the monoterpenes of peppermint essential
oil is produced by and stored in the peltate glandular trichomes (Turner et al., 2000a).
