Structure and histochemistry of the glandular trichomes on the leaves of Isodon rubescens (Lamiaceae)
The structure and histochemistry of the glandular trichomes on the surface of Isodon rubescens leaves were investigated. The main components in the secretion were identified. Morphological and histochemical study revealed one type of non-glandular and two types of glandular trichomes on the leaves of I. rubescens. These two types of glandular trichomes differed both anatomically and in the components of respective secondary metabolites. The glandular trichomes were classified into two subpopulations, namely the peltate and capitate glandular trichomes. The former was characterized by a short stalk and a large four-celled secretory head, while the latter was further subdivided into two groups; one has a short unicellular stalk and two-cellular head (type I), the other has a multicellular stalk and a globose unicellular head (type II). Peltate and capitate type I trichomes were constitutively present, whereas capitate type II was rarely found in all leaf samples. The secondary metabolites secreted by peltate trichomes were examined histochemically, which included terpenoids, flavonoids, carbohydrates, phenolics and alkaloids. The peltate trichomes are not only sites of oridonin accumulation, but also sites of oridonin biosynthesis in I. rubescens.
Key words: Isodon rubescens, Lamiaceae, glandular trichomes, histochemistry, secondary metabolites.