Morphology, histochemistry and ultrastructure of the floral glandular trichomes in three Doronicum species (Asteraceae).

Previously, it was found that four types of glandular trichomes (GTs) are developed on the surface of all aerial organs in Doronicum species. A detailed study of leaves had shown that only two types of GTs form in them. Nothing was known about any differences of GTs on vegetative and reproductive organs. Current work studies morphological, histochemical and ultrastructural features of two types of GTs arising on floral elements in three Doronicum species. The straight GTs there are on all elements of the inflorescence (peduncle, phyllaries, ray and disk florets); they have a short stalk and do not have a clearly visible head. The capitate GTs are located only on the inflorescence peduncle and phyllaries; the trichomes form a long stalk and a distinct head. The chemical composition of the secretion is confirmed by histochemical reactions. Both types of GTs have the specific ultrastructural features and secretory mechanisms. In capitate GTs, a smooth endoplasmic reticulum is the main organelle of cytoplasm and takes part in a synthesis of the phenolic substances. Phenols accumulate in the numerous small vacuoles and thick cell wall. In straight trichomes, leucoplasts of various shapes with plastoglobuli and peripheral reticulum predominate. Terpenes synthesized in leucoplasts are stored in the subcuticular cavity and are released when the cavity ruptures. The acidic polysaccharides are the additional components of a straight trichome secretion. The studied species of Doronicum differ from each other in localization, size and structure of the GTs.