Structure and functions of the glandular trichomes in three Arnica species (Asteraceae), depending on their location on leaves and flowers

The aim of this study was to characterize the structural and histochemical features of the glandular trichomes (GTs) of two types localized on the leaves and flowers in three Arnica species. GTs of the first type are found on leaves, peduncle, calyx, ovary, and petal base. They are biseriate, have a cylindrical form and 5 to 9 cell layers. GTs of the second type are located on the peduncle and calyx; they have a biseriate head of two to four cell layers and a long stalk. In each Arnica species differences between GTs of two types include dimensions, intensity of coloration by histochemical dyes, as well as ultrastructural features of the cells. According to the results of fluorescence microscopy and histochemical tests, phenols, terpenes, sesquiterpene lactones and polysaccharides are revealed in all GTs of the studied species; at the same time, the amount and composition of primary and secondary compounds are specific for trichomes of each organ. In the leaf GTs, phenols and terpenes prevail; in the GTs of the first type situated on the peduncle and calyx, acid polysaccharides are met in larger amount then in trichomes of other organs; in the GTs of the second type, sesquiterpene lactones are the main metabolites. At the secretory stage, the apical cells of GTs contain smooth and rough endoplasmic reticulum, diversiform leucoplasts with black inclusions in lamellae, chloroplasts, mitochondria, and Golgi apparatus. A single subcuticular cavity is formed in GTs of the first type, unlike GTs of the second type, where many small subcuticular cavities appear. In these GTs, the head surface looks porous. Although there are structural modifications between trichomes of the same type in different species of Arnica, the functional variations between trichomes of the different types are more significant.