Trichomes (from the Greek for “hair”) are fine outgrowths on plants such as hairs. They are highly variable in shape, cytology and function, and more than 300 types of plant trichomes have been described. Trichomes occur on different surfaces of almost all angiosperms and may alter the boundary layer above the leaf surface, contribute to light piping, protect against temperature stress, reduce water loss in transpiration or serve as the absorbing surface in roots.
Besides metabolically inactive or so-called non-glandular trichomes, biosynthetically active glandular trichomes also exist. They sequester or store plant metabolites that are often characteristic for specific taxonomic groups. The storage of bioactive compounds in in these structures is frequently associated with protection against herbivores and pathogens, but evaporation of volatile trichome metabolites also occurs and may have numerous physiological and ecological functions.
Previous studies have shown that the leaves of the sunflower (Helianthus annuus) have two different types of glandular trichome. A recent paper in AoB PLANTS describes detailed results on the morphology, localization and metabolic activity of glandular trichome in sunflowers and their occurrence in related taxa.
Linear glandular trichomes of Helianthus (Asteraceae): morphology, localization, metabolite activity and occurrence. (2013) AoB PLANTS 5: plt028 doi: 10.1093/aobpla/plt028
Capitate glandular trichomes of sunflower are well investigated, but detailed studies are lacking for the linear glandular trichomes (LGT), a second type of physiologically active plant hair present on the surface of sunflowers. Light, fluorescence and scanning electron microscopy as well as histochemical staining were used to investigate the structure and metabolite deposition of LGT. Consisting of 6–11 linearly arranged cells, LGT were found on the surface of most plant organs of Helianthus annuus. They were associated with the leaf vascular system, and also occurred along petioles, stems and the abaxial surface of chaffy bracts, ray and disc florets. The highest density was found on the abaxial surface of phyllaries. Phenotypically similar LGT were common in all species of the genus, but also occurred in most other genera of the Helianthinae so far screened. Brownish and fluorescent metabolites of an as yet unknown chemical structure, together with terpenoids, were produced and stored in apical cells of LGT. The deposition of compounds gradually progressed from the tip cell to the basal cells of older trichomes. This process was accompanied by nucleus degradation in metabolite-accumulating cells. The localization of these trichomes on prominent plant parts of the apical bud and the capitulum combined with the accumulation of terpenoids and other as yet unknown compounds suggests a chemo-ecological function of the LGT in plant–insect or plant–herbivore interaction.