Localization of Sesquiterpene Lactones Biosynthesis in Flowers of Arnica Taxa

Arnica montana is a valuable plant with high demand on the pharmaceutical and cosmetic market due to the presence of helenalin (H) and 11α, 13-dihydrohelenalin (DH) sesquiterpene lactones (SLs), with many applications and anti-inflammatory, anti-tumor, analgesic and other properties. Despite the great importance of these compounds for the protection of the plant and their medicinal value, the content of these lactones and the profile of the compounds present within individual elements of florets and flower heads have not been studied so far, and attempts to localize these compounds in flower tissues have also not been conducted. The three studied Arnica taxa synthesize SLs only in the aerial parts of plants, and the highest content of these substances was found in A. montana cv. Arbo; it was lower in wild species, and a very small amount of H was produced by A. chamissonis. Analysis of dissected fragments of whole inflorescences revealed a specific distribution pattern of these compounds. The lactones content in single florets increased from the top of the corolla to the ovary, with the pappus calyx being a significant source of their production. Histochemical tests for terpenes and methylene ketones indicated the colocalization of lactones with inulin vacuoles.