Surface flavonoids of Centaurea stenolepis and the local endemics Centaurea davidovii and C. parilica (Asteraceae, sect. Lepteranthus) from Bulgaria

The species of Asteraceae have been found to be a rich source of externally accumulated flavonoid aglycones. This type of compounds attracts the attention, besides for their importance as chemotaxonomic markers, also for their ecological role. In the present study, two local endemic species, Centaurea davidovii and C. parilica, and one widespread species from the same group, C. stenolepis, are analyzed for their external flavonoid composition. Flavonoid profiles of flower heads and leaves are analyzed separately. The exudates of flower heads have displayed more complex flavonoid profiles. Ten flavonoid aglycones are identified by thinlayer chromatography (TLC), with authentic markers, using three different sorbents (silica gel, polyamide, cellulose) and several combinations of mobile phases. Some differences are observed in the classes of flavonoids accumulating in the examined species: predominantly methylated derivatives of 6-hydroxyflavones in the exudates of C. davidovii and C. parilica, and methylated derivatives of 6-hydroxyflavonols of C. stenolepis. Similar flavonoid profiles are registered of the exudates of C. davidovii and C. parilica. They yielded apigenin, luteolin, scutellarein 6,4’-dimethyl ethers, 6-hydroxyluteolin 6-methyl ether, and kaempferol 3-methyl ether. Furthermore, in the exudates of C. davidovii, scutellarein 6-methyl ether and scutellarein 6,7,4’-trimethyl ethers are detected. Most flavonoids of C. stenolepis are methylated flavonols: kaempferol 3-methyl ether, 6-hydroxykaempferol 3,6-dimethyl ethers and quercetagetin 3,6,4’-trimethyl ethers. The species are studied for the first time for their flavonoid aglycone composition.