Comparative Assessment of the Phytochemical Composition and Biological Activity of Extracts of Flowering Plants of Centaurea cyanus L., Centaurea jacea L., and Centaurea scabiosa L.

The data on the phytochemical composition and biological activity for flowering plant extracts of the genus Centaurea (Knapweed)—cornflower (Centaurea cyanus L.), brown knapweed (Centaurea jacea L.), and greater knapweed (Centaurea scabiosa L.), which are typical representatives of the flora in the middle belt of the Russian Federation, were obtained. For the first time, biologically active substances such as pyranone, coumaran (2,3-dihydrobenzofuran), and 5-hydroxymethylfurfural were identified in ethanol and methanol extracts of Centaurea scabiosa L. by gas chromatography–mass spectrometry. Catechol and α-amyrin were the major components of the ethanol extract from Centaurea cyanus L., and flavone was the major component of Centaurea jacea L. flower extract. The greatest antimicrobial activity against phytopathogens was detected in Centaurea scabiosa L. when extracting freshly harvested flower biomass with methyl tert-butyl ether at room temperature: the minimum inhibitory concentrations were 60–120 µg/mL, the minimum fungicidal concentration was 120 µg/mL, and the minimum bactericidal concentration was 250 µg/mL. The low antioxidant activity of the studied plant extracts was established using the maximum values of Centaurea jacea L. Ethanol extract of Centaurea cyanus L. flowers had low antimicrobial and antioxidant activity. The extracts showed no phytotoxicity to garden cress germination but inhibited the growth of juvenile plants, especially roots. The greatest phytotoxic effect was revealed with methyl tert-butyl ether, where the depression of growth indicators was 35% or more.