Japanese knotweed (Fallopia japonica), Giant knotweed (Fallopia sachalinensis) and their hybrid, Bohemian knotweed (Fallopia × bohemica), are invasive plant species from the genus Fallopia and family Polygonaceae. The Japanese and Giant knotweeds were popular as ornamental garden plants and spreaded rapidly across Europe, becoming an ecological and economic problem in the last two centuries due to threats to biodiversity and infrastructure. Bioactive secondary metabolites from different compounds such as stilbenes, flavonoids, phenolic acids, carotenoids, chlorophylls and triterpenic acids have been identified in Japanese, Giant and Bohemian knotweed. Ethanolic extracts were prepared from freeze-dried shoots and root peels samples of three species of knotweeds. Total phenolic compounds were determined by the Folin-Ciocalteu method. Ethanolic extracts were further analysed by ultra-high-performance liquid chromatography coupled to a hybrid quadrupole-ion trap mass detector (UHPLC-MS/MS). The separation of phenolic compounds was achieved by reversed-phase chromatography and quantified after electrospray ionisation by tandem mass spectrometry. Seven polyphenolic compounds were quantified in at least one sample. This confirmed that the two methods used are suitable for the quantitative determination of polyphenolic compounds in knotweed extracts. The highest total polyphenolic compounds was with Folin-Ciocalteu method determined in the peels of Giant knotweed roots, while with UHPLC-MS/MS method we determined the highest amount of polyphenolic compounds in the shoots of Bohemian knotweed. We confirmed that the Folin-Ciocalteu and UHPLC-MS/MS methods are suitable for the quantitative determination of polyphenolic compounds in knotweeds and that they may give incomparable results.
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