Japanese knotweed, giant knotweed and Bohemian knotweed are three widely spread, invasive alien plants that represent an ecological and economic problem in Europe and North America. In order to exploit this biomass containing various bioactive compounds, high performance thin-layer chromatography (HPTLC) coupled with densitometry, image analysis and mass spectrometry (HPTLC-MS/MS) was used to study phenolic compounds flavan-3-ols and proanthocyanidins in leaves and rhizomes of these knotweeds. Image analysis of the HPTLC silica gel plates showed the highest proanthocyanidins content (based on the total peak areas) in rhizomes of Japanese knotweed. Using (-)-epicatechin and procyanidin B2 standards, the contents of monomers and dimers in leaves and rhizomes of all three knotweeds were determined. In leaves of Japanese, Bohemian and giant knotweed the contents of monomers were 0.84, 1.39, 2.36 kg/t of dry material, respectively, while the contents of dimers were 0.99, 1.40, 2.06 kg/t of dry material, respectively. In rhizomes of Japanese, Bohemian and giant knotweed the contents of monomers were 2.99, 1.52, 2.36 kg/t of dry material, respectively and the contents of dimers were 2.81, 1.09, 2.17 kg/t of dry material, respectively. With HPTLC combined with densitometry and image analysis we found qualitatively equal chromatographic profiles of proanthocyanidins in rhizomes of Japanese, Bohemian and giant knotweed, which was not the case with profiles of leaves, where giant knotweed's profile has three additional peaks. Within the same species, only giant knotweed has the same profile for both plant organs, while Japanese and Bohemian knotweed have two additional peaks in the profiles of rhizomes compared to leaves. Using HPTLC-MS/MS we were the first to identify all B-type proanthocyanidins from monomers to decamers in leaves of all knotweeds and in rhizomes of Bohemian and giant knotweed, as well as some of their gallates (from monomers to hexamers) and dimer digallates in rhizomes of Bohemian and giant knotweed. We were the first to identify proanthocyanidins in leaves of Bohemian knotweed. Among the leaves of all three knotweeds, the highest diversity in the qualitative profile of gallates was detected in leaves of giant knotweed. Monomer gallates and dimer gallates were identified in leaves of Japanese knotweed, while in addition to these digallate dimer was also confirmed in leaves of Bohemian knotweed. Leaves of Japanese, Bohemian and giant knotweed have the same chemical profiles of proanthocyanidins with respect to the degree of polymerization, although their chemical profiles with respect to gallates differ. Rhizomes of Bohemian and giant knotweed have the same chemical profiles of proanthocyanidins in terms of degree of polymerization and also in terms of gallates. Within each of the three individual species, only giant knotweed has the same chemical profile of proanthocyanidins in leaves and rhizomes, both with respect to the degree of polymerization and with respect to gallates.
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