Comparative metabolomics of Tilia platyphyllos Scop. bracts during phenological development

The medicinal plant drug Tiliae flos consists of the botanical flowers and bracts of Tilia sp., gathered almost exclusively during flowering. In this study, we examined the changes in the metabolome of specialized products in the bracts of Tilia platyphyllos from the appearance of the organ till the onset of senescence by LC-ESI-MS and data mining. A set of 504 natural products were detected, 241 of which showed significant seasonal variation (p < 9.92E-5). Seven compounds were quantified and an additional 45 were putatively identified. These included flavonoid glycosides, catechins, procyanidins, quinic acid derivatives (including chlorogenic acid) and coumarins. Compared to bracts during flowering, young tissues were characterized by a relatively high diversity of polyphenolic substances. Higher amounts of flavonol glycosides (quercetin, kaempferol), catechins and derivatives have been observed. Deoxyhexosides were almost exclusive to this phenological stage. Changes of about one order of magnitude were not uncommon. For some substances, 5-fold differences were observed (calibration with authentic standards). Some compounds (e.g. the coumarin fraxin) were more prominent at the late fruit growth stage. It was shown that bracts gathered before or after flowering could potentially be therapeutically useful. Changes are rapid during the early phase of bract development: three different groups of compounds presented their maxima during the first 32 days. Considering seasonal variation is of extreme importance during bioactivity tests and screening candidate sources for bioactive natural products. In the case of T. platyphyllos, young and old bracts can be of interest because of their high diversity of distinct specialized metabolites.