Spectrophotometric Estimation of Polyphenolic Compounds in Willowherbs (Epilobium angustifolium L. and E. hirsutum L.) and Implications for Genetic Resource Conservation

There is a growing interest in natural bioactive substances, particularly plant-derived secondary metabolites. Polyphenols constitute one of the largest and most significant groups of these metabolites. Rosebay willowherb (<i>Epilobium angustifolium</i>) is well known in traditional medicine and can serve as a reference species for studying its less-known congener, hairy willowherb (<i>E. hirsutum</i>), thereby expanding knowledge of medicinal plants. This study aimed to quantitatively estimate and compare the total phenolic content (TPC) and total flavonoid content (TFC) in the leaves and flowers of <i>Epilobium angustifolium</i> and <i>E. hirsutum</i>, and to identify populations with the highest phytochemical potential. TPC and TFC were quantified using the Folin-Ciocalteu and aluminium chloride (AlCl₃) colorimetric assays, respectively, with resulting values regarded as estimates due to the non-specificity of these assays. The results showed that, in terms of TPC, <i>E. angustifolium</i> leaves accumulated 132 ± 3.4 mg_GAE/g (milligrams of gallic acid equivalent per gram of dry plant mass), exceeding those of <i>E. hirsutum</i> by 16.8%; in flowers, the respective values were 153 ± 3 mg_GAE/g, a difference of 1.3%. Regarding TFC, <i>E. angustifolium</i> leaves contained 25 ± 1.4 mg_RE/g (milligrams of rutin equivalent per gram of dry plant mass), which was 20% lower than in <i>E. hirsutum</i>, whereas its flowers accumulated 44 ± 1.4 mg_RE/g, representing a 63% higher content compared with <i>E. hirsutum</i>. The study may contribute to the selection of the <i>Epilobium</i> populations for genetic resource conservation and sustainable utilisation.