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 (Epilobium angustifolium) is well known in traditional medicine and can serve as a reference species for studying its less-known congener, hairy willowherb (E. hirsutum), 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 Epilobium angustifolium and E. hirsutum, 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, E. angustifolium leaves accumulated 132 ± 3. 4 mgGAE/g (milligrams of gallic acid equivalent per gram of dry plant mass), exceeding those of E. hirsutum by 16. 8%; in flowers, the respective values were 153 ± 3 mgGAE/g, a difference of 1. 3%. Regarding TFC, E. angustifolium leaves contained 25 ± 1. 4 mgRE/g (milligrams of rutin equivalent per gram of dry plant mass), which was 20% lower than in E. hirsutum, whereas its flowers accumulated 44 ± 1. 4 mgRE/g, representing a 63% higher content compared with E. hirsutum. The study may contribute to the selection of the Epilobium populations for genetic resource conservation and sustainable utilisation.