Harpagophytum germplasm varies in tuber peel and pulp content of important phenylpropanoids and iridoids

Tubers of the xerophytic African plant Harpagophytum procumbens are used as an appreciated medicinal drug whereas the value of H. zeyheri remains uncertain. Due to large and sometimes continuous morphological variation, discrimination between these two species can be difficult. In addition, interspecific hybridization may occur and could further complicate the collection of proper plant material for medicinal purposes. Hence the objectives of this study were to 1) study the content of selected secondary metabolites in peel and pulp of Harpagophytum tubers of different taxa, 2) identify possible species-specific differences in content (profile and amount) of selected secondary metabolites, and 3) investigate associations between content of selected secondary metabolites and sample origin. Although composition and content of the studied compounds varied among populations within species, H. procumbens and H. zeyheri had significantly different phytochemical profiles, with putative hybrids sometimes being intermediate and sometimes transgressive. In both peel and pulp the content of isoverbascoside, verbascoside, acetylacteoside and pagoside were higher in putative hybrids and H. procumbens compared with H. zeyheri. The content of the main bioactive compound harpagoside was significantly higher in H. procumbens compared with putative hybrids and H. zeyheri, with both pulp and peel content of H. procumbens being approximately twice as high as the pulp and peel content of putative hybrids. Accurate identification, selection and multiplication of germplasm with superior qualities should therefore be performed to provide plant material with a high content of bioactive compounds.