Callus cultures of Harpagophytum procumbens (Burch.) DC. ex Meisn.; production of secondary metabolites and antioxidant activity

•Well-grown calli were cultured in the presence of picloram or NAA and BAP.•During long-term culture, calli were characterized by vigorous growth.•All calli were stable in terms of verbascoside and isoverbascoside production.•The level of harpagoside and harpagide depends on callus line and age.•The antioxidant activities of calli were correlated with total phenolic content.

Three cotyledon-derived callus lines of Harpagophytum procumbens were established on agar-solidified Schenk and Hildebrandt (SH) medium supplemented with 0.2 mg L− 1 α-naphthaleneacetic acid (NAA) and 1 mg L− 1 6-benzylaminopurine (BAP) (CNB line), 0.5 mg L− 1 picloram (CP0.5 line) or 2 mg L− 1 picloram (CP2 line). They were maintained in three different culture vessels: culture tubes, Erlenmeyer flasks and Magenta vessels. Hormonal treatment and type of culture vessel influenced callus growth and morphology. The highest biomass (64- to 130-fold increase in fresh and dry weights within 4 weeks) was achieved in Erlenmeyer flasks. The contents of major phenylethanoid and iridoid glycosides in the calli were determined by high performance liquid chromatography (HPLC). Harpagoside, harpagide, verbascoside and isoverbascoside were identified and the ability to produce these bioactive compounds sustained for over 3 years. H. procumbens calli were also analyzed by colorimetric methods for total contents of phenolics, flavonoids and anthocyanins. The extracts of CNB callus had the highest antiradical activity against DPPH and ABTS and capacity to reduce metal ions (Ferric Reducing Antioxidant Power—FRAP). The same extracts were also characterized by the highest total content of phenolics and flavonoids.