Effect of zinc on the production of phenolic acids and hypoxoside in micropropagated Hypoxis hemerocallidea

The present study was conducted to investigate the effect of zinc (Zn) on the synthesis of secondary metabolites in micropropagated Hypoxis hemerocallidea. In vitro raised plantlets of H. hemerocallidea were exposed to various exogenous concentrations of Zn (100, 200, 400 and 800 µM) for 6 weeks. The concentrations of phenolic acids and the accumulation of hypoxoside were determined using ultra-high performance liquid chromatography-tandem mass spectrometry. Application of Zn significantly increased the concentrations of hydroxybenzoic acid derivatives, namely gallic acid, p-hydroxybenzoic acid, protocatechuic acid, salicylic acid, syringic acid and vanillic acid. Variable concentrations of hydroxycinnamic acid derivatives were recorded with the highest concentration of caffeic acid and p-coumaric acid detected in plants grown on standard Murashige and Skoog (MS) medium containing the normal 30 µM-Zn concentration. Media supplemented with exogenous Zn (200 µM) stimulated the highest concentrations of chlorogenic acid and sinapic acid whereas Zn (100 µM) significantly increased the concentration of ferulic acid. Significantly high concentration of hypoxoside was recorded in the control and Zn (800 µM) treated cultures. The results revealed that exposure to Zn enhanced plant biomass and the accumulation of most phenolic acids, but significantly reduced the accumulation of hypoxoside, and antioxidant activity.