Effect of Ag+ and Cd2+ Elicitation on Polyphenol Production in Shoot Culture of Dracocephalum ruyschiana L.

Abiotic elicitation with heavy metals has demonstrated considerable potential to stimulate the production of industrially important secondary metabolites in plant in vitro cultures. The present study investigates the effect of exogenous silver nitrate and cadmium chloride supplementation on flavonoid and phenolic acid production, as well as other indicators of oxidative stress, in shoot cultures of Dracocephalum ruyschiana L. Owing to the presence of bioactive polyphenolic compounds, this Mongolian medicinal plant is traditionally used as an anti-inflammatory, antibacterial and antipyretic agent. The shoots were cultured for three weeks, and then, cadmium (Cd2+) and silver (Ag+) ions (50 or 100 µM) were added to the medium. The maximum proliferation rate was observed in the presence of 100 µM Ag+ (almost 5), the highest chlorophyll content in the presence of 100 µM Cd2+ (0.6 mg/g FW) and the highest biomass was observed with both these treatments (73.4–75.7 g FW and 7.53–7.72 g DW). UPLC-PDA-ESI-MS analysis revealed four phenolic acids and five flavonoid derivatives in the hydromethanolic extract of D. ruyschiana shoots. All treatments stimulated the production of rosmarinic acid (RA), which was the dominant compound in the analyzed culture; the highest level of RA, i.e., about three times higher than the control, was noted in shoots exposed to 50 µM Cd2+ (14.72 mg/g DW), whereas the level of most flavonoids in the culture increased most significantly when exposed to Cd2+ at a concentration of 100 µM. Moreover, the shoots grown in the presence of 100 µM Cd2+ exhibited significantly higher antioxidant potential in comparison to the control. Our findings indicate that heavy metals are able to stimulate phenolic compound biosynthesis in Dracocephalum shoots without any negative impact on their growth. These results could be of significant importance for the medical, nutraceutical and agronomic industries.