Bioactive Compounds in Medicinal Plants as Affected by the Level of Potentially Toxic Element Contamination in Soil

Medicinal plants represent important sources of bioactive compounds with beneficial effects on human health. However, many medicinal species are ruderal plants capable of growing in soils with elevated contents of potentially toxic elements (PTEs) such as Cd, Pb, and Zn. In addition to the potential accumulation of PTEs in plant biomass, the response of the plant metabolome—including bioactive substances with beneficial health effects—to elevated PTE levels in plants should also be considered. The potential impact of soil PTEs on the plant metabolome was investigated in three widely used medicinal plants, Taraxacum sp., Achillea millefolium, and Hypericum maculatum, sampled in an area polluted with PTEs. The total soil contents of the PTEs ranged between 7.7 and 65 mg/kg for Cd, 1541 and 3897 mg/kg for Pb, and 245 and 6553 mg/kg for Zn. A qualitative analysis of the whole plant metabolomes of the three plant species indicated close interrelationships between the selected metals and bioactive substances. Subsequently, a model pot experiment was conducted in which Taraxacum sp. plants were cultivated in three soils with stepwise increasing Cd, Pb, and Zn contents, and selected bioactive compounds were quantified. The results showed a decrease in the concentrations of some phenolic compounds in the aboveground parts of Taraxacum sp. grown in extremely polluted soil, supporting the hypothesis that stress induced by PTEs may affect the metabolic pathways of these compounds. In contrast, higher levels of phenolic compounds were observed in Taraxacum sp. roots grown in moderately contaminated soil, suggesting that milder soil contamination may activate defence mechanisms and stimulate phenolic metabolism. However, although the contents of bioactive compounds in plants indicate an improvement of the quality of these medicinal plants, the elevated element contents in the plant biomass can represent a potential risk for consumers.