Dual role of the hyperaccumulator Silybum marianum (L.) Gaertn. in circular economy: Production of silymarin, a valuable secondary metabolite, and remediation of heavy metal contaminated soils

In Mediterranean contaminated soils the weed Silybum marianum L. (Gaertn.) was cultivated in a two-year pot experiment. The study included two soils, from a rural and an urban area of Central Greece. In addition to the control samples, the soil samples were subjected to laboratory contamination with two levels of copper (70 and 140 mg Cu kg(-1) soil). The soils' water-soluble, exchangeable, DTPA-extractable, and pseudo-total Cu concentrations were determined. High correlations were found between soil and plant Cu. S. marianum which common name is milk thistle, appeared to be particularly resistant when cultivated on contaminated soils with heavy metals, such as copper (Cu). A relatively small transfer of the metal to its up-ground part was detected, when it was cultivated both in rural and urban Cu-loaded soils, as follows: stems > leaves > seeds. More specifically, in high contaminated (level B) rural soil the concentrations were 19.7 +/- 2.4 > 9.7 +/- 2.1 > 1.9 +/- 0.7 mg kg(-1), while in urban soil (level B) the concentrations were 13.4 +/- 2.7 > 8.2 +/- 1.8 > 1.7 +/- 0.5 mg kg(-1), respectively. On the other hand, the quality of silymarin, which was produced from the seeds of the plant, did not seem to be affected by Cu levels in the soils. Its eight components were quantitatively evaluated by High Performance Liquid Chromatography (HPLC), both in the control and in the soils contaminated with two levels of Cu. The yield of silymarin raised proportional to soil Cu levels, with 8.2-15% increase in urban, unfertilized, and contaminated soil, compared to rural soil. As a result, milk thistle appears to be a promising crop since likewise recovers heavily contaminated soils and enhances the production of silymarin, a valuable component with high medicinal value, even when cultivated in degraded and heavily polluted soils.