Varying concentrations of soil chromium (VI) for the exploration of tolerance thresholds and phytoremediation potential of the oregano (Origanum vulgare).

Varying concentrations of soil Cr(VI) were used in order to explore the tolerance thresholds and phytoremediation potential of Greek oregano (Origanum vulgare), in a pot experiment conducted outdoors. Oregano exhibited a rather exceptional capacity to bioaccumulate Cr in both the aerial part (up to 1200 mg of total Cr kg-1 DM) and the root-reaching 4300 mg kg-1 DM when grown in soil [Cr(VI)] of 150-200 mg kg-1. Plant responses indicated that there was a threshold set at 100 mg Cr(VI) kg-1 in the soil, above which the following results were recorded: (i) a restriction of Cr translocation from below- to above-ground plant part, (ii) a raise of the soil-to-root Cr transfer, and (iii) the Cr(III) evolution from the reduction of Cr(VI) was significantly decelerated in the root and accelerated in the aerial part. Soil [Cr] that surpassed this threshold challenged plant tolerance, resulting in a dose-dependent reduction of growth and antioxidant phenolics pool. Nonetheless, the significant Cr uptake capacity at plant level accounted for the considerably short remediation time (i.e., 29 years at soil [Cr(VI)] of 150 mg kg-1) calculated according to these results. The overall performance of oregano indicated that phytoremediation would be feasible at sites with Cr contamination levels ranging within the above-defined thresholds.