A pilot study to determine the substrate threshold for heavy metal toxicity in groundcover plants used in urban landscapes

Abstract

The objects of this pilot study were to determine the substrate threshold concentration for heavy metal toxicity, which is the highest permissible content in the substrates used in urban landscaping, and the maximum possible accumulation of heavy metals in shoots of Pachysandra terminalis and Vinca minor grown on substrate mixed with cadmium (Cd), lead (Pb), or zinc (Zn) for 90 days. Plants were transplanted into plastic containers filled with substrate, which had been previously treated with Cd (0, 25, 50, or 100 mg·kg-1), Pb (0, 250, 500 or 1,000 mg·kg-1), and Zn (0, 500, 1,000, or 2,000 mg·kg-1). The shoot dry weights of P. terminalis and V. minor were affected by the heavy metals in the substrate with the greatest degree by Cd. A 25 mg·kg-1 of Cd caused a reduction of about 63% and 30% of shoot dry weights of P. terminalis, and V. minor, respectively, compared to controls (Cd 0 mg·kg-1). There was no reduction in P. terminalis, and V. minor grown under both Pb 250 mg·kg-1 and 500 mg·kg-1 treatments. Under the Pb 1,000 mg·kg-1 treatment, a 3.7% of dry weight loss observed in P. terminalis compared to control (Pb 0 mg·kg-1); however, no reduction was found in V. minor. Zn-treated V. minor had no reduction of dry weight compared to controls (Zn 0 mg·kg-1), whereas in P. terminalis, about 11%, 26%, and 52% reductions were observed under Zn 500, 1,000, and 2,000 mg·kg-1 treatments, respectively. These results reveal that V. minor is more tolerant to Pb and Zn toxicity than is P. terminalis, and provide reference ranges for heavy metal concentrations in substrate that will be useful in further studies.