Effect of cadmium uptake and accumulation on growth and antibacterial activity of Merwilla plumbea — An extensively used medicinal plant in South Africa

Abstract

In South Africa, heavy metal contamination of agricultural soils is on the increase primarily due to excessive application of fertilizers, sewage disposal and mining activities. The aim of this study was to investigate the effect of cadmium (Cd) on plant growth and biological activity. The medicinal plant Merwilla plumbea [Syn. Merwilla natalensis (Syn. Scilla natalensis)] was selected due to the high demand for its bulbs in the traditional medicine markets. Low levels of Cd (2 mg/L) significantly reduced fresh mass of leaves, bulbs and roots in comparison to the control. Although most of the Cd was stored in the roots, the bulbs, which are used medicinally, accumulated 7.1, 5.9 and 11.6 mg/kg when grown in sand watered weekly with 2, 5 and 10 mg Cd/L respectively. The bulbs of M. plumbea contained 24-fold more Cd than the World Health Organization guideline of 0.3 mg Cd/kg, when irrigated with 2 mg Cd/L. The bulb extracts showed increased antibacterial activity against the Gram-positive bacterium Bacillus subtilis at 2 mg Cd/L. The plants treated with 10 mg Cd/L showed an increased antibacterial activity against B. subtilis, Klebsiella pneumoniae and Staphylococcus aureus in comparison to non-Cd-treated plants (control). However, there was no change in antibacterial activity of the various extracts against the Gram-negative bacterium Escherichia coli. The ability of M. plumbea to accumulate Cd not only raises concern for consumer safety, but also the quality of medicinal plants sold may be in question.