2.4. Plant tissue analysis
Dried plant samples (shoots and roots) were ground to powder form in an electric mill. The powdered samples were digested with concentrated HNO3 (16N) and HClO4 (11.6N) (5:1 v/v) (Allen, 1989). Concentrations of Cd, Cu, Pb and Zn in the digests were determined by ICP-OES. A standard reference plant material (SRM 1573a: Tomato leaves) from the US Department of Commerce National Bureau of Standards was used to verify the accuracy of metal determination, and the recovery rates were within 90 ± 10%.