Tolerance and co-tolerance of the grass Chloris barbata Sw. to mercury, cadmium and zinc.

Races of Chloris barbata Sw. from a mercury-contaminated site in the vicinity of a chloralkali plant and from a non-contaminated site were tested for their relative tolerance to mercury, cadmium and zinc. Pre-rooted tillers of each of the races were grown in calcium nitrate solution, Ca(NO ) , 4H O, l gl with or without HgCI (0.001, 0.01, 0.05, 0.1 and mgl ), CdSO .8H O (0.05, 1.0, 2.5 and 5 mg l and ZnSO 7H O (1, 5, 10, 20 and 40 mg l ) for a period of 5 d, Metal tolerance was evaluated by the effects of the metal on root elongation. Races from the contaminated site were found to be metal-tolerant, as indicated by higher tolerance indices (TIs) than races from the non-contaminated site. Mercury and cadmium at lower concentrations and zinc at all concentrations stimulated root growth in tolerant races, resulting in a TI of more than 100%. This indicated a requirement of the metal ion for better root growth. Compared to non-tolerant races, the tolerant races exhibited higher catalase, peroxidase and acid phosphatase activities as well as higher thiol levels. The over-all study thus indicated that races of C. barbata from the mercury-contaminated site were not only tolerant to mercury but also co-tolerant to cadmium and zinc. Biochemical studies provided evidence that the tolerant races were physiologically distinct from the intolerant races.