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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(NO3 )2 , 4H2 O, l gl-1 with or without HgCI2 (0.001, 0.01, 0.05, 0.1 and mgl-1 ), CdSO4 .8H2 O (0.05, 1.0, 2.5 and 5 mg l-1 and ZnSO4 7H2 O (1, 5, 10, 20 and 40 mg l-1 ) 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.