Tolerance and co‐tolerance of the grassChloris barbataSw. to mercury, cadmium and zinc

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Abstract
Summary: Races ofChloris barbataSw. 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, 4H2O, l gl−1with or without HgCI2(0.001, 0.01, 0.05, 0.1 and mgl−1), CdSO4.8H2O (0.05, 1.0, 2.5 and 5 mg l−1and ZnSO47H2O (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 ofC. barbatafrom 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.