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 v icinity of a chloralkali plant and from a non-contaminated site were t ested for their relative tolerance to mercury, cadmium and zinc. Pre-r ooted tillers of each of the races were grown in calcium nitrate solut ion, Ca(NO3)(2).4H(2)O, 1 g l(-1) with or without HgCl2 (0.001, 0.01, 0.05, 0.1 and 0.5 mg l(-1)), CdSO4.8H(2)O (0.05, 1.0, 2.5 and 5 mg l(- 1)) and ZnSO4.7H(2)O (1, 5, 10, 20 and 40 mg l(-1)) far a period of 5 d. Metal tolerance was evaluated by the effects of the metal on root e longation. Races from the contaminated site were found to be metal-tol erant, as indicated by higher tolerance indices (Tk) than races from t he non-contaminated site. Mercury and cadmium at lower concentrations and zinc at all concentrations stimulated root growth in tolerant race s, resulting in a TI of more than 100%, This indicated a requirement o f the metal ion for better root growth. Compared to non-tolerant races , the tolerant races exhibited higher catalase, peroxidase and acid ph osphatase activities as well as higher thiol levels, The over-all stud y thus indicated that races of C. barbata from the mercury-contaminate d site were not only tolerant to mercury but also co-tolerant to cadmi um and zinc. Biochemical studies provided evidence that the tolerant r aces were physiologically distinct from the intolerant races.