Aim of present study is to assess the tolerance of chlorophyll change in duckweed plant (Lemna minor). Tolerance level and toxic effect of copper sulphate (CuSO4) and mercuric chloride(HgCl2) in Lemna minor was investigated under laboratory conditions. For toxicity test plants of L. minor were exposed to different concentrations of CuSO4 (5ppm, 10ppm and 20ppm) and HgCl2 (1.0ppm, 3.0ppm and 5.0ppm) separately as well as in combination. The combination of metals (CuSO4 + HgCl2) were 2.5ppm Cu + 0.5ppm Hg, 5ppm Cu + 1.5ppm Hg and 10ppm Cu + 2.5ppm Hg. All experiments were conducted into two different seasons i.e. winter (December-January) and summer (May-June) and chlorophyll tolerance estimated at 2 days and 4 days post treatment with specific heavy metal/combination of metals respectively. There was decrease in the chlorophyll content with highest concentration being at 20ppm Cu and also when 5ppm Hg was added individually. Chlorophyll content was found decreased in all concentrations and was maximum at 20ppm Cu and 5ppm Hg concentration. Severity of symptoms was found toxicant concentration dependent. Possible mechanism of Cu and Hg toxicity to this bioindicator has been discussed in this paper. Tabulated data suggests the potential role played by L. minor in bioremediation of heavy metals within optimum concentrations
