GROWTH RESPONSE OF THE DUCKWEED LEMNA MINOR TO HEAVY METAL POLLUTION

To assess the tolerance and effect of heavy metals pollution on the duckweed lemna minor, the aquatic plants were exposed to different concentrations of copper (cu), nickel (ni), cadmium (cd) and zinc (zn) in a quarter coïc and lessaint solution at ph = 6.1 (± 0.1) and under a daily regime of 16 h light (101 μmol/m2.s1). copper at 0.2 mg/l and nickel at 0.5 mg/l promoted the growth of lemna fronds. at higher concentrations, cu and ni inhibited the growth of duckweed; the ec50 (concentration causing 50% inhibition) were 0.47 mg/l for cu and 1.29 mg/l for ni. cadmium and zinc decreased by 50% the growth of fronds when the medium contained respectively 0.64 and 5.64 mg/l (ec50). duckweed tolerated cu, ni, cd and zn at concentrations of 0.4, 3.0, 0.4 and 15.0 mg/l respectively without showing any visible signs of toxicity (chlorosis, frond disconnection and necrosis). on the basis of visible symptoms and the ec50 values, the toxicity of the metals on lemna. minor was in decreasing order of damage: cu > cd > ni > zn. it was concluded that the duckweed lemna. minor is very sensitive to copper and cadmium pollution.