Growth rate based dose-response relationships and EC-values of ten heavy metals using the duckweed growth inhibition test (ISO 20079) with Lemna minor L. clone St.

The duckweed Lemna minor L. clone St was used to investigate the effect of 10 heavy metals under the standardised test conditions of the ISO protocol 20079. By using growth rates derived from frond number (FN), fresh weight (FW), dry weight (DW), chlorophyll and carotenoid (Car) contents, concentration-response curves for all heavy metals and all growth parameters were classified. In addition, all data were fitted to obtain the inhibitions of growth rates (E(r)C(x)) at the level of 10%, 20% and 50% (E(r)C(10), E(r)C(20) and E(r)C(50), respectively) then used to evaluate the phytotoxicity of the different heavy metals. On the basis of the E(r)C(50) values (average ranking of all five growth parameters), the following series of phytotoxicity was detected by using molar concentrations: Ag(+)>Cd(2+)>Hg(2+)>Tl(+)>Cu(2+)>Ni(2+)>Zn(2+)>Co(2+)>Cr(VI)>As(III)>As(V).