OECD 221, Regulation (EC) No 440/2008, C.26, DIN EN ISO 20079
The duckweed species Lemna minor serves as model organisms for higher freshwater plants. Harmful effects of water-soluble substances and waste water on the growth of this plant are assessed based on the analysis of different parameters, such as number of fronds, frond area, chlorophyll content, fresh or dry weight.
A defined number of leaves, so-called fronds, are exposed to several concentrations of a substance over a period of seven days. As measurement variables the number of fronds and the frond area are determined by digital image processing at the beginning and at the end of the exposure period and at two additional point of time in between. The specific growth rates and the biomass integral (yield) are calculated for each test concentration and for a control group. For quantification of the toxic effects, the mean growth rates and the mean yields of the test concentrations are compared to those of the controls. The EC50 values as well as the NOEC and LOEC are calculated.
Test substance properties
The Lemna test can be conducted as static test without renewal of the test solution for soluble stable substances or as semistatic test with daily renewal of the test solution for non-soluble or instable substances. Especially for the testing of dark coloured test solutions, the Lemna test is preferable to the algae growth inhibition test (OECD 201), since light absorption and thereby resulting growth inhibition is irrelevant in this test.
The Lemna growth inhibition test is also applicable for the testing of waste water samples, especially for those of dark colour and/or fluorescence. The test is performed in the same way as described above and several dilutions can be tested. As result of the test the lowest ineffective dilution (LID), that means the lowest dilution with less than 10% growth inhibition, is reported. For analysis, the most sensitive parameter (frond number or frond area) is taken.