The duckweed growth inhibition test is internationally standardised by ISO (ISO 20079, 2005) and OECD (OECD 221) for the testing of chemicals, pesticides and a wide range of aquatic samples or substances that could be dissolved in water. These duckweed tests are generally performed with Lemna gibba (often preferred in pesticide screening for OECD, to comply with US guidelines) or – both for ISO and OECD testing – with Lemna minor. Especially Lemna minor ST is a very widely used clone that is extraordinarily suitable for ecotox testing. Duckweed is the best option for testing toxic substances and environmental samples with a higher plant. This provides very important complementary information in addition to the widely used algal tests.
According to the standards, it is a requirement to measure two parameters at least at the beginning and end of the test (+ two intermediate measurements for one parameter), as the growth rates of different parameters often do not correlate under toxic impact and the parameter with the greater sensitivity is important for the toxicity assessment. As a modern bio test, the duckweed growth inhibition test has alternative quantification options, which allow manual assessment (based on frond count and dry or fresh weight) as well as automated assessment based on image-based frond number and image-based frond area quantification. In addition to the two obligatory test parameters, the LemnaTec Lab Scananlyzer image-based analysis system is able to provide a comprehensive documentation of the test results, including qualitative parameters (frond discoloration, colony separation etc.) that need to be reported additionally. A summary of the method was recently given in the ISCDRA Newsletter in 2016.
Going beyond the standards, the Lab Scanalyzer units also provide a quantitative assessment of colour (and, even more important, color changes) and for example frond size distribution values. While this is not necessary for standard tests, it can prove advantageous if the test results are to be used to provide information for higher tier studies, for special concerns or in prolonged tests (including recovery), when only subsets of the incubated plants are cultivated for longer than the standard test duration of one week. The LemnaMiner software includes templates to calculate growth rate inhibition values automatically right after the measurement is completed. The results can be directly transferred to an ecotoxicological statistics software tool like ToxRat, in order to calculate effective concentration values (ECx).
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