OECD 221 – Lemna species; Growth Inhibition Test

OECD 221 assess the toxicity of substances to freshwater aquatic plants of the genus Lemna (duckweed).

OECD 221 assess the  toxicity of a test substance using Lemna gibba or  Lemna minor. For substances that rapidly precipitate out or degrade from solution as a result of volatilisation, photodegradation, precipitation or biodegradation, it is recommended to test with a renewal system that is semi-static or flow-through.

OECD 221 is performed by allowing exponentially growing plant cultures of the genus Lemna to grow as monocultures in different concentrations of the test substance over a period of seven days. The objective of the OECD 221 test method is to quantify substance-related effects on vegetative growth over this period by measurement variables.  The primary measurement variable is the number of fronds; at least one other measurement variable including total frond area, dry weight or fresh weight is also measured.

The test endpoint for OECD 221 is inhibition of growth where the determination of EC50, the lowest observed effect concentration (LOEC) and no observed effect concentration (NPEC) can by determined statistically. For selecting the best suitable test concentrations, prior knowledge of the toxicity of the test substance to Lemna from a range-finding test may help in the selection of at least 5 test concentrations.

Where a high risk to aquatic plants is identified, further information on the effects of the test substance to other aquatic plants like Myriophyllum or Glyceria may be useful. A recovery study can also be performed for materials that show considerable toxicity by transferring fronds to pure test water and observing the recovery in growth.

As with many toxicology test methods, information on the test substance’s performance, and physical and chemical properties may be useful in establishing the test conditions, which includes the structural formula, purity, water solubility (OECD 105)*, stability in water and light, dissociation constants in water (OECD 112)*, partition coefficient (OECD 117)*, vapor pressure and biodegradability.

Testing of Lemna is generally a requirement in many countries and by regulatory or third party labeling agencies for plant protection products acting as an herbicide or a plant growth regulator. The Lemna test is also generally considered a suitable alternative to the algal test (OECD 201*) for strongly colored substances. Depending on the project requirements, usage of the test substance and its environmental fate properties, testing duration and the number of exposure and recovery periods can be modified.

For any testing performed to meet regulatory or third party labeling requirements, it is strongly recommended to review requirements prior to testing with the appropriate agency to determine which tests should be performed for label claims or regulatory acceptance.

* OECD 105: Water Solubility

*OECD 112: Dissociation Constants in Water

*OECD 117: Partition Coefficient (n-octanol/water), HPLC Method

*OECD 201: Freshwater Alga and Cyanobacteria; Growth Inhibition Test

For more information on Toxicology testing, contact our Environmental test laboratory at 847-483-9950 or info@situtest.com.

*OECD Guideline 23: Guidance Document on Aquatic Toxicity Testing of Difficult Substances and Mixtures