Division 1.7.2 Test organism

The organism used for this test is either Lemna gibba or Lemna minor. Short descriptions of duckweed species that have been used for toxicity testing are given in Appendix 1. Plant material may be obtained from a culture collection, another laboratory or from the field. If collected from the field, plants should be maintained in culture in the same medium as used for testing for a minimum of eight weeks prior to use. Field sites used for collecting starting cultures must be free of obvious sources of contamination. If obtained from another laboratory or from a culture collection they should be similarly maintained for a minimum of three weeks. The source of plant material and the species and clone (if known) used for testing should always be reported.

Monocultures, that are visibly free from contamination by other organisms such as algae and protozoa, should be used. Healthy plants of L. minor will consist of colonies comprising between two and five fronds, whilst healthy colonies of L. gibba may contain up to seven fronds.

The quality and uniformity of the plants used for the test will have a significant influence on the outcome of the test and should therefore be selected with care. Young, rapidly growing plants without visible lesions or discoloration (chlorosis) should be used. Good quality cultures are indicated by a high incidence of colonies comprising at least two fronds. A large number of single fronds is indicative of environmental stress, e.g. nutrient limitation, and plant material from such cultures should not be used for testing.