The duckweed growth inhibition test is standardised in a number of states like France (AFNOR 1996), Sweden (SIS 1995, USA (ASTM 1991, EPA 1996, APHA 1992) and Canada (Environment Canada 1998). On the level of OECD (chemicals, plant protection) the final proposal for guideline 221 “Lemna sp. Growth Inhibition Test” is available (OECD 2000a). 37 participants successfully finished an international ring test in December 1998. The final version will be available soon.
The process of ISO standardization (waste water, soil, waste) started in summer 2000 with a complete draft developed and practically checked in Germany. The ISO standard ISO/CD 20079 is now available as “Water quality - Determination of the toxic effect of water constituents and waste water to duckweed (Lemna minor) - Duckweed growth inhibition test” (ISO 2001). Next decisive steps within the fixed schedule will be made as the beginning of 2002 in Philadelphia. Decision on a ring test will be made there. Comments on this document are invited and should be addressed together with detailed alternative wordings in English to the ISO´s member body in your country (http://www.iso.ch/iso/en/aboutiso/isomembers/MemberCountryList.MemberCountryList). The separations of standards for testing chemicals and environmental samples
partially vanished. According to the title of the ISO standard single chemicals may be tested as “water constituents”. Other water constituents may be elutriates of soil and waste. For soil elutriates the duckweed test was validated successfully. In the appendix of the ISO standard CD 15799 “Soil quality - Guidance on the ecotoxicological characterizations of soil and soil Materials” (ISO 1999) the listed AFNOR standard will be substituted by the ISO standard to give the formal basis to use duckweed with soil elutriates.
The use of the OECD Duckweed guideline for waste water has some practical concerns using the test-medium. It was developed for the test of substances free of additional nutrients. Additional nutrients of the environmental sample cause additional growth relative to the controls in some cases and reduce sensitivity of the test.
The Duckweed test is mentioned directly in the revised draft guidance document on aquatic toxicity testing of difficult substances and mixtures (OECD 2000b) to test suspensions or turbid and dyed test solutions.
All standards are in full compliance with measurements made by image-analysis as image-analysis include the observation parameter frond number. OECD and the ISO
standard include total frond area (alternatively to dry weight (OECD, ISO) or chlorophyll (ISO)) as a second observation parameter too. Most of the time consuming additional observations to be made in all standards (reduction of frond size, chlorosis, necrosis etc.) are quantified reproducibly without any additional workload by image analysis. Even special requirements can be achieved by image-analysis. For example the ASTM standard points out that only fronds with less than 50 percent chlorosis should be counted as living.

Standards and Guidelines

AFNOR 1996
Determination of the inhibitory effect on the growth of Lemna minor XP T 90-337

APHA 1992
Toxicity Part 8000 8-32-8-39 in: Standard methods for the examination of water and wastewater 18th ed., APHA, AWWA, WEF, Washington}
ASTM,1991

Conducting static toxicity tests with Lemna gibba. Guide E 1415-91. Annual book of ASTM standards. Section 11 Water and environmental technology. Vol. 11.04. ASTM, Philadelphia, U.S.

Environment Canada 1998
Biological test method: test for measuring the inhibition of growth using the freshwater
macrophyte lemna minor Report EPS 1/RM/37

EPA 1996
Ecological effects test Guidelines OPPTS 850.4400 Aquatic planttoxicity test using lemna ssp., Tiers I and II , EPA 712-C-96-156}

ISO 1999
ISO/CD 15799 1999 Soil quality - Guidance on the ecotoxicological characterizations of soil and soil materials

ISO 2001
“Water quality - Determination of the toxic effect of water constituents and waste water to duckweed (Lemna minor) - Duckweed growth inhibition test” ISO/CD 20079 August 2001

OECD 2000a
Proposal for Guideline 221 Lemna sp. Growth Inhibition Test OECD, October 2000

OECD 2000b
Revised draft guidance document on aquatic toxicity testing of difficult substances and mixtures, OECD, January 2000

SIS 1995
Swedish Institute of Standards, Water quality - determination of growth inhibition (7-d) Lemna minor , duckweed SS 02 82 13