Lemna as a Standard Model System for Environmental Testing
|Lemna growth assays are used in testing the
of pesticides and other environmental chemicals to higher
tests are highly standardized and specific
procedures have been laid out by regulatory agencies
testing. Standardization allows for valid
laboratories and tests on different chemicals.
to increase their efficiency and precision.
A recent EPA Scientific Advisory Panel Briefing recommends expanding phytotoxicity testing to a greater diversity of organisms and physical conditions. However, the document underscores the importance now attached to Lemna testing. Standardized duckweed bioassays expose the plants to chemicals dissolved or dispersed in their liquid media. In the future, it may be desired to also expose duckweeds to airborn (foliar) chemicals and to substances bound to sediments at the bottom of the culture dish. Researchers are currently studying these variations.
Video Image (right) courtesy of LemnaTec.
USEPA Scientific Advisory Panel Briefing, June 27-29, 2001
Excepts from http://www.epa.gov/oscpmont/sap/meetings/2001/june/sap14.pdf
Michael Davy, Richard Petrie, and Jerry Smrchek, EPA Ted
and Derek Francois, PMRA
Joint Presentation By: Health Canada - Pest Management Regulatory Agency and USEPA - Office of Prevention, Pesticides, and Toxic Substances
2.5.5 Floating Vascular Species
Of the floating aquatic plant species, duckweed (i.e., Lemna sp.) is perhaps the most commonly used in toxicity testing. Actually, “duckweed” can refer to both Lemna sp. and Spirodela sp. (greater duckweed), although, it is usually associated with Lemna sp. (Newmaster et al. 1997). Duckweeds are floating non-rooted aquatic plants with a reduced root system and lack stems and true leaves. It has been speculated that the roots serve as anchors to keep the fronds right side up and to form the tangled masses which are of some importance in dispersal and protection from water movement. Mats of duckweed are habitat for small invertebrates, for example, the ephydrid fly (Lemnaphila scotlandae) and the rhyncophorous beetle (Tansyphyrus lemnae). Other invertebrates such as hydras, flatworms and snails are common just beneath the duckweed mat (Hillman 1961). Waterfowl and marsh birds such as coots, black ducks, mallards, teals, wood ducks, buffleheads and rails eat duckweed in large quantities. Duckweed also provides food and shelter for fish (Newmaster et al. 1997).
Most toxicological studies with aquatic vascular plants expose the test organism to the toxicant through the exposure medium. Testing procedures can follow the ASTM (1998b), Environment Canada (1999), APHA (1998b) or the EPA (1996b) test method guidelines. The methodology recommended for use in pesticide registration is a static test with Lemna sp., similar to the flask method for algae. Briefly, the test is initiated by the introduction of Lemna fronds into test vessels. Colonies are inspected for changes in frond number and appearance at initial stages of exposure and at the end of exposure, the total number of living and/or dead fronds are enumerated.
Testing with other floating vascular plants is not well documented. Some of the obvious choices for other floating species are pond lily (Nuphar sp.), water lily (Nymphaea sp.) and greater duckweed (Spirodela sp.). There are however, no known test protocols for these species. There are currently no required data for the exposure of aquatic vascular plants to pesticide spray drift. Lemna species can easily be used for the examination of pesticide exposure through water, for the study of pesticide drift and research into the effects of surface films at the air-water interface (Swanson 1989, Taraldesen and Norberg-King 1990). Lockhart et al. (1989) [ summary ] have shown that the sensitivity of Lemna to glyphosate increased several-fold with a foliar exposure compared to the conventional exposure through the growth medium. In this method, glyphosate was first sprayed onto Petri dishes containing Lemna fronds in growth medium. The sprayer consisted of a spray nozzle which moved along a rigid track. The sprayer was calibrated using dyes to allow for selection of the desired application rate. After spraying, fronds were allowed to stand for 6-24 hours before being removed t o flasks containing clean culture medium. Subsequently, fronds were counted several times over a 2-week period.
The difficulty with recommending this type of test is that there is only one published report where exposure was examined through aerial deposit (Lockhart et al. 1989). In the case of herbicides, it may be useful to consider the mechanism of action before requiring this study. For example, if a herbicide is intended as a contact toxicant, then it would be useful to explore a foliar test. By contrast, it may not be necessary to examine systemic herbicides by this type of exposure provided that the mechanism of action is well characterized.
2.5.6 Submersed Vascular Species
Duckweed is the only aquatic vascular plant that is currently required for pesticide and chemical testing. Development of submersed vascular plant tests is recommended as no single species can be representative of the majority of vascular plant species even if Lemna has a global distribution. Lemna is somewhat unique in its floating, unrooted growth habit and exposure can be both aerial and aquatic. Furthermore, Lemna is a monocotyledon species. As there may be differences in herbicide sensitivity between monocot and dicot species, it may be beneficial to include dicot species in the testing scheme.
The references cited above can be found in the PDF file, http://www.epa.gov/scipoly/sap/2001/june/sap14.pdf
"A Sediment Toxicity Method Using Lemna Minor, Duckweed" a poster presentation (Society of Environmental Toxicology and Analytical Chemistry meeting, Nashville, Tennessee, November, 2000) by Lazorchak, J.M., Williams, D.E., Suszcynsky-Meister, E.M. and Smith, M.E., U.S. EPA and SBI Environmental, Cincinnati, OH
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Revised: August 5, 2006