name = Duckweed

image_width = 250px
image_caption = Common Duckweed ("Lemna minor")
regnum = Plantae
divisio = Magnoliophyta
classis = Liliopsida
ordo = Alismatales
familia = Araceae
subfamilia = Lemnaoideae
genus = "Lemna"
genus_authority = L.
subdivision_ranks = Species
subdivision = About 13, including:
*"Lemna aequinoctialis"
*"Lemna gibba" : Gibbous Duckweed
*"Lemna minuta"
*"Lemna minor" : Common Duckweed
*"Lemna trisulca" : Ivy Duckweed

"Lemna" is a genus of free-floating aquatic plants from the duckweed family. These rapidly-growing plants have found uses as a model system for studies in basic plant biology, in ecotoxicology, in production of biopharmaceuticals, and as a source of animal feeds for agriculture and aquaculture.

Taxonomy and growth habits

The duckweeds have been classified as a separate family, the Lemnaceae, but some researchers (the AGP II) consider the duckweeds members of the Araceae.

"Lemna" species grow as simple free-floating thalli on or just beneath the water surface. Most are small, not exceeding 5 mm in length, except "Lemna trisulca" which is elongated and has a branched structure. "Lemna" thalli have a single root, which distinguishes them from related genera "Spirodela" and "Landoltia"

The plants grow mainly by vegetative reproduction: two daughter plants bud off from the adult plant. This form of growth allows very rapid colonisation of new water. Duckweeds are flowering plants, and nearly all of them are known to reproduce sexually, flowering and producing seed under appropriate conditions. Certain duckweeds (e.g. "L. gibba") are long day plants, while others (e.g. "L. minor") are short day plants.

When "Lemna" invades a waterway, it can be removed mechanically, by the addition of herbivorous fish (e.g. grass carp) or treated with a herbicide.

The rapid growth of duckweeds finds application in bioremediation of polluted waters and as test organisms for environmental studies. It is also being used as an expression system for economical production of complex biopharmaceuticals.

Duckweed meal (dried duckweed) is a good cattle feed. It contains 25-45% proteins (depending on the growth conditions), 4.4% fat, and 8-10% fibre, measured by dry weight.

Assessing the toxicity of chemicals with "Lemna"

OECD [ [http://oberon.sourceoecd.org/vl=785298/cl=22/nw=1/rpsv/ij/oecdjournals/1607310x/v1n2/s22/p1 SourceOECD: issues ] ] and US EPA [http://www.epa.gov/opptsfrs/publications/OPPTS_Harmonized/850_Ecological_Effects_Test_Guidelines/Drafts/850-4400.pdf] guidelines describe toxicity testing using "Lemna gibba" or "Lemna minor" as test organisms. Both of these species have been studied extensively for use in phytotoxicity tests. Genetic variability in responses to toxicants can occur in Lemna, and there are insufficient data to recommend a specific clone for testing. The US EPA test uses aseptic technique. The OECD test is not conducted axenically, but steps are taken at stages during the test procedure to keep contamination by other organisms to a minimum. Depending on the objectives of the test and the regulatory requirements, testing may be performed with renewal (semi-static and flow-through) or without renewal (static) of the test solution. Renewal is useful for substances that are rapidly lost from solution as a result of volatilisation, photodegradation, precipitation or biodegradation.

Production of biopharmaceuticals

"Lemna" has been transformed by molecular biologists to express proteins of pharmaceutical interest. Expression constructs were engineered to cause "Lemna" to secrete the transformed proteins into the growth medium at high yield. Since the "Lemna" is grown on a simple medium, this substantially reduces the burden of protein purification in preparing such proteins for medical use, promising substantial reductions in manufacturing costs. [cite web| title=Biolex Corporate Website| url= http://www.biolex.com] [cite journal|last=Gasdaska|first= JR| coauthors= Spencer D and Dickey L| title=Advantages of Therapeutic Protein Production in the Aquatic Plant Lemna| journal=BioProcessing Journal| month= Mar/Apr| year= 2003| pages= 49–56] In addition, the host "Lemna" can be engineered to cause secretion of proteins with human patterns of glycosylation, an improvement over conventional plant gene-expression systems. [cite journal| last=Cox|first= KM|coauthors= Sterling JD, Regan JT, Gasdaska JR, Frantz KK, Peele CG, Black A, Passmore D, Moldovan-Loomis C, Srinivasan M, Cuison S, Cardarelli PM and Dickey LF| title=Glycan Optimization of a Human Monoclonal Antibody in the Aquatic Plant Lemna Minor| journal= Nature Biotechnology|month= December|year= 2006|volume=24 |issue= 12|pages= 1591–1597| doi=10.1038/nbt1260] Several such products are being developed, including monoclonal antibodies.

cientific farming

High amounts of duck weed with a high protein content can be achieved by careful control of the growing conditions. Even though duckweed can tolerate the temperature of 6 to 33 °C,the appropriate temperature range for a good harvest is 20 to 28 °C. The acceptable pH range is 5 to 9, although better growth is possible in the pH range of 6.5 to 7.5. A minimum water depth of 1 ft is required. A sample of 20m.M Urea provides a protein content of 45%. Water could contain 60 mg/L of soluble nitrogen and 1 mg/L of phosphorus. Fertiliser is required on a daily basis.

Duckweed can be farmed organically, with nutrients being supplied from for example cattle dung, pig waste, biogas plant slurry, or any other organic matter in slurry form. Because of the rapid growth, daily harvesting is necessary to achieve optimal yields. Harvesting is done such that less than a kilogram per square metre of duckweed remains. A duckweed farm can produce 10 to 30 tons of dried duckweed per hectare per year. [cite journal| first=R A | last=Leng| coauthors= J H Stambolie and R Bell| url=http://www.fao.org/ag/AGA/AGAP/FRG/lrrd/lrrd7/1/3.htm| title= Duckweed - a potential high-protein feed resource for domestic animals and fish| journal=Livestock Research for Rural Development| volume=7| issue= 1| month= October| year= 1995| format= dead link|date=June 2008 – [http://scholar.google.co.uk/scholar?hl=en&lr=&q=author%3ALeng+intitle%3ADuckweed+-+a+potential+high-protein+feed+resource+for+domestic+animals+and+fish&as_publication=Livestock+Research+for+Rural+Development&as_ylo=1995&as_yhi=1995&btnG=Search Scholar search] ]


General readings

*Cross, J.W. (2006). [http://www.mobot.org/jwcross/duckweed/ The Charms of Duckweed] .
*Landolt, E. (1986) Biosystematic investigations in the family of duckweeds (Lemnaceae). Vol. 2. The family of Lemnaceae - A monographic study. Part 1 of the monograph: Morphology; karyology; ecology; geographic distribution; systematic position; nomenclature; descriptions. Veröff. Geobot. Inst., Stiftung Rübel, ETH, Zurich.
*Lemna Ecotox testing [http://www.lemnatec.com/wasserlinsen_faq_en.htm Duckweed growth inhibition tests and standardisation]

External links

* [http://www.lemnatec.com/wasserlinsen_faq_en.htm Lemna Ecotox testing Duckweed growth inhibition tests and standardisation]
* [http://titania.sourceoecd.org/vl=5981511/cl=43/nw=1/rpsv/ij/oecdjournals/1607310x/v1n2/s22/p1 OECD Guideline for Lemna Test]
* [http://plants.usda.gov/java/profile?symbol=LEMNA USDA Plants Profile: North American Species]
* [http://ucjeps.berkeley.edu/cgi-bin/get_JM_treatment.pl?8328,8329 Jepson Manual Treatment: "Lemna"]

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