| Lemna | |
|---|---|
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| Common duckweed (Lemna minor) | |
Scientific classification | |
| Kingdom: | Plantae |
| Clade: | Tracheophytes |
| Clade: | Angiosperms |
| Clade: | Monocots |
| Order: | Alismatales |
| Family: | Araceae |
| Subfamily: | Lemnoideae |
| Genus: | Lemna L. |
| Synonyms[1][2] | |
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Lemna is a genus of free-floatingaquatic plants referred to by the common name "duckweed". They are morphologically divergent members of thearum familyAraceae. These rapidly growing plants have found uses as amodel system for studies incommunity ecology, basic plant biology,ecotoxicology, and production ofbiopharmaceuticals, and as a source of animal feeds foragriculture andaquaculture. Currently, 14 species ofLemna are recognised.[3]
These duckweeds were previously placed in a separate flowering plant family, theLemnaceae, but they are now considered to be members of theAraceae.[4]
Lemna species grow as simple free-floatingthalli on or just beneath the water surface. Most are small, not exceeding 5 mm in length, exceptLemna trisulca, which is elongated and has a branched structure.Lemna thalli have a single root, which distinguishes this genus from the relatedgeneraWolffia (lacks roots),Spirodela andLandoltia (have multiple roots).
The plants grow mainly byvegetative reproduction: two daughter plants bud off from the adult plant. This form of growth allows very rapid colonisation of new water. Duckweeds areflowering plants, and nearly all of them are known to reproduce sexually,flowering and producingseed under appropriate conditions. Certain duckweeds (such asL. gibba) arelong-day plants, while others (such asL. minor) areshort-day plants.
WhenLemna invades a waterway, it can be removed mechanically, by the addition of herbivorous fish (e.g.grass carp), or, inadvisedly, treated with aherbicide.
The rapid growth of duckweeds finds application inbioremediation of polluted waters, in municipal wastewater treatment[5] and as test organisms for environmental studies.[6] It is also being used as anexpression system for economical production of complexbiopharmaceuticals.
Duckweed meal (dried duckweed) is a good cattle feed.[citation needed] It contains 25–45% protein (depending on the growth conditions), 4.4% fat, and 8–10% fibre, measured by dry weight.[citation needed]
Organisation for Economic Co-operation and Development[7] andU.S. Environmental Protection Agency (US EPA)[8] guidelines describe toxicity testing usingL. gibba orL. minor as test organisms. Both of these species have been studied extensively for use inphytotoxicity tests. Genetic variability in responses to toxicants can occur inLemna, and data are insufficient 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 (semistatic 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.
Lemna has been transformed bymolecular biologists to express proteins ofpharmaceutical interest. Expression constructs were engineered to causeLemna to secrete the transformed proteins into the growth medium at high yield. Since theLemna is grown on a simple medium, this substantially reduces the burden ofprotein purification in preparing such proteins for medical use, promising substantial reductions in manufacturing costs.[9][10] In addition, the hostLemna can be engineered to cause secretion of proteins with human patterns ofglycosylation, an improvement over conventional plant gene-expression systems.[11] Several such products are being developed, includingmonoclonal antibodies.
High yields of duckweed with a high protein content for use inhuman nutrition,animal andfish feed can be achieved by careful control of growth conditions. Although duckweed can tolerate temperatures ranging from 6 °C (43 °F) to 33 °C (91 °F), the optimal growth range is 20 °C (68 °F) to 28 °C (82 °F). The acceptablepH range is 5 to 9, but better growth is obtained in the pH range of 6.5 to 7.5. A minimum water depth of 1 foot (30 cm) is desirable to prevent excessive temperature swings. High nitrogen levels, for example 20 mM urea, have provided a protein content in the range of 45% by dry weight. The water may typically contain 60 mg/L of solublenitrogen and 1 mg/L ofphosphorus. Fertiliser is required on a daily basis for optimal growth.
Duckweed can be farmed organically, with nutrients being supplied from a variety of sources, for example human urine,[12] cattle manure, pig waste,biogas plant slurry, or other organic matter in slurry form. Because of the rapid growth of duckweed, daily harvesting is necessary to achieve optimal yields. Harvesting is done such that less than 1 kg/m2 of duckweed remains. Under optimal conditions, a duckweed farm can produce 10 to 30 tons of dried duckweed per hectare per year.[13]
Infrageneric classification following Les et al. 2002.[14]
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