Lemnoideae

Lemnoideae
Close up of two different duckweeds: Spirodela polyrrhiza and Wolffia globosa. The latter are less than 2 mm long.
Scientific classification
Kingdom:	Plantae
(unranked):	Angiosperms
(unranked):	Monocots
Order:	Alismatales
Family:	Araceae
Subfamily:	Lemnoideae
Genus
Spirodela Landoltia Lemna Wolffiella Wolffia

Duckweeds, or water lentils, are aquatic plants which float on or just beneath the surface of still or slow-moving fresh water bodies. They arose from within the arum or aroid family, (Araceae), and therefore, often are classified as the subfamily Lemnoideae within the Araceae. Classifications created prior to the approximate end of the twentieth century tend to classify them as a separate family, Lemnaceae.

These plants are very simple, lacking an obvious stem or leaves. They consist of a small 'thalloid' or plate-like structure that floats on or just under the water surface, with or without simple rootlets. The plants are highly reduced from their earlier relatives in Araceae.

Reproduction is mostly by asexual budding, but occasionally three tiny 'flowers' consisting of two stamens and a pistil are produced and sexual reproduction occurs. Some view this 'flower' as a pseudanthium, or reduced inflorescence, with three flowers that are distinctly either female or male and which are derived from the spadix in Araceae. The flower of Wolffia is the smallest known flower in the world, measuring merely 0.3 mm long. The fruit produced through this occasional sexual reproduction is a utricle, and a seed is produced in a sac containing air that facilitates flotation.

Duckweed in various environments around the world

Duckweed is an important high-protein food source for waterfowl and also is eaten by humans in some parts of Southeast Asia (as khai-nam). As it contains more protein than soybeans, it is sometimes cited as an significant potential food source.[1] [2]

Some duckweeds are introduced into freshwater aquariums and ponds where they may spread rapidly. This introduction may be deliberate or unintended and once established in a large pond, may be difficult to eradicate. Occurring naturally by being carried on the feathers, shells, and coats of native species, the plant is introduced readily by birds, turtles, reptiles, and aquatic mammals visiting multiple ponds, rivers, and lakes. In water bodies with constant currents or overflow, the plants are carried down the water channels and do not proliferate greatly. In some locations a cyclical pattern driven by weather patterns exists in which the plants proliferate greatly during low water flow periods, yet are carried away as rainy periods ensue.

The tiny plants provide cover for fry of many aquatic species. The plants are used as shelter by pond water species such as bullfrogs and bluegills. They also provide shade and, although frequently confused with them, can reduce certain light-generated growths of photoautotrophic algae.

The plants can provide nitrate removal, if cropped, and the duckweeds are important in the process of bioremediation because they grow rapidly, absorbing excess mineral nutrients, particularly nitrogen and phosphates. For these reasons they are touted as water purifiers of untapped value.

The Swiss Department of Water and Sanitation in Developing Countries, SANDEC, associated with the Swiss Federal Institute for Environmental Science and Technology, asserts that as well as the food and agricultural values, duckweed also may be used for waste water treatment to capture toxins and for odor control, and, that if a mat of duckweed is maintained during harvesting for removal of the toxins captured thereby, it prevents the development of algae and controls the breeding of mosquitoes. [3] The same publication provides an extensive list of references for many duckweed-related topics.

These plants also may play a role in conservation of water because a cover of duckweed will reduce evaporation of water when compared to the rate of a similar size water body with a clear surface.

Taxonomy

The duckweeds long have been a taxonomic mystery, and usually have been considered to be their own family, Lemnaceae. They primarily reproduce asexually. Flowers, if present at all, are small. Roots are either very much reduced, or absent entirely. They were suspected of being related to the Araceae as long ago as 1876, but until the advent of molecular phylogeny it was difficult to test this hypothesis. Starting in 1995 studies began to confirm their placement in the Araceae and since then, most systematists consider them to be part of that family.

Their position within their family has been slightly less clear, but several twenty-first century studies place them in the position shown below. They are not closely related to Pistia, however, which also is an aquatic plant in the family Araceae.

	Gymnostachydoideae Orontioideae (skunk cabbages and golden club)
	Lemnoideae (duckweeds) most of the family Araceae

The genera of duckweeds are: Spirodela, Landoltia, Lemna, Wolffiella, and Wolffia.

Research

In July 2008 the U.S. Department of Energy (DOE) Joint Genome Institute announced that the Community Sequencing Program would fund the sequencing the genome of the giant duckweed, Spirodela polyrhiza. This was a priority project for DOE in 2009. The research is intended to facilitate new biomass and bio-energy programs.

Duckweed is being studied by researchers around the world as a possible source of clean energy. In the United States, in addition to being the subject of study by the DOE, both Rutgers University and North Carolina State University have ongoing projects to determine if duckweed might be a source of cost-effective, clean, renewable energy. Duckweed is a good candidate as a biofuel because as a biomass it grows rapidly, has 5 to 6 times as much starch as corn, and does not contribute to global warming. Duckweed is considered a carbon neutral energy source, because unlike most fuels, it actually removes carbon dioxide from the atmosphere.

Duckweed also functions as a bioremediator by effectively filtering contaminants such as bacteria, nitrogen, phosphates, and other nutrients from naturally occurring bodies of water, constructed wetlands and waste water. One study in Australia surrounding aquaculture suggests that although duckweed is initially effective as a nutrient filter, over time some nutrient build-up returns.

• 
  Common duckweed in Galicia, Spain
• 
  Lemna minor
• 
  Lemna trisulca
• 
  Lemna gibba.

Turning the canals of the Poitevin Marsh (Marais Poitevin, France) into the "Green Venice":

• 
  Spirodela polyrhiza
• 
  Canal green with duckweed
• 
  Duckweed-covered water edged with several bald cypress trees

See also

• Lemna
• Spirodela
• Landoltia punctata
• Wolffia

External links

• The Charms of Duckweed by John W. Cross.
• Wayne Armstrong's treatment of the Lemnaceae and allies
• Lemnaceae in Watson, L. and M.J. Dallwitz (1992 onwards). The families of flowering plants: descriptions, illustrations, identification, information retrieval. Version: 3 May 2006. http://delta-intkey.com
• The Duckweed Genome Project