Pistia is a genus of in the arum family, Araceae. It is the sole genus in the tribe Pistieae which reflects its systematic isolation within the family.
Description
Pistia stratiotes is a
Perennial plant monocotyledon with thick, soft leaves that form a rosette.
It floats on the surface of the water, its roots hanging submersed beneath floating leaves. The leaves can measure 2 – 15 cm long and are light green, with parallel venations and wavy margins.
The surface of the leaves is covered in short, white hairs which form basket-like structures that can trap air bubbles and increase the plant's buoyancy. The spongy parenchyma with large intercellular spaces in the leaves also aids the plant in floating.
The
are
plant sexuality, lack petals, and are hidden in the middle of the plant amongst the leaves.
Pistia stratiotes has a spadix inflorescence, containing one pistillate flower with one ovary and 2–8 staminate flowers with two stamens.
The pistillate and carpellate flowers are separated by folds in the spathe, where the male flowers are located above the female flowers.
Oval, green berries with ovoid seeds form after successful fertilization. The plant undergoes asexual reproduction by propagating through stolons, yet evidence of sexual reproduction has also been observed in the ponds of Southern Brazil.
Pistia stratiotes are found in slow-moving rivers, lakes, and ponds. The species displays optimal growth in the temperature range of 22–30 °C, but can endure extreme temperatures up to 35 °C.
Invasion
Water lettuce is among the world's most productive freshwater aquatic plants and is considered an
invasive species.
The species can be introduced to new areas by water dispersal, fragmentation, and hitchhiking on marine transportation or fishing equipment.
The invasion of
Pistia stratiotes in the ecosystem can lead to environmental and socio-economic ramifications to the community it serves. In waters with high nutrient content, particularly those that have been contaminated with human loading of sewage or fertilizers, water lettuce can exhibit weedy overgrowth. It may also become invasive in hydrologically altered systems such as flood control canals and reservoirs.
[Kasulo, V. 2000. The impact of invasive species in African lakes. In: The economics of biological invasions (eds. C. Perrings, M. Williamson and S. Dalmozzone). pp. 183–207. Cheltenham, UK: Edward Elgar.] The severe overgrowth of water lettuce can block gas exchange in the surface water, creating hypoxic conditions and eliminating or disrupting various native marine organisms.
Blocking access to sunlight, large mats of water lettuce can shade native submerged plants and alter communities relying on these native plants as a source of food.
The growth of these mats can also get tangled in boat propellers and create challenges for boaters or recreational fishermen.
Pistia stratiotes feature in the life cycles of certain insect vectors for malaria and filariasis.
Control
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Chemical control: Herbicides have been effective in controlling Pistia stratiotes: diquat, glyphosate, terbutryn, 2,4-D, among many others.
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Physical control: Pistia stratiotes can be controlled with mechanical harvesters that remove the water lettuce from the infested waters and transport it to disposal onshore.
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Biological control: Two species of are also being used as a biological control. Adults and larvae of the weevil Neohydronomous affinis feed on Pistia leaves, as do the larvae of the moth Spodoptera pectinicornis from Thailand.
The species is set to be banned in the EU from August of 2024 to prevent spreading.
Range
The center of origin of
Pistia stratiotes has long been a source of debate.
Nativity to northern Africa is indicated by Egyptian hieroglyphics and reports of plants meeting the description of
Pistia by Greek botanists, Dioscorides and
Theophrastus in the
Nile River.
In addition, the co-evolution of
Pistia stratiotes with various insects native to Brazil and Argentina, such as the
Neohydronomus,
indicates a long-term native tenure in South America.
Fossil specimens dating back to the late
Pleistocene (~12,000
Before Present) and early
Holocene (~3,500 BP) period are reported from Florida, indicating a native presence in the southeastern United States.
Recent genetic evidence also suggests that
Pistia is not actually a monotypic genus, as had been long assumed.
Instead,
Pistia appears to be composed of at least three genetically distinct, but morphologically and ecologically similar, species at a global scale.
Temperate occurrences
Though
Pistia stratiotes is intolerant of cold temperatures, it has been recorded growing at least temporarily in temperate areas of North America and Europe.
In the United States north of the Gulf of Mexico it has been found growing in Colorado, Connecticut, Delaware, Illinois, Kansas, Maryland,
Michigan, Minnesota, Missouri, New York, North Carolina, Ohio, Rhode Island, South Carolina,
and Wisconsin. One of these occurrences, in Idaho, survives in an area of a river fed by a hot spring. The rest are thought either to be completely eradicated by cold weather or possibly to survive by seed production.
Fossil record
Pistia-like plants appear in the
fossil record during the
Late Cretaceous epoch in rock
stratum from the western interior of North America. They were first described as †
Pistia corrugata by
Leo Lesquereux in 1876 based on specimens from the
Almond Formation of
Wyoming (late
Campanian age). However, based on more complete specimens from the Campanian Dinosaur Park Formation of southern
Alberta,
Canada, and other areas, they were redescribed as a separate
genus, †
Cobbania, primarily due to differences in leaf morphology.
Younger fossils attributed to
Pistia stratiotes have described from
Eocene strata in the southeastern United States,
and 350
fossil seeds of †
Pistia sibirica have been described from middle Miocene strata of the Fasterholt area near
Silkeborg in
Central Jutland,
Denmark. Fossils of this species have also been described from the
Oligocene and Miocene of
Western Siberia and from the Miocene of
Germany.
A specimen of Pistia from the Florida peninsula dating from at least 3,550 years Before Present and a report of Holocene Pistia fossils from a lake in south central Florida are consistent with genetic evidence indicating that some varieties of Pistia stratiotes are native to the southeastern United States.
Uses
Consumption
While considered edible,
Pistia stratiotes is not palatable as it is rich in
calcium oxalate crystals that are bitter in taste. Nevertheless, there are records of the plant being utilized as famine food in India during the Great Famine of 1876–1878.
The Hausa people people of Nigeria used the ash of the plant as a substitute for salt due to its high concentration of potassium chloride, a mineral salt.
Caution is advised when consuming Pistia stratiotes, as the plant is a hyperaccumulator, and can absorb and accumulate toxic heavy metals present in its environment.
In Singapore and Southern China, Pistia stratiotes is commonly grown or collected as animal feed for ducks and pigs.
Medical treatment
There are various medical uses of
Pistia stratiotes throughout regions in Asia and Africa. In Nigeria, the dried leaves are prepared into a powder form and are applied to wounds and sores
Disinfectant.
A similar use is present in
Ayurveda, where the powdered leaf is applied to
syphilitic eruptions and
skin infections.
In
Nigeria and
Gambia, the leaf is infused in water to create an
eyewash to treat allergic conjunctivitis.
The eyewash is known to have a cooling and
analgesic effect. Therefore, the plant is commonly called 'eye-pity' in Africa.
In addition, the leaves of
Pistia stratiotes can be burned into
ash, and in Indian and Nigerian traditional medicine, the ash is used in treating
Dermatophytosis infections of the
scalp.
Medicinal properties
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Anti-inflammatory properties: Extractions of the leaves of P. stratiotes reduces mast infiltration and degranulation in allergic reactions and presents anti-inflammatory properties.
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Antifungal properties: With the popular use of Pistia stratiotes as a traditional treatment for ringworms, researchers have tested P. stratiotes methanolic extracts on dermatophyte fungi. The results of the studies depicted significant Fungicide activity on T. rubrum, T. mentagrophytes, and E. floccosum.
Environmental remediation
The high sorption property of water lettuce makes it a great candidate for biodegradable oil sorbents in marine oil spills. Particularly, the leaves of
Pistia stratiotes can efficiently absorb significant amounts of hydrocarbons due to its large surface area and hydrophobicity.
As a hyper-accumulator, Pistia stratiotes has been studied as a potential candidate for wastewater treatment plants. The roots and leaves of the plant have been found to absorb excess nutrients and heavy metals, such as zinc, chromium, and cadmium in contaminated waters.
Pistia stratiotes can be grown in water gardens to reduce harmful algal blooms and eutrophic conditions.
See also
External links
