Pistia

Classification System: APG IV

Superregnum: Eukaryota
Regnum: Plantae
Cladus: Angiosperms
Cladus: Monocots
Ordo: Alismatales

Familia: Araceae
Subfamilia: Aroideae
Tribus: Pistieae
Genus: Pistia
Species: P. stratiotes
Name

Pistia L. (1753)

Type species: P. stratiotes L.

Synonyms

Heterotypic
Kodda-Pail Adans., Fam. Pl. 2: 25 (1763).
Zala Lour., Fl. Cochinch.: 405 (1790).
Apiospermum Klotzsch, Abh. Königl. Akad. Wiss. Berlin 1853: 351 (1853).
Limnonesis Klotzsch, Abh. Königl. Akad. Wiss. Berlin 1853: 24 (1853).

References

Linnaeus, C. 1753. Species Plantarum 2: 963.

Links

Govaerts, R. et al. 2018. Pistia in World Checklist of Selected Plant Families. The Board of Trustees of the Royal Botanic Gardens, Kew. Published on the internet. Accessed: 2018 Nov. 07. Reference page.
International Plant Names Index. 2018. Pistia. Published online. Accessed: Nov. 07 2018.
The Plant List 2013. Pistia in The Plant List Version 1.1. Published on the internet. Accessed: 2018 Nov. 07.
Tropicos.org 2018. Pistia. Missouri Botanical Garden. Published on the internet. Accessed: 2018 Nov. 07.

Pistia is a genus of aquatic plant in the arum family, Araceae. It is the sole genus in the tribe Pistieae which reflects its systematic isolation within the family.[4] The single species it comprises, Pistia stratiotes, is often called water cabbage, water lettuce, Nile cabbage, or shellflower. Its native distribution is uncertain, but probably pantropical; it was first discovered from the Nile near Lake Victoria in Africa. It is now present, either naturally or through human introduction, in nearly all tropical and subtropical fresh waterways and considered an invasive species as well as a mosquito breeding habitat. The genus name is derived from the Greek word πιστός (pistos), meaning "water," and refers to the aquatic nature of the plants.[5] The specific epithet is also derived from a Greek word, meaning "soldier," which references the sword-shaped leaves of some plants in the Stratiotes genus.[6]

Description
19th century illustration of Pistia stratiotes
Water lettuce in a home aquarium
Water lettuce spread over a Field in Kerala

Pistia is a perennial 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. They are light green, with parallel venations, and wavy margins. The surface of the leaves are covered in short, white hairs which form basket-like structures which trap air bubbles, increasing the plant's buoyancy. The spongy parenchyma with large intercellular spaces in the leaves also aid the plant in floating. The flowers are dioecious, lack petals, and are hidden in the middle of the plant amongst the leaves. Pistia 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 can also undergo asexual reproduction by propagating through stolons.
Invasion

Water lettuce is among the world's most productive freshwater aquatic plants and is considered an invasive species.[7] In waters with high nutrient content, particularly those that have been contaminated with human loading of sewage or fertilizers, water lettuce can often exhibit weedy overgrowth behavior. It may also become weedy in hydrologically altered systems such as canals and reservoirs.[8] The invasion of Pistia stratiotes in the ecosystem can lead to environmental and socio-economic ramifications to the community the community it serves.

Severe overgrowth of water lettuce can block gas exchange at the air-water interface, creating hypoxic conditions in the surface water and eliminating various native marine organisms. Large mats can also block access to sunlight by shading native submerged plants and alter immersed plant communities relying on these plants as a source of food.[9] The growth of these mats can also get tangled in boat propellers and create serious challenges for boaters or recreational fishermen.[10]

Mosquitoes of the genus Mansonia complete their life cycle only in the presence of aquatic plants such as Pistia, laying their eggs under the leaves. The emerging larvae fall into the water within 24 hours and stay attached to the Pistia root (which is rich with air sacs) with the help of a serrated siphon tube for respiration and develop into pupa. The pupa is also attached to the pistia root with the serrated piercing siphon tube. The egg to adult mosquito development is completed within 7 days.[11]

The moth Samea multiplicalis also uses Pistia as its primary host plant. Eggs are laid among leaves and stems of the host plant and larvae hatch and feed intensively as they develop.[12]
Control

Pistia can be controlled with mechanical harvesters that remove the water lettuce from the water and transport it to disposal on shore. Aquatic herbicides may also be used. Two species of insects are also being used as a biological control. Adults and larvae of the South American weevil Neohydronomous affinis feed on Pistia leaves, as do the larvae of the moth Spodoptera pectinicornis from Thailand. Both are proving to be useful tools in the management of Pistia.

In the Amazon basin, Pistia is a food source of freshwater turtles.[13]
Native range

The center of origin of Pistia stratiotes remains uncertain.[14] However, the plant is thought to be native to South America or Africa.[15] Described in Egyptian hieroglyphics and reported by Greek botanists, Dioscorides and Theophrastus, in the Nile River, the plant suggests an African origin.[16] In addition, the co-evolution of Pistia stratiotes with various insects native to Brazil and Argentina, such as the water lettuce weevil, proposes a South American origin.[17] It is a common aquatic plant in the southeastern United States, particularly in Florida. A more recent rationale for nativity in Florida explains botanist, William Bartram, found a fossil specimen in the St. Johns River basin in 1765, and dates back to the late Pleistocene (~12,000 BP) and early Holocene (~3,500 BP) period.[18] Yet, the rationale of southeastern, North American nativity remains controversial as disagreeing botanists claim the Spanish may have indirectly introduced into the plant into Florida's basins from their ship ballast.[19]
Fossil record

Pistia-like plants appear in the fossil record during the Late Cretaceous epoch in rock strata 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.[20][21] Younger fossils attributed to Pistia have described from Eocene strata in the southeastern United States,[19] 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.[22]

A specimen of Pistia from the Florida peninsula dating from at least 3,550 years Before Present, as well as a report of early Holocene Pistia pollen from a lake in Florida, have cast doubt on the claim that Pistia is not native to the southeastern United States.[19][23]
Uses
Consumption

While considered edible, Pistia 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 1867-1878.

The Hausa people of Nigeria utilized the ash of the plant as a substitute for salt due to its high concentration of potassium chloride, a mineral salt.[24] This salt substitute, also called 'zakankau,' was of high importance, especially when imported salt was unavailable.

Caution is advised when consuming Pistia, as the plant is an hyperaccumulator, and can absorb and accumulate toxic heavy metals present in its environment.[25] The presence of high concentrations of calcium oxalate crystals can induce various health concerns such as, inhibited mineral absorption and kidney stones.[26][27]

In Singapore and South-China, Pistia is commonly grown or collected as animal feed for ducks and pigs.[28] Pistia is also considered an alternative for poultry feed in, Indonesia due its high content of crude protein.[29]

Medical Treatment

There are various medical uses of Pistia throughout regions in Asia and Africa. In Nigeria, the dried leaves are prepared into a powder form and are applied to wounds and sores for disinfection.[30] A similar use is present in Indian traditional medicine, where the powdered leaf is applied to syphilitic eruptions and skin infections.[31] In Nigeria and Gambia, the leaf is infused in water to create an eyewash to treat allergic conjunctivitis.[32] The eyewash is known to have a cooling and analgesic effect. Therefore, the plant is commonly called ‘eye-pity’ in Africa.[33] In addition, the leaves of Pistia can be burned into ash, and in Indian and Nigerian traditional medicine, the ash is used in treating ringworm infections of the scalp.[31]

Medicinal Properties

Anti-Inflammatory Properties: Extractions of the leaves of P. stratiotes reduces mast infiltration and degranulation in allergic reactions and presents anti-inflammatory properties.[32][34] The ethanolic extracts have also been positively correlated with a reduction in inflammatory disorders such as, arthritis and fevers.[35]
Antifungal Properties: With the popular use of Pistia as a traditional treatment for ringworms, researchers have tested P. stratiotes methanolic extracts on dermatophyte fungi. The results of the studies depicted significant fungicidal activity on T. rubrum, T. mentagrophytes, and E. floccosum.[36][31]

Environmental Remediation

Pistia's high sorption property makes it a good candidate for biodegradable oil sorbents in marine oil spills. Particularly, the leaves of Pistia can efficiently absorb significant amounts of hydrocarbons due to its large surface area and hydrophobicity.[37]

As a hyper-accumulator, Pistia has been studied as a potential candidate for wastewater treatment plans. The roots and leaves of the plant has been found to absorb excess nutrients and heavy metals, such as zinc, chromium, and cadmium in contaminated waters.[38]

Pistia can be grown in water gardens to reduce the reduce harmful algal blooms and eutrophic conditions. The plant is able to control the growth of algae by restricting light penetration in the water column and competing for nutrients, with significant uptake of phosphorus and ammonia nitrogen.[39]
See also

Phytoremediation plants
Hyperaccumulators table – 3

References
Wikispecies has information related to Pistia stratiotes.

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"Biogeography of the Pistia clade (Araceae): Based on chloroplast and mitochondrial DNA sequences and Bayesian divergence time inference"