Pistia is a genus ofaquatic plants in thearum family,Araceae. It is the sole genus in the tribePistieae which reflects its systematic isolation within the family.[5] The single species it comprises,Pistia stratiotes, is often calledwater cabbage,water lettuce,Nile cabbage, orshellflower. Its native distribution is uncertain but is probablypantropical; it was first scientifically described from plants found on theNile nearLake Victoria inAfrica.[6] It is now present, either naturally or through human introduction, in nearly all tropical and subtropical fresh waterways and is considered aninvasive species as well as a mosquito breeding habitat. The specific epithet is derived from a Greek word, στρατιώτης, meaning "soldier", which references the sword-shaped leaves of some plants in theStratiotes genus.[7]
Pistia stratiotes is aperennialmonocotyledon with thick, soft leaves that form a rosette.[8] 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.[8] 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.[9] Theflowers aredioecious, 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.[10] The pistillate and carpellate flowers are separated by folds in the spathe, where the male flowers are located above the female flowers.[5] Oval, green berries with ovoid seeds form after successful fertilization. The plant undergoesasexual reproduction by propagating through stolons, yet evidence of sexual reproduction has also been observed in the ponds of Southern Brazil.[11]
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.[12] As a result,Pistiastratiotes do not grow in colder temperatures, beyond the tropics of Cancer and Capricorn. The species also require slightly acidic water in the pH range of 6.5–7.2 for optimal growth.[12]
Water lettuce is among the world's most productive freshwater aquatic plants and is considered aninvasive species.[13] The species can be introduced to new areas by water dispersal, fragmentation, and hitchhiking on marine transportation or fishing equipment.[14] The invasion ofPistia 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.[15] 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.[16] 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.[17] The growth of these mats can also get tangled in boat propellers and create challenges for boaters or recreational fishermen.[18]
Pistia stratiotes feature in the life cycles of certain insect vectors formalaria andfilariasis.[19] Mosquitoes of the genusMansonia can lay their eggs under the leaves of aquatic plants, such asPistia stratiotes. Twenty-four hours later, the emerging larvae attach to the plant's roots using its siphon tube for respiration. Within a week, larvae can develop into adult mosquitos, makingPistia stratiotes a potential breeding ground for vectors of infectious disease.[20] The mothSamea multiplicalis also usesPistia stratiotes 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.[21]
Chemical control: Herbicides have been effective in controllingPistia stratiotes: diquat, glyphosate, terbutryn, 2,4-D, among many others.[22] Yet, the use of herbicides must be critically assessed to prevent negative environmental impacts and possible toxic effects on marine life and human health.[23]
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.[24] Larger infestations can be removed with the aid of hydraulic excavators and tractors. To prevent the re-growth ofPistia stratiotes colonies, a long-term maintenance program should be implemented.
The center of origin ofPistia stratiotes has long been a source of debate.[29] Nativity to northern Africa is indicated byEgyptian hieroglyphics and reports of plants meeting the description ofPistia by Greek botanists,Dioscorides andTheophrastus in theNile River.[6] In addition, the co-evolution ofPistia stratiotes with various insects native to Brazil and Argentina, such as thewater lettuce weevil,[30] indicates a long-term native tenure in South America.[31] Fossil specimens dating back to the latePleistocene (~12,000BP) and earlyHolocene (~3,500 BP) period are reported from Florida, indicating a native presence in the southeastern United States.[32] Recent genetic evidence also suggests thatPistia is not actually a monotypic genus, as had been long assumed.[33] Instead,Pistia appears to be composed of at least three genetically distinct, but morphologically and ecologically similar, species at a global scale.[33]
ThoughPistia stratiotes is intolerant of cold temperatures, it has been recorded growing at least temporarily in temperate areas of North America and Europe.[4][34] 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.[35]
A specimen ofPistia from theFlorida peninsula dating from at least 3,550 yearsBefore Present and a report ofHolocenePistia fossils from a lake in south central Florida are consistent with genetic evidence indicating that some varieties ofPistia stratiotes are native to the southeastern United States.[33][38][40]
While considered edible,Pistia stratiotes is not palatable as it is rich incalcium oxalate crystals that are bitter in taste. Nevertheless, there are records of the plant being utilized as famine food in India during theGreat Famine of 1876–1878.[41]
TheHausa 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.[19] This salt substitute, also calledzakankau, was of high importance, especially when imported salt was unavailable.[42]
Caution is advised when consumingPistia stratiotes, as the plant is ahyperaccumulator, and can absorb and accumulate toxic heavy metals present in its environment.[43] The presence of high concentrations of calcium oxalate crystals can induce various health concerns, such as inhibited mineral absorption and kidney stones.[44][45]
In Singapore and Southern China,Pistia stratiotes is commonly grown or collected as animal feed for ducks and pigs.[46] Water lettuce is also considered an alternative for poultry feed in Indonesia due to its high content of crude protein.[47]
There are various medical uses ofPistia stratiotes throughout regions in Asia and Africa. In Nigeria, the dried leaves are prepared into a powder form and are applied to wounds and soresfor disinfection.[48] A similar use is present inIndian traditional medicine, where the powdered leaf is applied tosyphilitic eruptions andskin infections.[49] InNigeria andGambia, the leaf is infused in water to create aneyewash to treatallergic conjunctivitis.[50] The eyewash is known to have a cooling andanalgesic effect. Therefore, the plant is commonly called 'eye-pity' in Africa.[51] In addition, the leaves ofPistia stratiotes can be burned intoash, and in Indian and Nigerian traditional medicine, the ash is used in treatingringworm infections of thescalp.[49]
Anti-inflammatory properties: Extractions of the leaves ofP. stratiotes reduces mast infiltration and degranulation in allergic reactions and presentsanti-inflammatory properties.[50][52] The ethanolic extracts have also been positively correlated with a reduction in inflammatory disorders, such asarthritis andfevers.[53]
Antifungal properties: With the popular use ofPistia stratiotes as a traditional treatment for ringworms, researchers have testedP. stratiotes methanolic extracts ondermatophyte fungi. The results of the studies depicted significantfungicidal activity onT. rubrum,T. mentagrophytes, andE. floccosum.[54][49]
The high sorption property of water lettuce makes it a great candidate for biodegradable oil sorbents in marine oil spills. Particularly, the leaves ofPistiastratiotes can efficiently absorb significant amounts of hydrocarbons due to its large surface area and hydrophobicity.[55]
As a hyper-accumulator,Pistiastratiotes has been studied as a potential candidate forwastewater treatment plants. The roots and leaves of the plant have been found to absorb excess nutrients and heavy metals, such aszinc,chromium, andcadmium in contaminated waters.[56]
Pistiastratiotes can be grown in water gardens to reduceharmful algal blooms and eutrophic conditions.[57] 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 ofphosphorus andammonia nitrogen.[58]
^"Genus:Pistia L".Germplasm Resources Information Network. United States Department of Agriculture. 2006-02-23. Archived fromthe original on 2012-09-15. Retrieved2011-09-30.
^Neuenschwander, Peter; Julien, Mic H.; Center, Ted D.; Hill, Martin P. (2009), Muniappan, Rangaswamy; Reddy, Gadi V. P.; Raman, Anantanarayanan (eds.),"Pistia stratiotes L. (Araceae)",Biological Control of Tropical Weeds Using Arthropods, Cambridge: Cambridge University Press, pp. 332–352,doi:10.1017/cbo9780511576348.017,ISBN978-0-511-57634-8, retrieved2021-12-06
^Muniappan, Rangaswamy; Reddy, Gadi; Raman, Anantanarayanan (2009).Biological Control of Tropical Weeds Using Arthropods. Cambridge University Press. pp. 332–352.doi:10.1017/CBO9780511576348.017.ISBN9780511576348.
^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.
^Park, K. (2007).Park's Textbook of Preventive and Social Medicine (19th ed.). Jabalpur, India: Banarsidas Bhanot.
^Wheeler, G.S; Halpern, M.D (1999). "Compensatory responses ofSamea multiplicalis larvae when fed leaves of different fertilization levels of the aquatic weedPistia stratiotes".Entomologia Experimentalis et Applicata.92 (2):205–216.Bibcode:1999EEApp..92..205W.doi:10.1046/j.1570-7458.1999.00539.x.S2CID84780873.
^Hassler, Michael (16 January 2024)."Pistia".World Plants. Synonymic Checklist and Distribution of the World Flora. Version 18.3. Retrieved17 January 2024.
^Stockey, R.A.; Rothwell, G.R.; Johnson, K.R. (2007). "Cobbania corrugata gen. et comb. nov. (Araceae): A floating aquatic monocot from the Upper Cretaceous of western North America".American Journal of Botany.94 (4):609–624.doi:10.3732/ajb.94.4.609.JSTOR27733220.PMID21636430.
^Stockey, R.A.; Rothwell, G.R.; Johnson, K.R. (2016). "Evaluating relationships among floating aquatic monocots: A new species ofCobbania (Araceae) from the Upper Maastrichtian of South Dakota".International Journal of Plant Sciences.177 (8):706–725.doi:10.1086/688285.S2CID89477112.
Media related toPistia stratiotes at Wikimedia Commons