Pistia stratiotes (PROSEA)
- Protologue: Sp. pl. 2: 963 (1753).
- Family: Araceae
- Chromosome number: 2n= 28
- Water lettuce, tropical duckweed, Nile cabbage (En). Lettue d’eau (Fr)
- Indonesia: ki apu (Sundanese), kayu apu (Javanese), kiambang (West-Kalimantan)
- Malaysia: kiambang
- Philippines: kiapo, apon (Tagalog), loloan (Iloko)
- Thailand: chok (central), kaa kok, phak kok (northern)
- Vietnam: bèo cái, bèo tai tượng, dại phù bình.
Origin and geographic distribution
P. stratiotes has a pantropical distribution, and occurs throughout South-East Asia.
In China, Vietnam, Thailand, India, West Africa and Brazil, the leaves of P. stratiotes are used externally for skin diseases, such as boils, piles and syphilitic sores, and internally as a laxative, emollient and diuretic. They are also mixed with rice and coconut milk for dysentery, and mixed with rose water and sugar for cough and asthma. In Peninsular Malaysia, the leaves are applied in the treatment of gonorrhoea, probably because they act as a diuretic. They are also applied to haemorrhoids. In India, the ashes are used to treat ringworm, and a decoction of the plant for dysuria and as an expectorant. In Nigeria, the leaves are taken as a stomachic, but an overdose may cause acute diarrhoea. In Cuba, a decoction of the whole plant is drunk to relieve dyspepsia, and in Venezuela, a decoction is used in baths to reduce oedema.
In Java, P. stratiotes is tolerated because shrimps are known to take cover underneath. It is also good fodder for gourami, carp and goldfish. In India and China, it is cooked as a fodder for pigs and ducks, and can be used fresh to feed rabbits. In India and the Sudan, it has been used as a famine food, but humans do not generally find it very palatable. The Chinese eat the young leaves cooked, but their taste is first nondescript and then sharp because of the calcium oxalate.
The plant is also used as a manure, especially for its high potash content. The plant can be used with soap for taking stains out of clothing, probably because of its potash content.
Experiments have shown that P. stratiotes is a good waste water-cleaning agent, as it takes up nitrogen and phosphorous compounds, as well as heavy metals, and harbours active microbial organisms.
In the tropics, P. stratiotes is sometimes cultivated as an ornamental in fountains and aquatic gardens. In Europe and North America, P. stratiotes is sold as an indoor aquatic ornamental, but has been known to be introduced in fresh water illegally, where it dies back during winter, so it cannot become a noxious weed.
Production and international trade
P. stratiotes is not known to be traded as a medicinal plant.
The ash of P. stratiotes contains 75% potassium chloride and 23% potassium sulphate. It also contains small amounts of salts of sodium, magnesium, calcium, iron and aluminium. The fresh plant is a source of vitamins A, B and C, and also contains polysaccharides (galactose, glucose, mannose and arabinose residues), uronic acids, steroids, as well as the polyamines spermidine, spermine and some closely related compounds. Several potential allelochemicals have been extracted from the leaves, including linoleic acid,γ-linolenic acid, (12R,9Z,13E,15Z)-12-hydroxy-9,13,15-octadecatrienoic acid, (9S,10E,12Z,15Z)-9-hydroxy-10,12,15-octadecatrienoic acid and 24S-ethyl-4,22-chloestadiene-3,6-dione. They were found to inhibit the growth of several algae and cyanobacterial species in solid media. Also the phenylpropanoidα-asarone was isolated, which was found to be toxic to sensitive algal strains in liquid medium.
Several flavonoids were isolated as well, e.g. chrysanthin, lucetin, luceolin-7-glucoside, orientin, vicetin and vitexin. Other isolated compounds included the norterpene glucosides stratioside I and II.
A methanol extract of the whole plant showed calcium channel blocking activity in isolated segments of rabbit jejunum in vitro; this was confirmed by pretreatment with verapamil (a standard calcium channel blocker). Additionally, the extract inhibited dose-related bronchodilating activity in isolated guinea-pig trachea and dose-related neuromuscular blocking actions of reference compounds. It also caused a decrease in systolic and diastolic blood pressure in anaesthetised rats of 18% and 10% respectively, at a dose of 10μg.
In the brine shrimp lethality bioassay, some undefined P. stratiotes extracts exhibited moderate toxicity in low concentrations.
A small, monoecious, perennial, quick-growing, free-floating aquatic herb; stolons 10-60 cm long, stem almost absent; roots adventitious, 20-80 cm long, forming a dense tuft, root hairs plumosely spreading. Leaves rosulate, subimbricate, variable in shape and size, spathulate or tongue-shaped, 2-10 cm × 2-9 cm, patent or erect when crowded, pale to yellowish-green, whitish velvety hairy, blade succulent, veins flabellate; petiole absent. Inflorescence a spadix surrounded by a spathe, several in upper part of rosette, peduncle 12-15 mm long; male and female flower small, located separately on the spadix; spathe narrowly ovate, 7-9(-20) mm × 3 mm, acuminate, outside pubescent, white, inside glabrous, lower margins connate with each other and with ovary wall, free margins folded between stigma and stamens to form a constriction, just below spathe partition a thin pouch-shaped flap, green; spadix mostly adnate to spathe, at apex free, with 1 naked male flower, 4-6(-8) stamens united in a synandrium, at base subtended by a thin, cup-shaped ring, pores in anthers 4, in 2 superposed pairs; at base of spathe 1 naked female flower, ovary 1-celled, ovules many, in 4-6 rows on convex parietal placenta, style free from spadix, thick, stigma subglobose, bearded. Fruit a dry, ellipsoid berry, irregularly rupturing, with few to numerous seeds. Seed ovoid, 2 mm long, wrinkled, tapering towards the base, apex truncate and depressed in the centre, brown. Seedling germinating under water, emerging with a 2-lobed, cotyledon-like structure, filled with aerenchyma and floating; no primary root is developed, but 2-3 adventitious roots emerge soon from the base of the cotyledon; between the 2 lobes the first leaf appears, broadly ovate, densely hairy.
Growth and development
The leaves of P. stratiotes are able to float because the short depressed hairs on both surfaces trap air and repel water. Some leaves also have a conspicuous, ovoid swelling on the underside filled with spongy parenchyma, which enables the plant to float. The roots stabilize the rosettes, and when a portion of the roots is destroyed, part of the foliage may become partly or fully submerged.
Flowering and fruiting patterns of P. stratiotes vary by the region. Flowering seems to be markedly enhanced when vegetative reproduction is inhibited by crowding. In India, flowering starts in the hot season and continues up to the rainy season, while the fruits appear after the rainy season. In Indonesia, flowering can occur throughout the year. There are about 8 days between the appearance of the first flower buds and when the flowers open. At anthesis, first the pistil emerges, and after a few hours the stamens. P. stratiotes is pollinated by insects. Seeds reach maturity about 30 days after fertilization.
Other botanical information
Pistia belongs to the subfamily Pistoideae and is a monotypic genus.
P. stratiotes floats in stagnant or slowly flowing fresh water, ponds and tidal areas, from sea-level up to 1200 m altitude. It is cold sensitive and thus cannot exist far beyond the tropics of Cancer and Capricorn. It can become a part of dense aggregations of free-floating vegetation, called sudds, which are common in wide, slow-flowing rivers and in extensive swampy areas. Sudds are formed by thick, floating mats of P. stratiotes , and/or Eichhornia crassipes (Mart.) Solms, which are then colonized by hydrophytes, mainly grasses and sedges, which often invade from the shore. When the raft is large enough, it may be torn away from the shore by wind or flood, to become a floating island. It performs best at pH 4, and cannot grow at pH 3; its range of pH tolerance is much narrower than that of Eichhornia crassipes , which has its optimum growth at pH 7. P. stratiotes is a noxious weed like E. crassipes , obstructing navigation in rivers and grills of hydroelectric plants. It interferes with fisheries by hindering the nets and lowering the oxygen content of the water and the pH.
Propagation and planting
P. stratiotes is propagated by seed, or, more rapidly, by stolons. Seed production varies from 0-1 per flower in laboratory experiments in the United States, to several seeds per flower in Africa. The seeds float in the water for a few days, after which they sink and germinate. The seedling appears at the surface in 5 days.
Diseases and pests
P. stratiotes is most troublesome as a weed in Africa, but is also a serious problem in Southern Asia and the Caribbean islands. It is a serious competitor in irrigated rice fields, where it can root in shallow water.
The leaves of P. stratiotes serve as a niche for several mosquito species, which in turn serve as principal vectors of malaria, encephalomyelitis and filariasis.
Intensive research on biological control of P. stratiotes is being carried out. In several locations, it is periodically devastated by insects, of which the larvae of Proxenus hennia occur exclusively on P. stratiotes . Nymphula responsalis and the noctuid Spodoptera pectinicornis from South-East Asia also feed on other noxious water plants. In Indonesia, tropical Africa and tropical Australia, long-term control of P. stratiotes is done effectively by Neohydronomus affinis (Coleoptera), while areas with shallow water infested by P. stratiotes , which are regularly subjected to alternate wet and dry regimes, are best controlled chemically. Fungal pathogens have received much less attention as potential biological control agents, although some are promising, such as Cercospora pistiae , which causes leaf spot.
Plants of P. stratiotes are pulled out of the water when needed.
Handling after harvest
The leaves of P. stratiotes are used fresh or are dried in the sun or shade before use.
Genetic resources and breeding
As P. stratiotes is a fast growing, pantropical weed, propagating by seed and stolons, it is certainly not genetically endangered.
The fatty acids extracted from the leaves of P. stratiotes show interesing potential as allelochemicals, and therefore could be of use as a local source in the control of algae. More research is needed, however, to fully evaluate other possibly interesting activities, e.g. in developing new calcium channel blocking agents.
- Achola, K.J., Indalo, A.A. & Munenge, R.W., 1997. Pharmacologic activities of Pistia stratiotes. International Journal of Pharmacognosy 35(5): 329-333.
- Arifkhodzhaev, A.O & Shoyakubov, R.S., 1995. Polysaccharides of Eichhornia crassipes and Pistia stratiotes. Chemistry of Natural Compounds 31(4): 521-522.
- Cilliers, C.J., Zeller, D. & Strydon, G., 1996. Short- and long-term control of water lettuce (Pistia stratiotes) on seasonal water bodies and on a river system in the Kruger National Park, South Africa. Hydrobiologia 340(1-3): 173-179.
- Holm, L.G., Plucknett, D.L., Pancho, J.V. & Herberger, J.P., 1977. The world's worst weeds. Distribution and biology. East-West Center, the University Press of Hawaii, Honolulu, United States. pp. 379-384.
- Quisumbing, E., 1978. Medicinal plants of the Philippines. Katha Publishing Co., Quezon City, the Philippines. pp. 144-145.
- Soerjani, M., Kostermans, A.J.G.H. & Tjitrosoepomo, G. (Editors), 1987. Weeds of rice in Indonesia. Balai Pustaka, Jakarta, Indonesia. pp. 52-53.
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- Burkill, I.H., 1966. A dictionary of the economic products of the Malay Peninsula. Revised reprint. 2 volumes. Ministry of Agriculture and Co-operatives, Kuala Lumpur, Malaysia. Vol. 1 (A-H) pp. 1-1240, Vol. 2 (I-Z) pp. 1241-2444.
215, 246, 301, 384, 696, 912, 1123.
G.H. Schmelzer & N. Bunyapraphatsara