Mimosa pudica (also called sensitive plant, sleepy grass, sleepy plant, action plant, humble plant, touch-me-not, touch-and-die, shame plant or shy plant) is a creeping annual or perennial flowering plant of the pea/legume family Fabaceae. It is often grown for its curiosity value: the sensitive compound leaves quickly fold inward and droop when touched or shaken and re-open a few minutes later. For this reason, this species is commonly cited as an example of rapid plant movement. Like a number of other plant species, it undergoes changes in leaf orientation termed "sleep" or nyctinastic movement. The foliage closes during darkness and reopens in light. This was first studied by French scientist Jean-Jacques d'Ortous. In the UK it has gained the Royal Horticultural Society's Award of Garden Merit.
The species is native to the Caribbean and South and Central America, but is now a pantropical weed, and can now be found in the Southern United States, South Asia, East Asia, Southeast Asia, Micronesia, Australia, South Africa, and West Africa as well. It is not shade-tolerant and is primarily found on soils with low nutrient concentrations.
Taxonomy Mimosa pudica was first formally described by Carl Linnaeus in Species Plantarum in 1753. The species epithet, , is Latin for "shamefast", "bashful", or "shrinking", alluding to its shrinking reaction to contact.
Description
The stem is erect in young plants but becomes creeping or trailing with age. It can hang very low and become floppy. The stem is slender, branching, and sparsely to densely prickly, growing to a length of . The erect height of M. pudica usually reaches around ~30 cm (~1 ft).
The leaves are bipinnately compound, with one or two pinnae pairs, and 10–26 leaflets per pinna. The petioles are also prickly. Pedunculate (stalked) pale pink or purple flower heads arise from the leaf axils in mid-summer with more and more flowers as the plant gets older. A single flower survives for less than a day, and usually dies completely by the next day. Flowers of M. pudica are very brittle and soft. The globose to ovoid heads are in diameter (excluding the stamens). On close examination, it is seen that the floret petals are red in their upper part and the filaments are pink to lavender. Pollens are circular with approximately 8 microns in diameter.
The fruit consists of clusters of two to eight pods from long each, these being prickly on the margins. The pods break into two to five segments and contain pale brown seeds about long. The flowers are insect-pollinated and wind-pollinated. The seeds have hard seed coats which restrict germination and make osmotic pressure and soil acidity less significant hindrances. High temperatures are the main stimuli that cause the seeds to end dormancy.
The roots of Mimosa pudica produce carbon disulfide, an antifeedant that prevents certain pathogenic and mycorrhizal fungi from growing within the plant's rhizosphere. This allows the formation of nodules on the roots of the plant that contain endosymbiotic diazotrophs, which fix nitrogen.
Mimosa pudica is a tetraploid (2n = 52).
Plant movement
The leaflets close when stimulated in various ways, such as warming, touching, blowing, and shaking, which are all encapsulated within mechanical or electrical stimulation. Movements in response to these stimuli have been termed seismonastic movements. This reflex may have evolved as a defense mechanism to disincentivize predators, or alternatively to shade the plant in order to reduce water loss due to evaporation. The main structure mechanistically responsible for the drooping of the leaves is the pulvinus. The stimulus is transmitted as an action potential from a stimulated leaflet to the leaflet's swollen base (pulvinus), and from there to the pulvini of the other leaflets, which run along the length of the leaf's rachis. The action potential then passes into the petiole, and finally to the large pulvinus at the end of the petiole, where the leaf attaches to the stem. The pulvini cells gain and lose turgor due to water moving in and out of these cells, and multiple ion concentrations play a role in the manipulation of water movement.
Similar to the Venus flytrap's trigger hairs, M. pudica leaves are hypersensitive to touch. In line with the touch-sensing function used for tasks such as defense or nutrient maintenance, these parts have mechanoreceptors linked to mechanosensitive channels that can conduct calcium ions and indirectly relative anions upon touch stimulation, giving rise to depolarization and the initiation of an action potential (AP). They also have voltage-sensitive potassium channels that promote hyperpolarization and turgor formation. Such sensitive plants fire all-or-nothing type APs, similar to those seen in animals.
This movement of folding inwards is energetically costly for the plant and also interferes with the process of photosynthesis.
Distribution and habitat
Mimosa pudica is native to the tropical Americas. It can also be found in Asian countries such as Singapore, Bangladesh, Thailand, India, Nepal, Indonesia, Taiwan, Malaysia, the Philippines, Vietnam, Cambodia, Laos, Japan, and Sri Lanka. It has been introduced to many other regions and is regarded as an invasive species in Tanzania, South and Southeast Asia, and many Pacific islands. It is regarded as invasive in parts of Australia and is a declared weed in the Northern Territory, and Western Australia although not naturalized there. Control is recommended in Queensland.
It has also been introduced to Uganda, Ghana, Nigeria, Seychelles, Mauritius and East Asia but is not regarded as invasive in those places. In the United States, it grows in Louisiana, Florida, Hawaii, Tennessee, Virginia, Maryland, Puerto Rico, Texas, Alabama, Arkansas, Mississippi, North Carolina, Georgia, the territory of Guam, and the Virgin Islands.Distribution of Mimosa pudica in the United States of America, Natural Resources Conservation Service, United States Department of Agriculture.
Predators Mimosa pudica has several natural predators, such as the spider mite and mimosa webworm. Both of these insects wrap the leaflets in webs that hinder the responsive closing. Webbed leaves are noticeable as they become brown fossilized remnants after an attack. The Mimosa webworm is composed of two generations that arise at different seasons. This makes prevention difficult and requires proper timing of insecticides to avoid aiding other predators. Once the larvae become steel-gray moths, they are harmless to the plant but lay more eggs.
Agricultural impact The species can be a weed for tropical crops, particularly when fields are hand-cultivated. Crops it tends to affect are corn, coconuts, tomatoes, cotton, coffee, bananas, soybeans, papaya, and sugar cane. Dry thickets may become a fire hazard. In some cases, it has become a forage plant although the variety in Hawaii is reported to be toxic to livestock.
In addition, Mimosa pudica can change the physico-chemical properties of the soil it invades; total nitrogen and potassium, for example, have been seen to increase in significantly invaded areas.
Phytoremediation Thirty-six native Thai plant species were tested to see which conducted the most phytoremediation of arsenic-polluted soils caused by tin mines. Mimosa pudica was one of the four species that significantly extracted and bioaccumulated the pollutant into its leaves. Other studies have found that Mimosa pudica extracts heavy metals such as copper, lead, tin, and zinc from polluted soils. In principle, this accumulative behavior could be used to decontaminate some polluted soils.
Nitrogen fixation If nitrogen-fixing bacteria are present in the environment, Mimosa pudica may form root nodules for them to inhabit. These bacteria are able to convert atmospheric nitrogen into ammonia that plants can use for growth, i.e. nitrogen fixation.
As much as 60% of the nitrogen found in Mimosa pudica can be attributed to nitrogen fixation by its captive bacteria. Burkholderia phymatum STM815T and Cupriavidus taiwanensis LMG19424T are beta-rhizobial strains of diazotrophs that are highly effective at fixing nitrogen when coupled with M. pudica. Burkholderia is also shown to be a strong symbiont of Mimosa pudica in nitrogen-poor soils in regions like Cerrado and Caatinga.
Cultivation
In cultivation, this plant is most often grown as an indoor short lived perennial but is also grown for groundcover. Propagation is generally by seed. Germination should take place in 2-3 weeks. Mimosa pudica grows most effectively in nutrient-poor soil that allows for substantial water drainage. However, this plant is also shown to grow in scalped and eroded subsoils. Typically, disrupted soil is necessary in order for M. pudica to become established in an area. Additionally, the plant is shade intolerant and frost-sensitive, meaning that it does not tolerate low levels of light or cold temperatures. Mimosa pudica does not compete for resources with larger foliage or forest canopy undergrowth.
In temperate zones it must be grown under protection, where the temperature falls below .
Chemical constituents
Mimosa pudica contains the toxic alkaloid mimosine, which has been found to also have antiproliferative and apoptotic effects. The extracts of Mimosa pudica immobilize the filariform larvae of Strongyloides stercoralis in less than one hour. Aqueous extracts of the roots of the plant have shown significant neutralizing effects in the lethality of the venom of the monocled cobra (Naja kaouthia). It appears to inhibit the myotoxicity and enzyme activity of cobra venom.
Mimosa pudica demonstrates both antioxidant and antibacterial properties. This plant has also been demonstrated to be non-toxic in brine shrimp lethality tests, which suggests that M. pudica has low levels of toxicity. Chemical analysis has shown that Mimosa pudica contains various compounds, including "alkaloids, flavonoid C-glycosides, sterols, terenoids, tannins, saponin and fatty acids".Parasuraman S, Ching TH, Leong CH, Banik U (2019) Antidiabetic and antihyperlipidemic effects of a methanolic extract of Mimosa pudica (Fabaceae) in diabetic rats, Egyptian Journal of Basic and Applied Sciences, 6:1, 137–148, doi:10.1080/2314808X.2019.1681660 The roots of the plant have been shown to contain up to 10% tannin. A substance similar to adrenaline has been found within the plant's leaves. Mimosa pudica's seeds produce mucilage made up of D-glucuronic acid and D-xylose. Additionally, extracts of M. pudica have been shown to contain crocetin-dimethylester, tubulin, and green-yellow fatty oils. A new class of phytohormone turgorines, which are derivatives of gallic acid 4-O-(β-D-glucopyranosyl-6'-sulfate), have been discovered within the plant.
The nitrogen-fixing properties of Mimosa pudica contribute to a high nitrogen content within the plant's leaves. The leaves of M. pudica also contain a wide range of carbon to mineral content, as well as a large variation in 13C values. The correlation between these two numbers suggests that significant ecological adaptation has occurred among the varieties of M. pudica in Brazil.
The roots contain sac-like structures that release organic and organosulfur compounds including SO2, methylsulfinic acid, pyruvic acid, lactic acid, ethanesulfinic acid, propane sulfinic acid, 2-mercaptoaniline, S-propyl propane 1-thiosulfinate, and thioformaldehyde, an elusive and highly unstable compound never before reported to be emitted by a plant.
Research with Mimosa pudica