Mimosa pudica is an angiosperm plant, that is, it produces fruits that protect the seeds, which belongs to the legume family. Although the species is native to the Caribbean and South and Central America, it is known throughout the world for its ability to move quickly when touched or shaken. In fact, in English they call it touch-me-not , the plant ‘don’t touch me’.
But what mechanism is behind this swift response? Like many other plant species, the sensitive plant can change the orientation of its leaves depending on the light conditions, closing at night and opening during the day, in a movement called nictinastia or sleep movements, which are believed to be used to avoid heat losses by reducing solar radiation.
However, the movements induced in the leaves by mechanical or electrical stimuli are known as sismonastia , only described in carnivorous plants with bilobed leaves, such as the Venus flytrap ( Dionaea muscipula ), and in mimosa, such as Mimosa pudica . Sensitive mimosa has compound leaves that, when in contact with a stimulus, pass the action that causes a chain reaction through the leaflets, until reaching the pulvino, a widening in the form of a bearing at the base of the leaf or petiole. , which stops the momentum so that more leaves do not fold than have been stimulated.
The biochemical explanation is that upon receiving the action potential (the stimulus signal), the pulvino responds by expelling potassium and chlorine ions, and absorbing calcium ions, something that results in an osmotic gradient that draws water out of the cells of the affected leaflets, causing them to shrink temporarily. It is an energetically expensive process that can interfere with the photosynthesis process.
Now that we know the biochemical reason for the process, the question would be , what is it for? What evolutionary advantages does this rapid movement bring to the plant? The answer is not simple, it seems that the fact of being able to close the petioles gives it a certain protection against herbivores, since it makes it appear withered and, therefore, predators avoid eating it. There is also the hypothesis that both herbivores and harmful insects could be scared by seeing the rapid movement of the plant and would prefer to eat others, more immobile. Another proposal from the scientific community is that closing them helps them lose less water through evaporation.
The most interesting thing, according to a study published in the journal Plant Ecology , is that the sensitive mimosa can act with a certain plasticity when it comes to reopening the petioles and returning to their initial position. So, depending on whether that area of the plant is more or less exposed to possible predators, the plant would take longer to open; and the opposite in areas close to the inflorescences, which is important for them to be accessible as long as possible for their pollination.
Referencias: Amador-Vargas, S., Dominguez, M., León, G., Maldonado, B., Murillo, J., & Vides, G. L. (2014). Leaf-folding response of a sensitive plant shows context-dependent behavioral plasticity. Plant Ecology, 215(12), 1445–1454. https://doi.org/10.1007/s11258-014-0401-4
