Nastic

Nastic movements are plant movements that occur in response to external stimuli but are non-directional with respect to the stimulus. Unlike tropisms, which involve directional growth toward or away from a stimulus, nastic responses are not oriented toward the source of stimulation. They are often rapid or reversible and are typically driven by changes in turgor pressure within specialized cells, such as those in the pulvinus at the base of leaves or leaflets.

Nastic movements are categorized by the type of stimulus involved. Nyctinasty refers to daily or circadian movements tied to light-dark cycles, such as leaves folding at night. Photonasty describes movements triggered by light intensity rather than direction. Thermonasty involves movements in response to temperature changes. Thigmonasty (also called seismonasty) encompasses responses to mechanical stimulation, such as touch or vibration; the rapid leaf folding of the sensitive plant (Mimosa pudica) and the closing of the Venus flytrap are well-known examples.

These movements are typically driven by rapid ion fluxes and water shifts across cell membranes, causing reversible changes in cell turgor. They do not involve growth on the time scale of hours to days, distinguishing them from many tropic reactions. Nastic responses can be observed across a variety of plant groups, including tropical and temperate species, and contribute to adaptations such as reducing water loss, deterring herbivory, or regulating light capture.