nyctinasty

Nyctinasty is a rhythmic nastic movement of plant parts in response to the onset of darkness. In many plants, leaves or leaflets fold, droop, or reposition themselves at night, returning to their daytime position with dawn. The movement is typically circadian, maintaining a roughly 24-hour pattern even under short-term constant conditions, and is coordinated by an endogenous clock synchronized by the light-dark cycle.

Physiologically, nyctinastic movements result from changes in turgor pressure in specialized pulvini at the base of leaf stalks or leaflets. During the night, ion fluxes, notably of potassium and anions, lead to water efflux from pulvinus cells, causing the petiole to shorten and the leaf to fold. In many species, the action is gradual and reversible, occurring over minutes to hours.

Nyctinasty is widespread across flowering plants, with well-known examples including the leaves of the prayer plant (Maranta leuconeura), Oxalis species, and several legumes such as Mimosa and Desmodium. Some plants display nyctinastic flower movements as well, opening in daytime and closing at night.

Potential adaptive advantages include reducing water loss by lowering transpiration, reducing exposure to nocturnal herbivores, and protecting photosystem components during cooler night temperatures. The exact function can vary among species and environments, and not all plants exhibit nyctinasty.