nyctinastia

Nyctinastia, often written nyctinasty in English, is the circadian movement of plant leaves, typically characterized by nocturnal folding or drooping of leaflets and sometimes leaves, in response to darkness. This phenomenon is observed in many dicot plants, especially within the legume family, though it occurs in some other groups as well. The movement is driven by pulvini—the swollen joints at the base of leaflets or petioles—where rapid changes in turgor pressure cause bending. These turgor changes are linked to ion fluxes and water movement across parenchyma cells, mediated by plant hormones and the plant's endogenous circadian clock. The cycle can be entrained by light-dark cycles and often persists in constant conditions with a period near 24 hours.

Common illustrative examples include Mimosa pudica (the sensitive plant), Desmodium gyrans (the telegraph plant), and many tropical legumes; but the intensity and timing vary widely.

Possible functions proposed include reducing transpiration, protecting photosynthetic tissues from late-day radiation, reducing exposure to herbivores at night, or improving recovery after daytime closures—though the exact adaptive value remains debated. Nyctinastic movements are generally slow on a timescale of minutes to hours, unlike thigmonasty, which is rapid and stimulus-specific.

The term nyctinastia is a variant spelling used in some languages and texts; the more widely accepted English form is nyctinasty. There is no separate botanical genus widely recognized as Nyctinastia; if you are seeking information on a plant genus by that name, please provide context or check spelling.