nyctinastic

Nyctinastic refers to circadian rhythm–driven movements of plant organs, typically leaves or leaflets, that occur with the daily cycle of night and day. The most familiar example is the daily folding, drooping, or closing of leaves at night and their reopening by day, a set of movements often called sleep movements. Nyctinastic movements are common in a variety of plant groups, especially among herbaceous species and some legumes and tropical shrubs.

Mechanism and regulation: These movements are mainly produced by motor cells in the pulvini, the swollen junctions at the base of leaves or leaflets. Changes in turgor pressure within these cells, driven by rapid fluxes of ions such as potassium and chloride, cause the pulvini to lose or gain water and bend. The timing is controlled by the plant’s internal circadian clock and is entrained by light-dark cycles; environmental factors like temperature can modulate the amplitude and timing of movement, but the core rhythm tends to persist under constant conditions.

Examples and scope: Nyctinasty occurs across many plant families. Well-known examples include Oxalis species, whose leaflets close at night, and various legumes. Some plants, such as the telegraph plant (Desmodium gyrans), show conspicuous leaf movements related to light and dark, though not all movements in a given species are strictly nyctinastic and may respond to other stimuli as well.

Function and interpretation: Proposed adaptive benefits include reduced water loss by lowering leaf surface exposure at night, protection of photosynthetic tissues, or deterrence of herbivory. The precise ecological value varies among species, and in some cases movements may reflect an evolved circadian output with multiple contributing factors.