Thermonasty

Thermonasty is a plant movement in which organs such as leaves or leaflets rapidly change position in response to temperature changes, without growth. It is a non-growth or nastic response, meaning the movement is reversible and not due to tissue enlargement or elongation. Thermonasty is distinguished from movements driven by circadian rhythms or light, and from growth-oriented tropisms that involve directional growth over longer periods.

The underlying mechanism involves rapid changes in turgor pressure within specialized motor tissues, such as pulvini at the base of leaflets or petioles. Temperature fluctuations can alter membrane fluidity, ion channel activity, and osmotic gradients, leading to water movement and bending of the organ. The result is a swift, reversible reorientation that helps the plant cope with environmental temperature changes. While heat and cold can trigger thermonastic movements, the response can vary in timing and magnitude among species.

Thermonasty has been observed in a range of flowering plants, including some legumes and ornamental species, where leaves or leaflets adjust their angle in response to thermal cues. In contemporary literature, the term is used to describe temperature-induced movements that are not tied to growth. It is often considered alongside other rapid plant movements such as thigmonasty (touch-induced) and seismonasty (vibration-induced), as part of the broader study of plant motor responses.

See also: plant movement, nyctinasty, thigmonasty, seismonasty, tropism.