pulvini

Pulvini (singular: pulvinus) are specialized swellings at the base of leaves or leaflets, often at the juncture between the petiole or rachis and the blade. They function as motor organs that enable reversible leaf movements without growth, by changing the turgor pressure within their cells. Pulvini are common in many dicotyledonous plants, notably within the Fabaceae family, where they play a key role in leaf orientation and mobility.

Anatomically, a pulvinus consists of a layer of cortex surrounding a central vascular bundle, with two groups of motor cells on opposite sides. The cells on one side lose turgor while those on the opposite side maintain or increase turgor, causing the pulvinus to bend toward the low-turgor side. The motor cells are rich in organelles for energy production and contain ion transport machinery that drives osmotic water movement into or out of the cells.

Movement in pulvini is primarily hydraulic and can be triggered by electrical signals, such as action potentials or variations in membrane potential, followed by rapid ion fluxes (e.g., potassium and chloride) and water movement. This results in bending of the leaf or leaflet within seconds to minutes in some species, with longer-term adjustments possible in others.

Functions of pulvini include nyctinasty (diurnal leaf-opening and closing), thigmonasty (movement in response to touch or vibration), and adjustments to light capture or water use. In several species, rapid leaf movements deter herbivory or help minimize damage in windy environments.

Notable examples include Mimosa pudica (sensitive plant), Desmodium spp., and Albizia julibrissin. Pulvini illustrate a common plant strategy for fast, reversible movement driven by turgor changes rather than growth.