Pulvinus

A pulvinus (pl. pulvini) may refer to a joint-like thickening at the base of a plant leaf or leaflet that facilitates growth-independent movement. Pulvinus is also a botanical term for the persistent peg-like bases of the leaves in the coniferous genera Picea^[1] and Tsuga. Pulvinar movement is common, for example, in members of the bean family Fabaceae (Leguminosae)^[2]: 185 and the prayer plant family Marantaceae.^[2]: 381

Pulvini may be present at the base of the leaf stalk or on its other end (apex), where the leaf is attached, or in a compound leaf at the place where the leaflets are joined to its middle stem. They consist of a core of vascular tissue within a flexible, bulky cylinder of thin-walled parenchyma cells. A pulvinus is also sometimes called a geniculum (meaning a knee-like structure in Latin).^{[citation needed]}

Pulvinar movement is caused by changes in turgor pressure leading to a contraction or expansion of the parenchyma tissue. The response is initiated when sucrose is unloaded from the phloem into the apoplast. The increased sugar concentration in the apoplast decreases the water potential and triggers the efflux of potassium ions from the surrounding cells. This is followed by an efflux of water, resulting in a sudden change of turgor pressure in the cells of the pulvinus. Aquaporins on the vacuole membrane of pulvini allow for the efflux of water that contributes to the change in turgor pressure. The process is similar to the mechanism of stomatal closure.

Common examples for pulvinar movements include the night closure movement of legume leaves and the touch response of the sensitive plant Mimosa pudica. Sleep movements (nyctinastic movements) are controlled by the circadian clock and light signal transduction through phytochrome. Touch response (thigmonastic) movements appear to be regulated through electrical and chemical signal transduction spreading the stimulus throughout the plant.