A pulvinus (pl. ''pulvini'') is a joint-like thickening at the base of a plant

leaf A leaf ( : leaves) is any of the principal appendages of a vascular plant stem, usually borne laterally aboveground and specialized for photosynthesis. Leaves are collectively called foliage, as in "autumn foliage", while the leaves, ...

or leaflet that facilitates growth-independent movement. Pulvini are common, for example, in members of the bean family

Fabaceae The Fabaceae or Leguminosae,International Code of Nomenc ...

(Leguminosae) and the prayer plant family

Marantaceae The Marantaceae are a family, the arrowroot family, of flowering plants consisting of 31 genera and around 530 species, defining it as one of the most species-rich families in its order.Kennedy, H. (2000). “Diversification in pollination mech ...

. 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 A geniculum is a small '' genu'', or angular knee-like structure. The term is often used in anatomical nomenclature to designate a sharp knee-like bend in a small structure or organ. For example, in the facial canal, the genicular ganglion is sit ...

(meaning a knee-like structure in Latin). Pulvinar movement is caused by changes in

turgor Turgor pressure is the force within the cell that pushes the plasma membrane against the cell wall. It is also called ''hydrostatic pressure'', and is defined as the pressure in a fluid measured at a certain point within itself when at equilibri ...

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 Water potential is the potential energy of water per unit volume relative to pure water in reference conditions. Water potential quantifies the tendency of water to move from one area to another due to osmosis, gravity, mechanical pressure and ...

and triggers the efflux of

potassium Potassium is the chemical element with the symbol K (from Neo-Latin '' kalium'') and atomic number19. Potassium is a silvery-white metal that is soft enough to be cut with a knife with little force. Potassium metal reacts rapidly with atmos ...

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 A vacuole () is a membrane-bound organelle which is present in plant and fungal cells and some protist, animal, and bacterial cells. Vacuoles are essentially enclosed compartments which are filled with water containing inorganic and organic m ...

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 A circadian clock, or circadian oscillator, is a biochemical oscillator that cycles with a stable phase and is synchronized with solar time. Such a clock's ''in vivo'' period is necessarily almost exactly 24 hours (the earth's current solar day ...

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.

Pulvinus in ''Mimosa pudica''

In '' Mimosa pudica'', the internal biological clock mediates the closing of leaflets at night and opening during day. Rapid (''seismonastic'') movement of leaves is triggered in response to touch and temperature. A pulvinus is located at the base of each leaflet of the plant. Mechanical stimulation via touch is perceived and is translated to electrical stimulation causing the flow of ions out of the pulvinus cells. An upregulation larification neededof water channel proteins (''

aquaporin Aquaporins, also called water channels, are channel proteins from a larger family of major intrinsic proteins that form pores in the membrane of biological cells, mainly facilitating transport of water between cells. The cell membranes of a ...

s'') and membrane proteins which move solutes across a cell membrane (H+ -

ATPase ATPases (, Adenosine 5'-TriPhosphatase, adenylpyrophosphatase, ATP monophosphatase, triphosphatase, SV40 T-antigen, ATP hydrolase, complex V (mitochondrial electron transport), (Ca2+ + Mg2+)-ATPase, HCO3−-ATPase, adenosine triphosphatase) are ...

) allows for the rapid flux of water out of these motor cells. Water flux out of the cell's symplast and into its surrounding apoplast results in a decrease in turgor pressure, and the characteristic closing of the leaves of ''Mimosa pudica''. The drop in turgor pressure is reversible but slow. Leaves slowly open to their initial position after 20 minutes of lack of stimulation. It has been demonstrated that seismonastic movement can be inhibited with the use of anaesthetics. Using nuclear magnetic resonance, upward movement of water within the pulvinus joint in response to electrical stimulation was observed in the pulvinus at the base of the petiole (=the leaf stalk). Movement of water to the upper or lower part of the pulvinus causes asymmetric swelling, therefore causing the stalk to either droop or rise.

Pulvinus in ''Mimosa pudica''

References

Further reading