Turgor Pressure

Turgor pressure is the pressure of the fluid inside a cell pushing against its cell wall. It keeps plant tissues firm, and when it drops, as in a dry day, the leaves and stems wilt.

A plant cell holds dissolved sugars, salts and other solutes in its central vacuole. Water therefore enters by osmosis across the cell membrane until the stretched wall pushes back and stops further uptake. The resulting pressure is typically 0.5 to 1 megapascal, several times the pressure in a car tire. The wall prevents the cell from bursting, and this is one reason the extracellular matrix of plants is so important.

Turgor supports soft tissues that have no skeleton and powers growth, because cells expand when the wall is loosened and water rushes in. It opens and closes the stomata: when guard cells take up water and swell, the pore opens, and when they lose water, it closes. It also drives the quick movements of the Venus flytrap and the sensitive plant, and pollen tubes growing through the style.

When the soil is dry or salty, or when transpiration is faster than the roots can supply water, cells lose water and turgor falls. Eventually the membrane pulls away from the wall in a state called plasmolysis. Watering usually restores turgor, but severe or long drought can cause permanent damage. The same problem affects crops under drought, and the physical principle is the balance of osmotic and pressure potential in plants.

• Stomata — pores controlled by turgor
• Transpiration — the water loss that lowers turgor
• Cell — the unit in which it operates