Turgor pressure is the hydrostatic pressure exerted by water inside the plant cell. This internal force gives non-woody plants their stiffness and is responsible for their ability to maintain an upright, firm posture against gravity. This pressure is a fundamental mechanical principle that underpins nearly all aspects of plant life, from growth to movement.
Defining Turgor Pressure at the Cellular Level
Turgor pressure originates from the interplay between water, dissolved solutes, and the physical structure of the plant cell. The process begins with osmosis, the movement of water across a semipermeable membrane from an area of low solute concentration to an area of high solute concentration. Plant cells maintain a higher concentration of solutes, such as sugars and ions, inside their central vacuole compared to the surrounding environment.
This difference in solute concentration causes water to flow continuously into the cell. As the vacuole swells, the water-filled protoplast pushes the cell membrane outward against the inner surface of the cell wall. This outward push is the hydrostatic force defined as turgor pressure.
The cell wall, a rigid outer layer composed primarily of cellulose, converts this internal hydrostatic force into measurable pressure. Unlike animal cells, which would burst from this influx of water, the tough cell wall resists the expansive force. It exerts an equal and opposite pressure, effectively containing the cell’s contents and creating the firmness characteristic of healthy plant tissue.
Essential Roles in Plant Structure and Growth
The pressure generated by turgor is foundational for the structural integrity of herbaceous plants, acting much like the air pressure that inflates a rubber tire. This internal force provides the necessary mechanical support to hold up stems, keep leaves expanded for maximum sunlight capture, and maintain the shape of flowers. Without this internal support, the non-woody parts of the plant would become flaccid and collapse under their own weight.
Turgor pressure is also the primary driving mechanism for plant growth. Cell enlargement, a prerequisite for a plant to increase in size, occurs when turgor pressure pushes the cell wall outward. For a cell to expand, the cell wall must temporarily loosen its structure, allowing the sustained internal pressure of the turgid cell to drive the irreversible expansion of the cell volume. This process is particularly evident in rapidly growing areas like root tips and pollen tubes.
Turgor and Specialized Plant Movements
Turgor pressure is finely controlled to power dynamic, reversible movements in specialized plant parts. The opening and closing of stomata, the tiny pores on the leaf surface that regulate gas exchange and water loss, are directly governed by turgor changes in their flanking guard cells.
During the day, guard cells accumulate potassium ions, which increases their internal solute concentration. Water follows the ions by osmosis, causing the guard cells to swell and curve outward, which opens the stoma to allow carbon dioxide uptake for photosynthesis. Conversely, when the guard cells lose these ions, water flows out, the cells become flaccid, and the pore closes to conserve water during drought or at night.
This rapid control mechanism is also responsible for nastic movements, such as the dramatic leaf folding seen in the sensitive plant, Mimosa pudica. When the plant is touched, an electrical signal triggers a sudden efflux of ions, particularly potassium, from specialized motor cells located in the pulvini at the base of the leaflets. The rapid loss of solutes causes water to rush out of the motor cells, resulting in a sudden loss of turgor. This turgor decrease on one side of the pulvinus causes the leaflets to collapse almost instantly, serving as a defense against herbivores.
What Happens When Turgor Is Lost?
When a plant cannot take up enough water to compensate for the water lost through transpiration, the cells’ internal water content decreases, leading to a drop in turgor pressure. The most visible symptom of this water stress is wilting, where the entire plant structure or its leaves become limp and droop. Wilting occurs when the internal pressure drops to zero, a point known as the turgor loss point, and the cells lose their ability to support the plant’s structure.
If water loss continues, the cellular condition progresses to plasmolysis, a more severe state where the cell membrane visibly shrinks and pulls away from the rigid cell wall. While wilting can often be reversed simply by watering the plant, prolonged or severe plasmolysis can result in permanent damage and cell death.
By rehydrating, the cells take in water, the central vacuole refills, and the pressure against the cell wall is reestablished. Reversible wilting serves as a temporary protective response, reducing the leaf surface area exposed to the sun and slowing down further water loss until the internal pressure is regained.