Vessel cavitation is the formation of a vapor-filled bubble, called an embolism, within a plant’s water transport tissues. This embolism breaks the flow of water, much like a vapor lock in a fuel line, and can have serious consequences for the plant’s health. The process begins when tension in the water column causes dissolved gases to expand and form a blockage that disrupts water transport.
The Plant’s Plumbing System
Plants possess a water-conducting tissue called xylem, which moves water from the roots to the leaves. This network consists of interconnected cells called vessels and tracheids that form continuous conduits for water. The transport of water is explained by the cohesion-tension theory, which states that evaporation from leaves, a process called transpiration, creates a pulling force, or tension.
This tension extends down the entire length of the xylem. Water molecules have an attraction to one another, a property known as cohesion, allowing them to be pulled up in a continuous stream. The water molecules also adhere to the walls of the xylem vessels, which helps counteract gravity. The system operates under negative pressure, and its efficiency depends on this uninterrupted column of water.
Triggers for Cavitation Events
The primary trigger for cavitation is water stress from drought conditions. As soil dries, a plant must pull harder to extract water, which increases the tension within the xylem. This high tension can cause “air-seeding,” where air is pulled from adjacent spaces into a vessel through microscopic pores in its walls. Once inside, the air bubble expands under the negative pressure, creating an embolism.
Another cause is exposure to freeze-thaw cycles, a danger for plants in colder climates. When water inside the xylem freezes, dissolved gases are forced out of the solution and form small bubbles. When the ice thaws, these bubbles can coalesce or expand, especially if the water column is under tension. This process creates a large embolism that obstructs the vessel.
Consequences of a Broken Water Column
An embolism renders an affected xylem vessel non-functional, reducing the plant’s overall hydraulic conductivity, which is its capacity to move water. While a small number of embolisms can be bypassed by rerouting water through adjacent vessels, widespread cavitation can lead to water deficits in the leaves.
This internal water shortage produces visible symptoms:
- Leaves may wilt as they lose turgor pressure.
- The plant’s growth will slow or stop.
- Extensive hydraulic failure can lead to the death of entire branches, a condition known as dieback.
- Photosynthesis is limited, as the plant closes its stomata to conserve water, which also stops the intake of carbon dioxide.
How Plants Prevent and Repair Damage
Plants have developed structural features to resist cavitation, especially those native to dry environments. These plants may possess xylem vessels with narrower diameters, which are less vulnerable to embolism formation. Additionally, their pit membranes can have smaller pores, increasing the tension required for air-seeding to occur.
Some plants can also actively repair embolized vessels, most commonly through the generation of root pressure. Occurring at night when transpiration is low, the roots pump solutes into the xylem. This influx draws in water through osmosis, creating a positive pressure that rises up the stem.
This pressure can compress the air bubbles within embolized vessels, forcing the gases to re-dissolve into the surrounding water and restoring flow. However, not all plants, particularly tall trees, can generate sufficient root pressure to overcome gravity and repair embolisms high in their stems.