Final answer:
Cavitation in plants is caused by excessive tension pulling water from the xylem, forming air bubbles that disrupt water transport. This increases resistance to water flow and creates a positive feedback loop, leading to embolism. Structural adaptations help mitigate risk, but extreme stress from drought or freezing can exacerbate cavitation and embolism.
Step-by-step explanation:
Understanding Cavitation and Embolism in Plants
In vascular plants, water stress can lead to a phenomenon known as cavitation, where water is forcefully pulled from the vessels and tracheids due to high tension resulting from water loss. Cavitation causes the formation of air bubbles within the xylem, disrupting the water column and leading to the loss of part of the xylem's conducting system. As the resistance to flow increases, more tension is required to pull water, creating a positive feedback loop that makes further cavitation events more likely.
This cycle of dysfunction increases the likelihood of embolism, where a break in the continuous stream of water in the xylem vessels occurs, ultimately resulting in these vessels being plugged and becoming non-functional. The physical structure of xylem vessels, with rings to maintain shape and small perforations between vessel elements, is adapted to handle pressure changes, but under extreme tension, even these adaptations may not prevent cavitation and embolism. Large trees, requiring more tension to pull water to greater heights, are particularly susceptible to these phenomena.
Environmental stressors like drought and freezing can exacerbate these issues. Drought reduces water potential, increasing the risk of air bubble formation and interruption of water flow. Freezing causes the exclusion of dissolved air from ice, and upon thawing, residual air bubbles can expand causing cavitation.