Question

What are the limitations of root pressure theory?

Hint:

Think of root pressure as a "small push from below" and transpiration pull as a "powerful pull from above." For a small plant, a push might be enough, but for a tall tree, you need the strong pull.

Answer 1

The root pressure theory suggests that a positive hydrostatic pressure generated in the roots can push water up the xylem. While root pressure does exist, it has several limitations as the primary mechanism for water transport in plants:

Insufficient Force: The magnitude of root pressure is typically low (around 1-2 atmospheres), which is only enough to push water up a few meters. It cannot account for the ascent of sap in tall trees, which can exceed 100 meters in height.
Not a Universal Phenomenon: Root pressure is not observed in all plants. For example, gymnosperms and many other tall trees do not generate any significant root pressure.
Absent During Active Transpiration: Root pressure is highest at night or in conditions of high humidity when transpiration is low. During the day, when water demand is highest due to active transpiration, the water column in the xylem is under tension (negative pressure), not positive pressure.
Slow Rate: The rate of water movement attributed to root pressure is much slower than the actual rates measured in the stems of actively transpiring plants.
Therefore, while root pressure can explain guttation (exudation of water droplets from leaf margins) in small herbaceous plants, the main driving force for water movement over long distances is the cohesion-tension or transpiration pull theory.