Explain the transpiration pull theory of ascent of sap

Water and minerals from the soil enter a plant through the epidermis of the roots. Moving radially across the root cortex, they reach the xylem, where the xylem sap (water with dissolved minerals) flows upward. This upward movement, essential for transporting water and minerals from the roots to the uppermost leaves against gravity, is called the ascent of sap.

Scientists agree that this upward flow is driven by a transpiration pull. Transpiration from the leaves generates negative pressure in the xylem, pulling water upward. The most widely accepted explanation is the Cohesion-Tension Theory proposed by Dixon and Jolly.

This theory is supported by the physical properties of water:

• Cohesion: Water molecules stick to each other.
• Adhesion: Water molecules adhere to the walls of xylem vessels.
• Surface tension: Helps maintain a continuous water column.

These properties give water high tensile strength (resistance to breaking under pull) and high capillarity (ability to rise in narrow tubes). In plants, the small diameter of tracheids and vessel elements increases capillarity.

When water evaporates from stomata, it creates a thin continuous film of water on cell surfaces. This evaporation pulls nearby water molecules from the xylem into the leaves. Additionally, since the atmosphere has a lower water vapour concentration than the air spaces inside leaves, water diffuses outward, reinforcing the transpiration pull.