Question

Most accepted theory of ascent of sap is

• A. Capillarity theory
• B. Root pressure theory
• C. Pulsation theory
• D. Transpiration pull theory

Answer 1

The correct option is D Transpiration pull theory

Most accepted theory of ascent of sap is transpiration pull theory. Upward conduction of water in the form of a dilute solution of minerals ions (sap) from roots to aerial parts is called ascent of sap. It occurs through the tracheary element of xylem. Many theories were put forward to explain the mechanism of the ascent of sap but the most accepted theory is Cohesive force or transpiration pull or cohesion-tension theory.

The transpiration driven ascent of xylem sap depends mainly on the following three physical properties of water:

Cohesion: mutual attraction between water molecules.

Adhesion: Attraction of water molecules to a polar surface (such as the surface of tracheary elements).

Surface tension: Water molecules are attracted to each other in the liquid phase more than to water in the gas phase.

Water column does not break its connection from the tracheary elements (vessels and tracheid) because of another force called adhesive force between the wall (lignocellulosic) and water molecules. It produces surface tension by high capillarity through tracheids and vessels. As water evaporates through the stomata since the thin film of water over the cells is continuous, it results in the development of strong negative water potential which further results pulling of water, molecule by molecule, into the leaf from the substomatal cavity and intercellular spaces, water diffuses into the surrounding air.