Question

The greatest contribution of root pressure is

• A. to re-establish the continuous chains of water molecules in the xylem which often break under the enormous tensions created by transpiration.
• B. to play a major role in water movement up tall trees over 50 feet in height.
• C. to act as a main driving force to push water from the roots in the xylem towards the leaves.
• D. to give water high tensile strength and high capillarity action.

Answer 1

Answer: (A)

A positive pressure called root pressure is developed as various ions from the soil are actively transported into the vascular tissues of the roots, water follows its potential gradient and increases the pressure inside the xylem. It is responsible for pushing up water to small heights in the stem. It can't push water over long distances. The greatest contribution of root pressure may be to re-establish the continuous chains of water molecules in the xylem which often break under the enormous tensions created by transpiration. Root pressure does not account for the majority of water transport; most plants meet their need by transpiratory pull. Cohesion, adhesion and surface tension give water high tensile strength, i.e., an ability to resist a pulling force, and high capillarity, i.e., the ability to rise in thin tubes.