Question

The most widely accepted theory for the ascent of sap in the tree is
A. Capillarity
B. Role of atmospheric pressure
C. Pulsating action of a living cell
D. Transpiration pull and cohesion theory of Dixon and Joly

Answer 1

The upward movement of water from root to aerial parts like leaves, stems, etc. is called the ascent of sap. The most widely accepted theory for the ascent of sap in the tree is the continuous movement of water from the stomata of the leaves to the atmosphere.

Complete answer: Transpirational pull and cohesion theory of Dixon and Joly is the most widely accepted theory for the ascent of sap in trees. This theory is based on three assumptions; cohesion between water molecules, continuity of water column and the transpiration pull. Cohesion is the property of molecules of the same substance to remain together. This theory has two essential features: first, cohesion between water molecules and adhesion of the water molecules to xylem tissues, and the transpirational pull. There is the presence of strong adhesive force between water molecules due to which these molecules cannot separate. Further, these water molecules remain attracted to the inner wall of the xylem duct. Water column cannot pull away from the xylem wall due to strong adhesion. So, the water column remains continuous from roots to leaves. Now, during transpiration (water loss) there is negative pressure developed in the root which causes transpirational pull in plants.
Hence, the correct answer is option D.

Note: Plant uses only 1.2% of absorbed water in metabolic activities, other 98% is lost to the atmosphere by transpiration. Transpiration mostly occurs during the night. The transpirational pull helps plants to absorb nutrients from the soil and it also maintains the temperature of plants.