Question

Explain the transpiration pull theory for the ascent of sap?

Answer 1

Transpiration pull:

1. The force that assists in pulling water up from roots to leaves is known as the transpiration pull or the suction force.
2. A portion of the water in leaves is used for photosynthesis, and any extra water is discharged into the atmosphere through stomata.

Transpiration pull theory

1. Transpiration and Cohesion Pull Theory is based on the following characteristics and was initially proposed by Dixon and Joly (1894).
   Cohesion and adhesion I
2. Cohesion is the mutual attraction of water molecules.
3. Lignin and cellulose make up the tracheid and xylem vessel walls, which have a great attraction for water (adhesion).
4. As a result of transpiration pull, the xylem sap creates a negative pressure or tension that is conveyed to the root.
5. Through stomata and cuticular transpiration, the wet walls of mesophyll cells of leaves lose water vapour.
6. Mesophyll cells transpire a sufficient amount of water through their intercellular gaps.
7. This is so because the leaf's wet air has a higher water potential than the dry air outside.
8. Water diffuses from the stomata as a result, and the diffusion pressure deficit (DPD) rises.
9. As a result, adjacent mesophyll cells and ultimately vascular tissue are drained of their water.
10. The roots receive a transmission of this strain.
11. Water molecules exhibit cohesion, and water molecules and vessel walls exhibit adhesion.
12. The force known as the transpiration pull is responsible for pulling the water column upward because cohesive and adhesive forces prevent it from breaking.