Question

Please explain – in detail - the Cohesion/Tension Theory of water transport (aka transpiration).

Answer 1

Transpiration is the inevitable loss of water happening through the aerial parts of a plant. The transpiration stream assists in the delivery of minerals and other substances from roots to the shoots and leaves. Transpiration also results in evaporative cooling, which can lower the temperature of a leaf by as much as 10-15 oC compared with the surrounding air. The transpiration occurs due to the opening of stomata in response to the turgor pressure in the guard cells. Water transport, the movement of water from roots to leaves in plants even up to the height of 100 meters can be explained by the cohesion adhesion theory. This theory relies on the fact that the water is a polar molecule.  

The capillary action and the inherent surface tension of water are the primary mechanisms of water movement in plants. a continuous water column is forming from roots to leaves through xylem. Since the water is constantly lost by transpiration from the leaf, this creates a transpiration pull. The transpiration pull is the negative pressure creates in the leaves as a result of the loss of water due to transpiration. Due to this negative pressure happened in the upper regions or leaves, the water column moves upward to balance the pressure. During this upward movement, the water can enter into the plants from the soli to roots through the process of diffusion. Since the negative pressure caused by the transpiration pull is the major mechanism involved in the upward movement of the water column in the xylem, this process will occur continuously during the day times.