Transpiration pull is established in plants primarily through the evaporation of water from leaf cells.
The transpiration pull mechanism relies on the cohesive properties of water and the plant's structure to transport water from the roots to the leaves. Here's a breakdown:
- Evaporation from Leaf Cells: Water evaporates from the surface of mesophyll cells inside the leaves. This evaporation occurs through the stomata, which are small pores on the leaf surface.
- Water Potential Gradient: As water evaporates, the water potential in the leaf cells decreases (becomes more negative). This creates a water potential gradient between the leaf cells and the xylem, the plant's water-conducting tissue.
- Cohesion and Adhesion: Water molecules are cohesive, meaning they stick together due to hydrogen bonds. They are also adhesive, meaning they stick to the walls of the xylem vessels.
- Tension in the Xylem: The evaporation of water from the leaves pulls water upwards through the xylem vessels. This creates tension, or negative pressure, throughout the xylem, from the leaves all the way down to the roots. The reference mentions that this proper flow of water helps protect the plant from an embolism, which is a blockage in the xylem.
- Water Uptake from the Roots: The tension in the xylem pulls water from the soil into the roots and up into the plant.
In essence, transpiration creates a "pull" that draws water up the plant, utilizing the cohesive and adhesive properties of water within the continuous network of xylem vessels.
