Pathway along apoplast
Pathway through symplast
Plants lose a large volume of water from transpiration, the evaporation of water from a plant’s surface. This creates a negative pressure at the stomate opening (where water was lost).
Water is replaced by the bulk flow of water and minerals, called xylem sap, from the steles of roots to the stems and leaves.
Is sap mainly pushed up from the roots, or pulled up by the leaves?
Pushing Xylem Sap: Root Pressure
At night, when stomates are closed, transpiration is very low. Root cells continue pumping mineral ions into the xylem of the vascular cylinder, lowering the water potential.
Water flows in from the root cortex, generating root pressure.
Root pressure sometimes results in guttation, the exudation of water droplets on tips or edges of leaves … usually in small plants.
Positive root pressure is relatively weak and is a minor mechanism of xylem bulk flow.
Pulling Xylem Sap: The Transpiration-Cohesion-Tension Mechanism
Water is pulled upward by negative pressure in the xylem
Water vapor in the airspaces of a leaf diffuses down its water potential gradient and exits the leaf via stomata. (This creates a low - a negative pressure).
Transpiration produces negative pressure (tension) in the leaf, which exerts a pulling force on water in the xylem, pulling water into the leaf. ( H --> L).
Generation of transpiration pull
Microfibrils in cell wall of mesophyll cell
Microfibril (cross section)
Cohesion and Adhesion in the Ascent of Xylem Sap
The transpirational pull on xylem sap is transmitted all the way from the leaves to the root tips and even into the soil solution.
Transpirational pull is facilitated by cohesion of water molecules to each other (so water column rises unbroken) and adhesion of water molecules to the xylem vascular tissue.
Drought stress or freezing can cause cavitation, the formation of a water vapor pocket by a break in the chain of water molecules. This can be fatal to the plant.