There is continuous loss of water through stomata of the leaves in the form of water vapor. This process is called transpiration.
The water lost due to transpiration is taken up from the xylem vessels and tracheids in the leaves. This loss of water during transpiration creates a suction pressure which pulls water from the xylem cells of root.
As a result, there is enhanced absorption and upward movement of water and dissolved minerals from roots to the leaves due to transpiration.
The effect of root pressure in transport of water is more important at night when stomata are closed.
When the stomata are open during the day, the transpiration pull is the major force in the movement of water and dissolved minerals in the xylem, especially in trees.