Root pressure is a transverse osmotic pressure required for the plants for the movement of water in an upward direction.
Thus, root pressure is a force required to drive the fluid upward in the xylem.
In higher plants, movement of water mainly occurs by either transpiration or root pressure process.
Transpiration pull is more during the daytime because stomata are opened in the daytime.
Thus, only root pressure helps in the movement of water at night.
Root pressure is caused by the absorption of nutrients from the soil.
The effect of root pressure is observed more at night or early in the morning since stomata are closed at night because of the absence of sunlight at night.