The pressure that can build up in the root systems of plants so as to force water upwards through the xylem vessels. The force is a function of the *osmotic potential
of the root cell contents and is evidenced by the continuing flow of water from the cut surface of a recently severed stem. Positive root pressures tend to build up at night, when the rate of transpiration is very low, and lead to the process of *guttation
. Root pressure is not of great importance in water uptake and varies seasonally, being lowest in magnitude during the summer.