If the stem of a young seedling is cut off just above the soil, the stump will often exude sap from the cut xylem for many hours. If a manometer is
sealed over the stump, positive pressures can be measured. These pressures can be as high as 0.05 to 0.5 MPa.
Roots generate positive hydrostatic pressure by absorbing ions from the dilute soil solution and transporting them into the xylem. The buildup of solutes in the xylem sap leads to a decrease in the xylem osmotic potential and thus a decrease in the xylem water potential. This lowering of the xylem provides a driving force for water absorption, which in turn leads to a positive hydrostatic pressure in the xylem. In effect, the whole root acts
like an osmotic cell; the multicellular root tissue behaves as an osmotic membrane does, building up a positive hydrostatic pressure in the xylem in response to the accumulation of solutes.
Root pressure is most likely to occur when soil water potentials are high and transpiration rates are low. When transpiration rates are high, water is taken up so rapidly into the leaves and lost to the atmosphere that a positive pressure never develops in the xylem.
Plants that develop root pressure frequently produce liquid droplets on the edges of their leaves, a phenomenon known as guttation.
Positive xylem pressure causes exudation of xylem sap through specialized pores called hydathodes that are associated with vein endings at the leaf margin. The “dewdrops” that can be seen on the tips of grass leaves in
the morning are actually guttation droplets exuded from such specialized
pores. Guttation is most noticeable when transpiration is suppressed and
the relative humidity is high, such as during the night.