Root pressureis the transverse osmotic pressure within the cells of a root system that causes sap to rise through a plant stem to the leaves.

Root pressure occurs in thexylemof somevascular plantswhen the soil moisture level is high either at night or whentranspirationis low during the day. When transpiration is high, xylem sap is usually under tension, rather than under pressure, due totranspirational pull. At night in some plants, root pressure causesguttationor exudation of drops of xylem sap from the tips or edges of leaves. Root pressure is studied by removing the shoot of a plant near the soil level. Xylem sap will exude from the cutstemfor hours or days due to root pressure. If a pressure gauge is attached to the cut stem, the root pressure can be measured.

Root pressure is caused by active distribution of mineral nutrient ions into therootxylem. Without transpiration to carry the ions up the stem, they accumulate in the root xylem and lower thewater potential. Water then diffuses from the soil into the root xylem due toosmosis. Root pressure is caused by this accumulation of water in the xylem pushing on the rigid cells. Root pressure provides a force, which pushes water up the stem, but it is not enough to account for the movement of water to leaves at the top of the tallesttrees. The maximum root pressure measured in some plants can raise water only to 6.87 meters, and the tallest trees are over 100 meters tall.