what is root pressure

Root pressure is a force or hydrostatic pressure generated in the roots that helps drive fluids and other ions from the soil in an upward direction into the xylem vessels. Root pressure occurs in the xylem of some vascular plants when the soil moisture level is high. It is caused by the active distribution of mineral nutrient ions into the root xylem. Without transpiration to carry the ions up the stem, they accumulate in the root xylem and lower the water potential. Water then diffuses from the soil into the root xylem due to osmosis. Root pressure provides a force that pushes water up the stem, but it is not enough to account for the movement of water to leaves at the top of the tallest trees. The maximum root pressure measured in some plants can raise water only to 6.87 meters. Root pressure is considered to be the lesser force and is important mainly in small plants at times when transpiration is not substantial, e.g., at nights. The main contribution to the movement of water and mineral nutrients upward in vascular plants is considered to be the transpirational pull.