Root pressure

[1] Root pressure occurs in the xylem of some vascular plants when the soil moisture level is high either at night or when transpiration is low during the daytime.

At night in some plants, root pressure causes guttation or exudation of drops of xylem sap from the tips or edges of leaves.

Xylem sap will exude from the cut stem for hours or days due to root pressure.

The Casparian strip prevents mineral nutrient ions from moving passively through the endodermal cell walls.

The Casparian strip substitutes for their lack of cell membranes and prevents accumulated ions from diffusing passively in apoplast pathway out of the endodermis.

The main contributor to the movement of water and mineral nutrients upward in vascular plants is considered to be the transpirational pull.

Some trees "bleed" xylem sap profusely when their stems are pruned in late winter or early spring, e.g. maple and elm.

Such bleeding is similar to root pressure only sugars, rather than ions, may lower the xylem water potential.