Root Pressure

Root Pressure

As mineral ions from the soil are actively pumped into the root's vascular tissue, water follows along its own water-potential gradient, and this raises the internal pressure of the xylem — a positive pressure called root pressure, capable of pushing water a modest distance up the stem.

Demonstrating root pressure: on a small, soft-stemmed plant, on a humid morning, cutting the stem horizontally near its base with a sharp blade causes drops of solution to ooze out from the cut — driven by positive root pressure. Fitting a rubber tube over the cut as a sleeve lets the exudate be collected and its rate and composition measured.

Root pressure is also what produces guttation — at night and in the early morning, when transpiration (evaporative loss) is minimal, excess water is instead pushed out in liquid form as droplets from special pores near the tips of grass blades and the leaves of many herbaceous plants. …