The roots of a plant use reverse osmotic forces to overcome the matrix potential of the soil. The stomates on the leaves of a plant transpire at different rates throughout the day. The resulting tension in the xylem column can be measured using a plant water status console device / water potential meter. For more details details.
A pressure bomb or pressure chamber or Scholander bomb is an instrument with which it is possible to measure the approximate water potential of plant tissues. A leaf attached to a stem is placed inside a sealed chamber and pressurised gas is added to the chamber slowly. As the pressure increases at some point sap will be forced out of the xylem and will be visible at the cut end of the stem. The pressure that is required to do so is equal and opposite to the water potential of the leaf.
PSP