If pressure if applied to pure water or a solution, its water potential increases. This is because the pressure is tending to force the water from one place to another. Such a situation may occur in living cells. For example, when water enters plant cells by osmosis, pressure may build up inside the cell making the cell turgid and increasing the pressure potential. (Also, water potential of blood plasma is raised to a positive value by the high blood pressure in the glomerulus of the kidney). Pressure potential is usually positive, but in certain circumstances, as in xylem when water is under tension (negative pressure) it maybe negative.
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