This video is a visual 10 step instruction manual how to use a pressure bomb for measuring water pressure in the xylem of transpiring leaves. It is geared towards undergraduate students working in plant physiology labs as well as viewers who are interested in plant biology.
Water in the xylem is under negative pressure. This tension can be measured by a “pressure bomb”. Tension pressure in a non-transpiring leaf is “equivalent” to water potential of cells surrounding the xylem (mesophyll cells), because the xylem’s osmotic potential is negligible.
A single leaf is sealed in the pressure chamber with the cut surface protruding through a rubber stopper. Pressure is applied to the shoot from a tank of compressed air (or nitrogen) until xylem sap appears at the cut ends of the xylem. The hydrostatic pressure inside the xylem elements is then identical to the applied external pressure. The amount of pressure that must be applied to force water out of the leaf cells into the xylem is regarded as equal to the xylem tension when the leaf was cut from the plant.
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