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In simplest terms, the pressure chamber can be thought of as
measuring the "blood pressure" of a plant, except for plants it
is water rather than blood, and the water is not pumped by a
heart using pressure, but rather pulled with a suction force as
water evaporates from the leaves. Water within the plant mainly
moves through very small, interconnected cells, collectively
called xylem, which are essentially a network of pipes carrying
water from the roots to the leaves. The current model of how
this works is that the water in the xylem is under tension, and
as the soil dries, or for some other reason the roots become
unable to keep pace with evaporation from the leaves, then the
tension increases. Under these conditions you could say that the
plant begins to experience "high blood pressure."

Simply put, the pressure chamber is just a device for applying
air pressure to a leaf (or small shoot), where most of the leaf
is inside the chamber but a small part of the leaf stem (the
petiole) is exposed to the outside of the chamber through a
seal. The amount of pressure that it takes to cause water to
appear at the petiole tells you how much tension the leaf is
experiencing on its water: a high value of pressure means a high
value of tension and a high degree of water stress. The units of
pressure most commonly used are the Bar (1 Bar = 14.5 pounds per
square inch) and the Mega Pascal (1 MPa = 10 bars).

Because tension is measured, negative values are typically
reported. An easy way to remember this is to think of water
stress as a "deficit:" the more the stress, the more the plant
is experiencing a deficit of water. The scientific name given to
this deficit is the "water potential" of the plant. The actual
physics of how the water moves from the leaf within the pressure
chamber to the cut surface just outside the chamber is more
complex than just "squeezing" water out of a leaf, or just
bringing water back to where it was when the leaf was cut. In
practice, however, the only important factor is for the operator
to recognize when water just begins to appear at the cut end of
the petiole.

Additional information available at

[Summary provided by the Fruit and Nut Research and Information Center.]