Measuring Stem Water Potential (SWP)
How it works
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 cut surface of 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 unit
of pressure most commonly used is Bar. (1 Bar = 14.5 PSI)
Principle of Operation
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 inter-connected cells,
collectively called xylem, which are essentially a network of pipes carrying
water from the roots to the leaves. The water in the xylem is under tension.
As the soil dries or humidity, wind or heat load increases, it becomes
increasingly difficult for the roots to keep pace with evaporation from
the leaves. This causes the tension to increase. Under these conditions
you could say that the plant begins to experience "high blood pressure."
Since 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 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. However, in practice,
the only important factor is for the operator to recognize when water just
begins to appear at the cut end of the petiole.
Stem Water Potential
Stem water potential is a reading of what is going on within the xylem
of the plant. To take this reading a reflective plastic bag is placed
on a lower canopy, shaded leaf, and the bag is left on long enough (as
little as 10 minutes may be sufficient) to allow the water tension in the
leaf to come to equilibrium with the water tension in the stem or trunk
of the plant. The leaf is removed from the plant and tested in the
pressure chamber while still enclosed by the bag. For
more information about Stem
Water Potential click here.
1. Cut a leaf from plant to be tested. Use a sharp blade to make
a clean cut. Avoid breaking the sample.
2. Insert the leaf into the hole in the lid so that the end of the sample
barely protrudes through the lid. Twist the Compression Screw clock-wise
to seal the sample.
3. Put leaf inside chamber and lock down the lid into chamber. Place
pins completely through holes so they are locked.
4. Ensure you are wearing eye protection in case sample slips out of chamber.
Place foot on foot-rest, swing eye lens in place and begin pumping instrument.
Instrument should increase pressure about 1/2 bar per stroke. If
instrument does not increase pressure, check sample seal and also check
for obstructions between chamber lid O-ring and chamber wall.
5. While pumping on the down stroke watch sample through eye lens for
a film of water to appear. When water appears, stop pumping and record
pressure indicated on gauge.
6. Use Pressure Relief Valve to release the pressure completely, remove
pins and lid and you are now ready to measure another sample.
CAUTION: Ensure you have released
all the pressure from the chamber before removing the Chamber Pins and
the Chamber lid. Failure to do this could cause injury. Always wear
eye protection when using the instrument!
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