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

Pump Up Chamber Artist's Concept PMS Instruments

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. h3. 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.
Cutting a leaf sample for the pump up chamber PMS Instruments

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.
Inserting the stem on a pump up chamber PMS Instruments

3. Put leaf inside chamber and lock down the lid into chamber. Place pins completely through holes so they are locked.
Inserting the bag into the pump up chamber PMS Instruments

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.

Intake Stroke
Lift up instrument to take in air
Using the pump up chamber how to stroke PMS Instruments

Compression Stroke
Push down instrument to compress air into chamber
Using the pump up chamber how to stroke PMS Instruments

Look to see if water has come to surface of cut stem
Using the pump up chamber how to stroke PMS Instruments


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!