When selecting a tree from an irrigation block, choose a tree that is a good representative of the block. Choose a tree that is in good condition. The tree should be at least 3-4 rows from the outside of the orchard. This will ensure good representation of the irrigation block. Trees on the edge of the orchard normally have additional stress on them due to more exposed canopy.
Select a leaf from the lower canopy and close to the trunk of the tree. The leaf should be on the shady side of the tree. An established limb and not new growth.
Place the leaf into a Stem Water Potential Bag. Close the top of the bag so that just the petiole is sticking through.
The bag should stay on the leaf for at least 10 minutes. You can go throughout the whole orchard and put on all the bags first, then come back and make the readings. It is OK to put the bags on earlier in the morning and then come back for sampling in the afternoon (12:30-2:30 PM Pacific Time) – Normally 1 or 1/5 hours each side of “solar noon”.
Return to the leaf when you are ready to make the measurement. You can just pull the leaf from the tree by pulling upward against the petiole.
Sometimes cutting the petiole on a 45 degree angle will help it fit through the lid easier…..
Slide the petiole through the lid. Twist the Compression Screw to tighten the gasket around the sample.
Focus on the 1/2 moon crescent. This is the area where the xylem comes to the surface and where you want to watch for water to appear.
Small amount of water begins to appear………..
Notice the color change in the crescent. This is the official end-point. You should stop the increase of pressure at this point and record the reading. This is the true water potential of the plant.
With a little more pressure added the water begins to bubble or boil. This is past the end point. To recheck the sample, you can simply release a small amount of pressure from the chamber and allow the water to suck back down into the xylem. Re-pressurize the sample slowly and check it again.
Many thanks to Dr. Ken Shackel for helping with this tutorial and his extensive research and work with prune!