Description of how the pressure bomb works

Stem parts, leaves, branches or whole plants are placed into the chamber so the cut end protrudes through the specimen holder. Pressure (nitrogen) is applied to the plant part until a drop of sap is observed at the cut end. The pressure required to force a drop of sap from the sample is equivalent to the force with which water is held to plant tissues by forces of adsorption and capillarity.

Sample selection

No matter what type of plant tissue (a leaf, twig, a leaf with or without petiole or the whole plant) is going to be selected according to the species, the sample should be taken from the part of the plant stratum that was previously selected on the calibration run. The tissue should be healthy and well developed, with no damages by insects, plant pathogens, or mechanical injuries. Also, such plant tissue should be representative of the plant in terms of sunlight and age.

Sample preparation

When possible, cut the samples with a razor blade and run measurements in the field. If this is not possible, cut each sample and place it in a ziploc plastic bag, and immediately put them in a cooler with ice. Plant samples should not be in contact with ice, however. They should be taken to the laboratory as soon as possible, where all measurements will be taken.

Basic steps to obtain the plant water potential (PWP) or plant moisture stress (PMS) measurements:

• Step 1. Close the metering valve, turning it clockwise.

• Step 2. Turn the control valve to "OFF" position.

• Step 3. Open compressed gas cylinder sealing valve by turning it counterclockwise.

• Step 4. Take out the specimen holder from pressure vessel and put it in the hole of sample              preparation board.

• Step 5. Choose the sample to run (for example a branch). For initial operation a round rod of             1/8" to 1/4" in diameter for the sample can be replaced to close the specimen holder.              If the diameter to be closed is less than 1/8", choose the support washer with 9/64"              hole. If the diameter to be sealed is bigger than 1/8", select the support washer with              the 1/4" opening. Then, choose the rubber sealing sleeve that best matches the stem              diameter.

• Step 6. Adjust the sealing sleeve on the stem, and push the stem into the specimen holder.             Then turn the closing knob clockwise until the compressing metal eyelet (or ring)             squeezes the sealing sleeve enough around the stem to maintain it securely. This             should be done gently to avoid stem disruption. Make sure the cut edge of the stem             protrudes enough up through the specimen holder.

• Step 7. Maintaining the specimen holder by the sealing cap, put the branch sample into the             pressure vessel and then push the specimen holder down into place in the pressure             vessel. Use both thumbs to push the specimen holder down. After this, turn the             specimen holder 45º clockwise, in order to lock it into the cams of the pressure             vessel.

• Step 8. Rotate the control valve to the "PRESSURIZE" position.

• Step 9. Open the metering valve slowly rotating it counterclockwise and observe the pressure             buildup on the read-out gauge. Put the valve to the rate of increase in the rate you             want for this test.

• Step 10. As the pressure is going up into the pressure vessel, watch carefully the cut end of             the stem or petiole of the plant tissue. If you notice some pressure leak, tighten the             sealing knob forward to produce higher sealing pressure and stop the pressure             movement. CAUTION: Take with your left hand a magnifying glass and place it next             to the protruding cut stem (never place your face too close to the chamber to prevent             an accident).

• Step 11. When the sap starts flowing at the cut end of the stem, the equilibrium pressure for             the sample is obtained, and the reading is recorded.

• Step 12. Pressurized nitrogen is released slowly, using the control valve by turning it to the             "EXHAUST" position.

• Step 13. Take away the sample by slightly loosening slightly the locking knob when the             specimen holder is still in the pressure vessel. After that, turn the closing cap of the             specimen holder 1/8" turn (45º) in a counterclockwise direction. Then, go to the             next sample.

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