Computer Interfacing

Transpiration

Procedure - Prepare the Plant

1. Position the ring stand and the large utility clamp so that the pressure sensor is approximately 35 cm above the surface of the work area, as shown in Figure 1. The sensor's cord should be positioned above the pressure sensor while the blue 3-way valve should be positioned down towards the table top.

2. Using the scalpel or razor blade carefully cut the plant one inch above the soil. Place the plant down on a hard surface and make a new cut at a 45 degree angle near the base of the stem.

3. As one team member is cutting the plant, another should partially fill plastic tubing with water:

• Place two small utility clamps on the ring stand below the one holding the pressure sensor, as shown in Figure 1.
• Select an 18-24 cm piece of plastic tubing. If the tubing will not hold the plant cutting tightly, obtain a second length of tubing of the correct diameter and attach it to the first piece with a tubing connector.
• Bend the plastic tubing into a U shape. The tubing should not be attached to the pressure sensor. Make one side of the U about 3-4 cm longer than the other side. The short side will connect to the plant and the long side will connect to the valve on the pressure sensor.
• Use the Beral pipette to add water to the tube. Add water until there is about 3-4 cm of empty space at the short end of the tubing.

4. Insert the stem into the tubing. To insert the stem:

• Place a generous amount of petroleum jelly or vinyl tape 1.5 cm above the cut end of the stem. Make sure that none of the petroleum jelly covers the fresh cut on the stem. If it does, make a new cut just above the previous one.
• Raise the long end of the plastic tubing until water is at the top of the short end.
• Now carefully force the seedling down into the water and tubing until the petroleum jelly or tape connects with the end of the tubing. Important: make certain that no air bubble forms over the cut end of the stem.
• Spread the jelly around the stem and the tubing where they meet. This will allow for an air tight seal to prevent any loss of pressure during the experiment.
• If the stem is too large for the tubing, cut the stem higher at a point where the diameter is small enough to fit into the tubing. The tighter the fit the better the seal will be.

Note: Be sure that the tubing is filled to the edge with water. The water column must be continuous with the stem. If water slides down the tube away from the stem after the stem has been inserted, check to see that the seals are air tight. Do not allow petroleum jelly to clog the stem.

5. Secure the seedling in an upright position with one of the utility clamps. It should be positioned so that the cut stem is 3-4 cm below the level of the blue plastic valve on the pressure sensor.

6. Place the valve on the pressure sensor in the closed position. Connect the loose end of the plastic tubing to the pressure sensor valve and secure it with the remaining utility clamp. Be extra cautious to ensure that no water enters the valve of the pressure sensor. Once connected there should be about 10 cm of air in the tubing between the water level and the blue sensor valve. As long as the base of the plant is below the horizontal position of the 3-way valve there will be no chance of water being forced back into the valve.

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