IB Biology, IHS
Transpiration Assay: Gas Pressure Sensor
Adapted from the Vernier Lab Manual, LabQuest Lab 10 – revised 2014-15
IB Bio Practical # 7
In this assay you measure transpiration, the evaporation of water from a plant through open stomata, using a Vernier ™ Gas
Pressure Sensor and a LabQuest interface.
MATERIALS
Equipment
Vernier™ Gas Pressure
Sensor
Vernier™ LabQuest
Ring stand
2 utility clamps
Plastic syringe
Plastic tubing
Plastic tubing clamp
Valve with Luer connector
Lamp with 60-W bulb
Electronic balance
Tub or basin of water
Ruler
Consumables
Plant cutting with 8-10 leaves: laurel, holly, Viburnum…
Select a cutting without torn or missing leaves.
Consider bringing gardening gloves, as
some leaves are sharp!
Fig. 1
PROCEDURE
1. Position a ring stand, 2 utility clamps, and a Gas Pressure Sensor as shown in Figure 1. Set aside.
2. Obtain and submerge the following in a tub of tap water: tubing, tubing clamp, valve, syringe, and plant
cutting.
3. Connect a plastic syringe to one end of the tubing and use the syringe to slowly draw water into the tubing
until it is full.
Tubing clamp,
shown OPEN
4. Slip a plastic tubing clamp onto the tubing (see Fig. 2) and slide it around to the end that is
connected to the syringe, but don’t tighten the clamp yet.
Fig. 2
5. Fit a valve in the open position (the release is vertical), on the other end of the tubing.
6. Inspect the tubing while it is still immersed – it should be completely full of water.
7. Make a fresh cut at a 45-degree angle across the bottom of the cutting while it is under water. This minimizes the
introduction of air bubbles into the xylem. Plan the cut so the end of the cutting will fit the diameter of your tubing.
8. Still working under water, disconnect the syringe and insert the cutting in its place. Gently push the plant cutting down as
far as it will go without damaging it – aim for at least one centimeter of stem in the
tubing.
Fig. 3
9. Squeeze the tubing clamp shut, as shown in Fig. 3.
10. Close the valve (turn the release so it is horizontal).
11. Lift the plant/tubing from the tub. With a gentle half turn, attach the Luer connector on
the valve to the Luer lock on the Gas Pressure Sensor. See Fig.1. Now open the
valve! Caution: Do not allow water to enter the Gas Pressure Sensor.
12. Attach the plant/tubing to the ring stand as shown in Fig. 1. Note: the bottom of the
gas pressure sensor needs to be about 8 cm above the top of the tubing (where the cutting is inserted).
13. Check: √ …that the end of the plant is in direct contact with the water in the tubing, not blocked by an air bubble.
√ …that the leaves are dry; gently dab off excess water.
14. Illuminate the plant with a 60 W light source from approximately 30 cm away.
15. Allow the system 5 minutes to adjust to the environment.
16. Connect the Gas Pressure Sensor to a LabQuest and choose New from the File menu.
17. On the Meter screen, tap Length. Change the data-collection length to 900 seconds. (This is 15 minutes.) Select OK.
Select a sampling rate that will give you about 30 data points in those 15 minutes.
18. Start data collection.
19. When data collection has stopped, transfer the data to your data table (or, if time is short, print the table at home).
20. Then, dismantle the potometer but save the cutting!
21. Cut off and save all the leaves. Mass them. Record in a second data table.
22. Carefully cut out and determine the mass of a 4-cm2 (2 cm x 2 cm) section of leaf. Try not to include big veins. Record in
the second data table.
Data analysis (show all calculations!)
 Use the data in (21) and (22) to estimate the surface area in cm2 of your cutting.
 Express surface area in m2.
 Divide every pressure water loss value in your raw data table by surface area in m2, generating a column containing values
of cumulative pressure loss per square meter of leaf (kPa/m2). ‘kPa’ stands for kiloPascals.
 Make a line graph of cumulative pressure loss per square meter of leaf (mL/m2) vs. time (min).
 Calculate transpiration rate (kPa/m2min) from the slope of a linear portion of the graph. Write this value on the graph.