Required Practical 7: Rate of transpiration using a potometer Flashcards
Research question
How does the rate of transpiration in plants vary between different environmental conditions?
OR
Which environmental conditions allow for the greatest rate of transpiration by plants?
Independent variable
Different environments (mist, wind, bright light)
Dependent variable
Transpiration rates (as measured by potometers)
Safety
-Plant sap is a potential irritant, so wash your hands afterwards.
-Take care when handling the scalpel or scissors.
-Seek medical attention if you cut yourself.
Method
1) Submerge the glass pipette part of the potometer underwater.
2) Tip slightly to remove air bubbles. If air bubbles do not exit then use a pipette to suck them out directly. Figure 1 shows the technique for steps 1 and 2.
3) Repeat steps 1 and 2 with the rubber tubing.
Keep all parts submerged underwater at all times.
4) Remove the stem or leaf that you intend to use and quickly submerge underwater.
5) Using a scalpel or scissors, cut the tip of the stem underwater.
6) Insert the stem into the rubber tubing and the rubber tubing into the glass pipette.
7) Depending on the size of your equipment, you may also insert the glass pipette into the rubber tubing.
8) Make sure that the connection is airtight to prevent any new air bubbles forming. Use petroleum jelly to seal the connection points between the stem and the rubber tubing, and the rubber tubing and the glass pipette. You may also use a clip if your science technician has them.
9) Attach the potometer and plant to the boss clamp and stand, as shown in Figure 2.
10) Check for any air bubbles and remove them using a pipette.
11) Start the stopwatch.
12) Mark the water level of the glass pipette and take a reading after 1 hour (this can be shorter or longer depending on the type of plant that you have used).
13) Repeat all steps as many times as required for your research question.
Diagram of potometer
How do you calculate the rate of transpiration?
By dividing the average distance travelled by the time taken
Suggest why the above method is not 100% reliable and accurate for measuring the water loss from a plant.
-This is an indirect measurement that measures water uptake rather than water loss from the leaves.
-The water taken up will be used for other processes, such as photosynthesis, so the actual transpiration rate might be different from that calculated.
Did your leaves all have the same surface area? How might this affect the results?
If the surface area of the leaf was smaller than the other leaves, this might account for the difference in rate.
