9.2 Applications and skills
9.2.2 Pratical 7: Factors affecting transpiration
Transpiration is the process by which water travels through xylem tissue from the roots to the leaves and eventually into the atmosphere. The transpiration stream is maintained by two important forces: the push of root pressure from below, and the pull of evaporation from the leaves above. We know that root pressure is an active process because if roots are deprived of oxygen, guttation does not occur. Evaporation at the leaves does not require energy – it is a physical process that occurs because of the adhesive and cohesive properties of water (See: Chapter 2.2). It follows that transpiration rates can be affected by physical factors:
- Light – stomata are normally open during the day to allow carbon dioxide to diffuse in. Transpiration tends to increase when conditions are brighter.
- Humidity – moist air surrounding leaves prevents transpiration. Water does not evaporate into saturated air.
- Temperature – higher temperatures increase evaporation.
- Wind – increases evaporation.
Measuring rates of transpiration using a potometer
Using a potometer, one can directly measure the rate of uptake of water in plants under different atmospheric conditions. The most important assumption made when using a potometer is that all the water taken into the plant enters into the transpiration stream. However, some of the water may escape the xylem, entering cells in order to increase turgidity or aid in photosynthesis. These assumptions mean that transpiration rates can be measured only indirectly using a potometer.
There are many ways to set up a potometer. Each set-up includes a leafy shoot, a length of rubber tubing and a calibrated glass tube.
Method 1: A simple potometer
You will need:
- Shallow tray
- Leafy shoots (e.g. Phaseolus)
- Sand and two clamps
- Parafilm or petroleum jelly
- Rubber tubing
- Calibrated glass tubing (a pipette)
- Fill the shallow tray with water at room temperature.
- Attach the rubber tubing to the calibrated pipette and seal with parafilm. Submerge the tubes completely in the water, filling the tubes so that no air bubbles are present. You may use the syringe to pull water through the tubes if they are very thin.
- Submerge the plant cutting in the water. Cut the stem at an angle while the plant is submerged. Make sure the stem is entirely submerged, otherwise air bubbles will get into the xylem tube.
- Without removing the cutting from the water, insert the cut end of the stem into the rubber tubing. Seal with parafilm.
- Carefully remove the entire apparatus from the water. Attach it to the clamp and stand as shown above. You may lose some water from the top of the pipette. This is acceptable.
- Allow the apparatus to stand and equilibrate for 10 minutes before taking your first reading.
- Take a reading once every 5 minutes for 30 minutes.
Method 2: The bubble potometer
A simple potometer like the one shown in Method 1 is very easy to set up and does not require any specialised equipment. However, results may be skewed because evaporation can occur from the tip of the pipette. A more accurate way of measuring transpiration rates is described below.
You will need:
- Capillary tube
- Shallow plastic tray
- Syringe or reservoir cup
- Submerge the glass tube until it is full of water.
- Cut the leafy stem under water and insert it with a bung into the glass tube. Seal with parafilm.
- Remove the tubing from the water, keeping your finger on the bottom to prevent water from escaping the tube. Make sure the reservoir valve is closed.
- Very quickly release your finger from the bottom of the tube in order to allow an air bubble in the tube. Immediately, put the end of the tube into the beaker and set up as shown.
- Carefully open the reservoir to adjust the position of the bubble. Close the valve.
- Fill the reservoir with water. Allow the apparatus to equilibrate for 10 minutes before beginning your trials.
- Measure the distance the bubble travels over time.
- After each trial, reset the bubble by opening the reservoir valve. This will push the bubble to its original position.
In the lab
To prevent evaporation from the pipette tip, place a drop of oil at the tapered end of the pipette.
- If your glass tube is not calibrated, use felt-tip pen to mark the distance the bubble has travelled every 5 minutes.
- Don’t be discouraged if you lose the bubble during the set-up. Be patient – it takes practice to set up a potometer properly.
Figure 9.2.2a – A simple potometer
As the plant transpires, measurements can be read from the pipette.
Figure 9.2.2b – A bubble potometer
The reservoir is used to reset the bubble for the next trial.
Figure 9.2.2c – Bubble potometer variation
A syringe can be used to reset the bubble position. These types of potometer can be easily purchased from school science suppliers.