- Know how to determine water uptake in a leafy shoot
- Be able to investigate the effect of environmental conditions on water uptake
- Care should be taken not to break the capillary tubing when pushing the cutting into the connecting tubing
- Students should let you know if they find the plant sap irritating to their skin
- Ensure the room is well ventilated to remove small amounts of noxious gases that are given off by some plant materials
- Large leafy shoot – the end must be cut and kept under water
- Capillary tubing with a short rubber connection tube attached at one end
- Grease pencil or fine marker pen
- Ruler
- Large bag
- Spray bottle
- 250cm3 Beaker
- Petroleum jelly
- Rope / thread
- Scissors
- Graph paper to sketch
- Stopwatch
- Independent
- The humidity in the environment around the plant
- Dependent
- The volume of water (cm3) taken up in transpiration, from which we can calculate the transpiration rate. This was done over a period of 3 minutes
- Control
- The surface area of the leaves / the plant itself as different plants would have different amounts of respiration and photosynthesis
- The increase in humidity per experiment, we added a spray of water each time. A spray is equivalent to 10/9ml
- In this investigation water uptake is measured under different humidity conditions using a bubble potometer. Our potometer had a reservoir to allow for easy refill, the length the bubble travels along the tubing is assumed to be equal to the volume of water utilised by transpiration. Because of this we can infer and calculate the transpiration rate.
- In our experiment we were investigating how humidity affected the transpiration rate. To do this we added one spray each experiment as this would give a regular reading which would lead to more in depth interpretation of the results
- Lay the capillary tube and rubber connector under water and fill both parts
- Carefull select a leaft shoot which has a stem as close in diameter to that of the rubber connector as possible. Keep the end of the shoot under water at all times while you select and trim it as necessary
- Quickly insert the shoot into the rubber connector, making sure the fit is as tight as possible. Carry out this step under the water
- Firmly clamp the capillary tube to the stand with the shoot at the top end. Place the bottom end of the capillary tube into the beaker of water. Two people may need to carry out this step to avoid the leafy shoot becoming detached
- Smear petroleum jelly around the join between the shoot and the rubber connector to ensure an airtight seal. Dry the leaves by blotting them gently with paper towels
- Leave the apparatus for 5 minutes to allow water to be drawn up into the end of the capillary tube. A small air bubble should be present at the end of the capillary tube from the time spent out of water in step 4. If there is no air bubble, remove the tube and draw water out and then replace
- Then at each condition you have chosen, using the ruler record (cm) how far the bubble has travelled (measure from the same part of the bubble both times) after a set amount of time. For my experiment I chose 3 minutes
- To reset the bubble open the reservoir slowly pushing the bubble back to the start of the potometer
- When your investigation is complete, work out the leaf area of you shoot by drawing around each leaf on graph paper and counting up the squares
Volume of cylinder = ½ x π x r2 x h
If we take 3 squirts as an example, h is height which is the same as distance the bubble travelled as the water displaced would have been equal.
h = 0.8
r = 0.5
We can work out the rate of water uptake by dividing volume of water taken up over the course of the experiment divided by the time the experiment went on for
We can see from these graphs that as the humidity of the air around the plant decreases the distance of the bubble travelled and therefore the rate of water uptake. This is because the transpiration stream works by having perpetual motion up the plant. To do this water needs to come in by the capillary tube and out the stoma of the leaves. The water exiting the stoma is done by a passive process known as diffusion. The rate of diffusion depends largely on the concentration difference between the leaf and the surroundings. By increasing the water content of the air around the plant we reduce the concentration difference. This causes the process to slow down which is reflected in the result. The rate of water uptake reduces from 0.5585 to 0.06108.
Questions
1.Why is it necessary to form an airtight seal?
It is necessary to form an airtight seal between the shoot and the capillary tubing. This is to make sure that there is no loss of pressure. The transpiration stream has a pull on the water from the potometer to draw it up into the xylem which requires closed conditions that means that you maintain suction. The purpose of this experiment is to simulate transpiration from the leaves to the roots in a natural plant. In real world conditions there would not be holes which would be detrimental to the turgor pressure of the stream.
2.The limitations of an investigation are factors that reduce accuracy and reliability of results. They may arise from variables that are difficult to control. What are the limitations of this procedure?
The first, and most impactful limitation was that to maintain accuracy we used the same plant for every experiment. We also used the same bag and because of the time constraints we only had a brief interval between humidity conditions. This meant that some residual increased humidity would affect the true value of humidity. So when I say 2 squirts it means 2 squirts and the remaining water in the air from the first experiment. The next limitation is that our bag was sealed by tying it at the base of the stem. Although this stops a majority of water escaping it doesn’t create a microclimate within the bag as there is still a gap in which water can escape. This happened for all experiments sop this limitation is consistent throughout all of the experiments but I cannot reliably estimate the volume of water retained in the bag.
3.Suggest ways of reducing the limitations to give more accurate and reliable data
I should make sure to do the experiments at different conditions at regular intervals of 24 hours. This would eliminate the effect of residual humidity. You would need to make sure that the conditions are similar on all occasions, this includes temperature, wind and light intensity. I would also use cable ties and make sure the base of the stem was covered off by the bag to make sure there was very little to no gap for water to escape through.