I intend to investigate only one factor, so to make it a fair test, all the other conditions must be kept constant. As I have decided that the best way to measure the rate of transpiration is with a potometer, and I want to get continuous results, the best factor to investigate would be one of temperature, light intensity, or wind speed. However, water is used up in photosynthesis, and both temperature and light intensity have a direct effect in the rate of photosynthesis. This would mean that the results would be showing the effect of light intensity on transpiration and photosynthesis combined which would be pretty useless. Therefore, I have chosen to investigate the effect of wind speed on the rate of transpiration, as I can fairly easily control the wind speed with a fan, and apart from any cooling effect, it has very little effect on photosynthesis, unless you increase it until the stomata close.
Therefore, I must make sure that all the other factors listed above are kept constant; as long as I use the same shoot, the stomatal density, surface area of the plant, and the species of plant will remain the same, and as long as I treat the plant properly (see method) when attaching it to the potometer, then the plant will never be water stressed, and so that will have minimal effect, and the experiment will be conducted in a room with only artificial light, so the light intensity will remain constant. I will monitor the humidity and the temperature throughout the experiment, and be careful not to breathe on the plant too much.
Preliminary Work
In order to check that everything would work, and there would not be any unexpected errors, I did a trial experiment. I set up the potometer (see method) with the plant, and measured the wind speed in the middle of the plant’s leaves (this would give an approximate for the average wind speed over the leaves of the plant). I realised that it takes a while for the rate of transpiration, as determined by the uptake of water by the plant, to react to a change in the conditions. This means that when conducting the experiment, I will have to leave the experiment running with all the conditions constant before I take a reading of how fast the water is being taken up. I measured the time for the plant to react to a change in the conditions, and found that when the only changing condition was the wind speed, it took about five minutes to react. I will take account of this in the method.
During this experiment, after some measurements with my anemometer, I noticed that the wind speed was not directly proportional to the distance from the fan. I put this down to the spreading of the wind, and realised that it would not be easy to form an equation relating the wind speed and the distance from the fan. Therefore, I decided that in order to be accurate, instead of measuring the distance of the fan from the plant, I would measure the wind speed at the centre of the plant, and that would be my dependant variable. When the wind speed was very high, the rate seemed to level off and even drop a little. From what I have read in the Jones &Jones Biology Textbook, I can attribute this to the closure of the stomata, a mechanism the plant has to minimise water loss when it is water stressed, or the rate of transpiration is very high. I realised that if I started to take the readings from the fastest wind speed to the slowest, then some of the stomata may close, and so the results would not be accurate. Therefore, I will do the experiment starting with the slowest wind speed, and increasing gradually.
I also conducted an experiment in which I produced a replica of the surface of some leaves with varnish, and examined this under a microscope to count the density of stomata on the upper and undersides of a privet and of a laurel leaf. I found that almost all of the stomata were on the underside of the leaf – could not see any on the top, and also that privet leaves had a stomatal density more than three times greater than that of laurel. That is why I decided to do this experiment with a privet shoot, because more stomata will mean more transpiration, and so the results can be more accurate (it is easier to accurately measure a large amount of anything than a small amount). I also considered that a high density of stomata suggests that privet has adapted to live in conditions where water is plentiful, this might suggest that a privet shoot would be less likely to close its stomata than a laurel shoot, however I have no evidence for this.
Prediction
I predict that as I increase the wind speed, the rate of transpiration will increase, until the rate becomes so great that the stomata close, (and the leaves blow off). This is because an increase in the wind speed, means faster removal of any moisture from just below the stomata, and so means that the gradient of water from inside the stomata to the outside is steeper, and therefore the rate of transpiration increases. However, I think I can go further than this by saying that the steepness of the water gradient is directly proportional to the wind speed, and the rate of evaporation is directly proportional to the steepness of the water gradient. On the other hand, if there were no wind at all, there would still be some transpiration. This is because the molecules of air are still moving, there is just no net movement. Therefore, it would be incorrect to say that the rate of transpiration is directly proportional to the wind speed, so my prediction is that as the wind speed increases, so does the rate of transpiration. I predict that the final graph will be a straight line that does not go through the origin, and levels off, or even dips when the wind speed gets very high, and the stomata close.
Apparatus
A shoot of Privet
A potometer. (Thistle reservoir with tap at bottom, capillary tube, ruler, thicker tube with joint and bend, rubber connections between tubes, Vaseline to help make seals airtight, and water.)
An Anemometer
Some wire – to secure the plant-tube seal.
A micrometer
A thermometer
A humidity measurer
Method
- Set up the potometer as above.
- Go and cut a large shoot of privet from the healthiest looking bush.
- Immediately bring this to the lab and place its base under water, but keep the leaves dry.
- Cut a shoot with a diameter just under that of the tube, but be careful with the scissors, and keep the base of the shoot under water at all times. Then cover the sides of the base of the shoot with Vaseline, but do not get any on the cut bottom of the shoot.
- Put the end of the shoot into the rubber seal, making sure no air could get in, and tighten the seal with some wire.
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The whole of the apparatus should now be filled with water, and airtight, so now take the apparatus out of the water, checking the thistle reservoir is full of water, and leave it for a couple of minutes to get going. As the plant transpires, air should come into the end of the capillary tube as the water is used up. Do not allow the air to get all the way to the plant as it will interfere with the uptake of water into the xylem, and can cause problems if it gets in there.
- Close the blinds, so that no natural or changing light could get in, so only artificial light is lighting the room. Shut the doors, and make sure that there are no draughts with the anemometer. Set up the thermometer the humidity meter and make sure that both remain constant throughout the experiment.
- Set up the fan at a distance so that the wind speed in the middle of the plant, as measured by the anemometer, is 0.1m/s . Leave this running for five minutes, if necessary, opening the tap to the thistle reservoir so that the air does not get too close to the plant, and then measure how far the air water meniscus moves in two minutes, trying to be as accurate as possible, (the nearest millimetre). Repeat this twice more.
- Move the fan, and if necessary turn up the power on the fan, so that the wind speed in the middle of the plant is 0.25m/s, then do as above, waiting five minutes and repeating three times.
- Repeat this for the wind speed at 0.5, 0.75, and 1m/s. Then wash your hands.
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It is obviously not very useful to find out that a plant transpires 15mm of water in a minute as this means nothing. Therefore, you need to measure the diameter of the capillary tube with the micrometer. Once you know the diameter, divide that by 2 to get the radius. Then put that into the equation πr2 in order to find the area of a cross section of the tube. Multiply this by the distance the water-air meniscus travelled in the two minutes, and that is the volume of water transpired.
- Take the average of the three readings of each wind speed, and plot them on a graph.
Take care throughout this experiment not to break any glass, if you do, sweep it up and tell the master in charge.
Summary
This plan is as precise as possible. However, water is used up in photosynthesis, but this should be at a constant rate so would not affect the increase in the uptake of water (I already know that the graph is not going to go through the origin, so it will just affect how far up the graph starts). It also will be fairly inaccurate as it is not easy to read how far the meniscus has travelled to any degree of accuracy smaller than 1mm, but yet this will have a large effect on the results, and 0.5mm could be as much as 5% of the distance travelled, this inaccuracy would then be multiplied up by the area of a cross section of the capillary tube. It would not be very easy to improve this, unless I used a very accurate ruler, and a magnifying glass, but this seems unnecessary, as the inaccuracy would not make that great a difference to the graph.
This plan is valid as I am confident that I have taken into account all the factors that could affect the rate at which a plant takes up water. Water is used for transpiration, making the cells turgid, and for photosynthesis, it is produced by respiration. I have made sure in my plan that the plant is water stressed at no point, so all the cells would already be turgid. The factors that affect photosynthesis, light intensity, temperature and the concentration of carbon dioxide in the atmosphere should all remain constant, and the of the factors that affect transpiration, only the wind speed will change. Therefore, I am confident that any changes in the rate of transpiration can be attributed to the change in the wind speed.
It is reliable only as long as you use a very similar piece of privet that had been subjected to similar conditions before the experiment. But with the same piece of privet, under the same conditions, providing the privet hasn’t died, and the stomata haven’t closed, then similar results should be obtained every time.
I know light intensity does from a previous experiment, in which I measured the rate at which oxygen was produced from Elodea when a lamp was positioned at certain distances from the plant. I found out that the rate of photosynthesis was inversely proportional to the square of the distance the lamp was from the plant, hence was directly proportional to the light intensity. I found out that temperature has an effect on photosynthesis in the Jones and Jones Biology Textbook.