Biology HL IA - Transpiration lab report
Candidate name: Ieva Drūlytė
Investigation into the transpiration rate of maple dependency on the area of leaves
Transpiration is the evaporation of water particles from plant surfaces, especially from the surface openings, or stomata, on leaves. Stomatal transpiration accounts for most of the water loss by a plant, but some direct evaporation also takes place through the surfaces of the epidermal cells of the leaves. The amount of water given off depends somewhat upon how much water the roots of the plant have absorbed. It also depends upon such environmental conditions as sunlight, humidity, winds and temperature. A plant should not be transplanted in full sunshine because it may lose too much water and wilt before the damaged roots can supply enough water. Transpiration occurs as the sun warms the water inside the blade. The warming changes much of the water into water vapor. This gas can then escape through the stomata. Transpiration helps cool the inside of the leaf because the escaping vapor has absorbed heat.
Aim: to investigate the effect of altering the leaves area on the rate of transpiration of Acer (Maple).
Hypothesis: with decreasing area of leaves provided, the rate of transpiration would also decrease as transpiration occurs through stomata which are located commonly on the lower epidermis of leaves.
Variables: the rate of transpiration (the amount of H2O taken, mm) (dependent variable) will be altered by reducing the area of leaves (cm2) (independent variable).
Table 1 Controlled variables.
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- Maple branch
- 1 mL pipette; 0.01 divisions (±0.005)
- Test tube
- Glass tube
- Rubber bung with hole
- Stand with clamps and bosses
- Razor blade
First, maple tree branch of the appropriate size to fit photometer was collected. It had to be cut under the water to prevent oxygen getting into the xylem. Then, the photometer was constructed from the test tube, plastic tube, and measuring pipette and filled up with water so that any air bubbles were removed from the tubing. After that, the end of the branch of maple tree was placed through rubber bang into the test tube filled with water. All the procedure had to be taken underwater. The space between the hole of rubber bung and plants was isolated with plasticine.
Second, the photometer and branch were attached to the clamp stand and the plant was left to rest for 10 minutes. To the same clamp stand the sensors of humidity and temperature were attached in order to control the controlled variables.
Third, after 10 minutes the amount of water in the pipette was recorded and stop-watch was started. The amount of water that was taken by a plant was recorded every 5 minutes for 25 minutes, which was done due to reliable results necessity. Then, approximately one third of all leaves were cut, cutting places were immediately covered with nail polish. After 10 minutes of rest records were started to be taken every 5 minutes for a period of 25 minutes. Then, the same procedure was made with second third and last third of all leaves until the plant had no leaves at all.
Finally, the surface area of leaves cut was calculated with a scanning program ImageJ, collected data was processed and graphed.
Data collection and processing
Table 2 Raw date; H20 transpired after every reduction of the area of leaves per period of 5 min.
In order to found transpiration rate per unit of surface area of leave, leaves, which were cut during every reduction of total surface area of leaves of a branch, were scanned and their surface area was calculated using a computer program ImageJ. Results were: area of first third – 153722 mm2; second third – 196203 mm2; last third – 179602 mm2. Hence, the surface area of leaves provided at every step was calculated by subtracting results above from the total surface area of three thirds of leaves. For example:
Area of leaves after first reduction = total area – first third = (153722+196203+179602) – 153722
→ 2/3 of leaves = 375805 mm2 = 0,376 m2.
When the surface area of leaves was calculated, the rate of transpiration was found for every surface area of leaves tested by dividing average amount of water transpired per 1 min to the surface area of leaves in that step.
Table 3 Processed data; transpiration rates: mL/min and mL/m2min1.
Conclusion and evaluation
A branch of maple was placed in the hole of the rubber bung and inserted into the water filled test tube. As the plant takes up water, the amount of water in the measuring pipette decreases. This experiment results proved hypothesis that with decreasing area of leaves, the rate of transpiration would also decrease. Transpiration rate was calculated according to the volume of water taken by a plant and the decrease of it in different steps of reducing the total area of leaves can be seen from the Chart 1. The tendency is that for every third of leaves cut, the amount of water taken by a plant decreased for at least 0.02 mL, for example, when no leaves were cut it was 0.113 and then one third was cut it was 0.077, etc. Although tendency seems to be quite reliable, an anomaly occurs when all the leaves are cut. Even after 10 minutes of rest water was still being absorbed by a plant. This may be due to that the stem, where leaves were cut, might not have been isolated properly and vaporization still occurred. Further data processing, when transpired amount of water was compared with the exact area of surface of leaves in every step of reduction, showed some more contrary results. As can be seen from the Chart 2, the values of transpiration rate (mL/m2min1) decrease from first step to second (from 0.043 to 0.041) but there is a steep increase in the third step (from 0.041 to 0.068). This could be explained that, although transpiration rate depends straightly on the area of leaves, e.g. number of stomata, plant may take water at the same rate even after leaves are cut. This could happen as a result of continuous transpiration pull. Maybe if the measurements would have been taken the following day after all the leaves were cut they would be similar as expected, e.g. more equal to zero.
The method of investigation might be discussed as having some limitations and weaknesses. First, the anomalous results of water having been absorbed even when the plant had no more leaves at all may not occur if the leaves would have been not cut (which might cause air bubbles getting into xylem) but covered with an oily material, for instance, nail polish. This would work as stomata are usually located on the lower and sometimes upper epidermis of leaves surface and nail polish would cover them. Second, as the area of leaves reduced every time depended on leaves’ shape, it was difficult every time to reduce the same area and that might have caused an influence on results. Counting leaves and cutting the same number is not reliable method, therefore, for example, leaves size and shape could be replicated on the paper and the area could be calculated before the experiment. However, one should be very careful not to harm leaves, hence, their xylems. Third, the results that transpiration at some level occurred even when all leaves were cut might come from inappropriate length of periods for which the plant was left to rest. A method improvement would be to measure a volume absorbed in every 10 minutes for an hour minimum and to perform an experiment continuously a few days. Finally, the method could be discussed at terms of precision and accuracy. This includes already mentioned aspect of counting area of surface of leaves and, what’s more, raw data recording. As a pipette taken was only of 1 mL capacity, sometimes it was really hard to decide the exact amount of water absorbed. Pipette with smaller graduations should be used, however, plant would transpire water from pipette more quickly and this would lead to the re-filling the mechanism, which could result in the errors in the results as well. As these are so called minor improvements for the method of investigation, a few suggestions could be made for other possible experimental designs, for example, more types of plants could be used or other environmental conditions such as temperature or humidity could be altered together with the surface area of leaves in order to investigate more precisely the effects on transpiration rate.