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Rose Macomber
AP Biology
Period E/F
4/22/11
The Effect of Dark, Light, Fan, and Mist on Whole Plant Transpiration
Abstract:
The purpose of this experiment was to test the effects of environmental variables on rates
of transpiration using a controlled experiment. From highest to lowest, the amount of
transpiration due to the environments will be light, fan, room, dark, and mist conditions. To
collect the data, each plant was measured daily using the same scale and recorded in a table,
which was then shared with the class. After three days, all plants were recorded to have lost
mass, with the fan and light conditions having lost the most mass, and the dark and misted
conditions having lost the least amount of mass. The room condition was used as a constant and
remained to be in the middle in terms of transpiration and mass lost of all the different
conditions. It can be concluded that plants will transpire more with lower water potentials
outside the plants and with a need of water for photosynthesis.
Introduction:
Transpiration is the loss of water by evaporation of water, usually through the stomata in
the leaves. This process is usually initiated by a higher water potential inside the cells of a plant
than in the environment surrounding it. This higher water potential inside the plant launches the
process of osmosis to occur, moving water into the mesophyll cells. Although in cases with a
lower water potential in the cell might partially prevent transpiration, the stomata of a plant must
remain open to allow the entry of CO2 used in photosynthesis.
All plants do not transpire at the same rates while under different conditions. The
purpose of this experiment was to test the effects of environmental variables on rates of
transpiration using a controlled experiment. Since transpiration is the plants main source of water
loss, placing the plant in different conditions will test the way different environments affect
water loss and the closing or opening of the plant’s stomata.
The dependent variable in this experiment is the mass lost through transpiration for each
variable. The independent variables in this testing include soil and plant type. For each condition,
the independent variables changed, but were constant within the condition. For example, both
plants placed in the light condition were the same distance from the light and had the same
amount of water in the heat bath to absorb heat from the light. This is the same for each trial: in
the fan trial, both plants were placed at the same distance from the fan, both plants in the mist
condition were misted with the same amount of water each day, and both plants in the dark
condition were in the same amount of dark. The variable held constant would be the room
condition tests because there were no influences to promote or postpone the natural rate of
transpiration from occurring.
Heat and humidity play a large role in rate of transpiration. If a plant is continually left in
the light or in the path of a fan (wind), then it will lose the most mass through transpiration out of
all of the plants because light promotes more photosynthesis to occur, which requires the intake
of CO2 and the release of water through the stomata. Also, with the wind blowing on a plant, all
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of the humid air is blown away, leaving the air around the plant dry; therefore giving the plant
more water potential to leave the stomata and evaporate into the surrounding environment. With
the wind constantly blowing the humid air away from the plant, there is always a high water
potential, making the plant lose more water through transpiration. If a plant is left in the dark or
misted with water daily, then it will lose the least amount of water through transpiration. If a
plant is in the dark, it will not have the light initiating photosynthesis, which will conserve water
and keep the amount of transpiration to a minimum. In addition, a plant being misted with water
on a daily basis will either lose the least amount of mass through transpiration or gain mass. This
will happen because when a plant is misted, it is creating lower water potential in the cells,
which will stop osmosis while the water is on the plants, and prevent transpiration. Lastly, the
plant kept at a room condition will lose a middle amount of mass due to transpiration because
there are no variables inhibiting or initiating transpiration more than average.
Materials/Methods:
See AP Biology Lab 9 Alternative: Whole Plant Transpiration for full set of materials and
procedure.
Results:
Table 9.1: Daily plant mass recordings for various environmental conditions.
Condition
mass0 (g)
m1 (g)
m2 (g)
Room 1
249.5
210.8
188.4
Room 2
118.7
95
87.6
Room Avg
184.1
152.9
138.0
31.2
14.9
Avg change in
0
mass (g)
Dark 1
131.7
116.9
101.3
Dark 2
260.9
250.2
238.3
Dark Avg
196.3
183.6
169.8
12.7
13.8
Avg change in
0
mass (g)
Light 1
268.6
189.5
134.8
Light 2
277.8
227.7
191.2
Light Avg
273.2
208.6
163.0
64.6
45.6
Avg change in
0
mass (g)
Fan 1
230.6
167.7
130.9
Fan 2
291.8
212.5
166.8
Fan Avg
261.2
190.1
148.9
71.1
41.2
Avg change in
0
mass (g)
Mist 1
298.9
292.3
286.5
Mist 2
229.7
217.4
210.7
Mist Avg
264.3
254.9
248.6
0
9.4
6.3
Avg change (g)
m3 (g)
137.5
77
107.3
30.7
92.3
227.2
159.8
10.0
84.0
125.5
104.8
58.2
90.9
102
96.4
52.5
270.0
192.4
231.2
17.4
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Table 9.2: Leaf surface area (SA) of plants used in experiment
Condition
Room Room Dark
Dark Light Light
1
2
1
2
1
2
Surface
178.2
109
476.8
287.8
375
304.5
Area (cm2)
Average SA
231.4
382.3
339.8
(cm2)
Fan 1
Fan 2
Mist 1
Mist 2
343.5
551.9
497
349.3
447.7
423.2
Graph:
Discussion:
After testing multiple conditions for plant transpiration, the results can determine whether
or not the hypotheses listed above were correct. As you can see in data table 9.1 above, each
plant in every condition lost at least some mass through the process of transpiration, although it
is evident that some plants lost much more mass than others. The total average of mass lost by
each condition is shown above, with room condition having a loss of 76.8 grams, dark having
lost 36.5 grams, light lost 168.4 grams, fan lost 164.8 grams, and finally the mist condition lost
33.1 grams.
These results support the hypotheses because it was predicted that the light and fan
variables would lose the most mass through transpiration and the fan and mist variables would
lose the least amount, with room condition in the middle. Light and fan variables lost the most
amount of mass because sunlight promote photosynthesis to occur and wind causes the emission
of water to occur due to a lower water potential outside the flower. Also, the dark and mist
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variables lost the least amount of mass because transpiration was kept to a minimum due to
lower water potential in the plant and no need to follow through with photosynthesis. The room
condition remained in the middle of the mass loss because it was the constant variable and there
were no influences for or against transpiration, allowing the natural rate of transpiration to take
place.
The data from this experiment is valid because the data clearly supports the hypotheses,
although some changes to the tests could have made the data more valid. To make this
experiment more reliable, the tests could have been more closely monitored and the sources of
mass loss could have been more controlled. One possible source of error is that the variables
were not held completely constant. For example, for the light condition, it is possible that the
plants were not kept the same distance from the light every day because the distance was not
measured. Also, for the light condition, the heat baths were not refilled every day, which could
have lead to the heat also affecting the results and caused transpiration to happen at a faster rate.
The data could be more valid if the source of mass lost was more controlled. In this experiment,
each plant was wrapped in a plastic bag and tightened with a rubber band to prevent any water
loss from anything other than the leaves. It is possible that in this process, water could have
escaped through the soil and roots and exited through the small gaps in the bag which were not
securely closed off by the rubber band.
The results for this experiment are reliable. There were two trials done for each condition
and all results were similar to the other plant in its same condition. The rate of transpiration for
each trial was controlled by the treatment on each plant by being kept at the same exact
conditions as the other plants in its condition. The plants in each treatment were kept the same
distance from the light or fan, given the same amount of water, and kept in the same amount of
darkness; therefore making this experiment reliable.
Conclusion:
The hypotheses were correct according to the data that was collected by the class. It was
found that more transpiration of the plant will occur when there is lower water potential outside
of the plant, or through the process of photosynthesis, but not when photosynthesis is delayed
and there is lower water potential inside of the cell.