Carbon Dioxide Consumption in Drought-Tolerant vs. Non-Drought-Tolerant Plants
Lyndsey Bennett
Department of Biological Sciences
Saddleback College
Mission Viejo, CA 92692
Abstract
Global warming is a growing concern for Earth, and rising levels of carbon dioxide in
the atmosphere is a major contribution. With many parts of the country falling into a state
of drought, individuals are replacing their gardens with plants that consume less water.
This experiment is to prove non-drought-tolerant plants are more effective at consuming
carbon dioxide from the environment than are drought-tolerant plants. Six carbon dioxide
(CO2) probes were individually placed into airtight terrariums containing one of three
drought-tolerant plants of the same species, and one of three non-drought-tolerant plants
of the same species. Concentration readings of CO2 levels were recorded hourly, on three
separate occasions. Over twenty-four hours, drought-tolerant plants consumed on average
361.1 ± 23.06 SEM parts per million (ppm) of CO2, while non-drought-tolerant plants
consumed on average 804.78 ± 17.87 SEM ppm, proving to be significantly more
efficient (p = 5.54 x 10-5). These results predict that levels of CO2 in the atmosphere will
increase at a greater rate than currently occurs due to a lower abundance of efficient CO2
eliminating non-drought-tolerant plants.
Introduction
Two of the most critical concerns regarding today’s changing environment
include the apprehension for potential drought in many regions around the world, as well
as increasing levels of carbon dioxide (CO2) residing in the atmosphere and contributing
to global warming (Beardsley, 2008). These issues go hand in hand, as limited water
availability affects plants’ capacity to convert existing atmospheric carbon dioxide into
oxygen through photosynthesis, thereby leaving higher amounts of CO2 to disrupt the
Earth’s ozone layer (Beardsley, 2008).
According to the National Centers for Environmental Information, nearly 18
percent of the United States was in a moderate to extreme drought at the end of October
2015. In response to this issue, an increasing number of individuals are replacing their
typical, plush lawns and gardens with drought-tolerant plants, in an effort to conserve
water. While these steps may result in lower water consumption, possibly alleviating
immediate drought concerns, greater long-term damage in regards to raised CO2 levels in
the atmosphere may result.
Water is a vital component in photosynthesis (Osterhout and Haas, 1918). Less
water, as during periods of drought, means less carbon dioxide uptake from the
environment. The commonly accepted chemical reaction for photosynthesis shows the
interaction between water and carbon dioxide: 6CO2 + 6H2O (+ light energy) →
C6H12O6 + 6O2.
This experiment was conducted to determine whether replacing non-droughttolerant plants with drought-tolerant plants is likely to result in a change of carbon
dioxide levels in the atmosphere, thereby contributing to further and more rapid erosion
of Earth’s ozone. It is expected the non-drought-tolerant plants are significantly better at
converting carbon dioxide into oxygen through photosynthesis than are drought-tolerant
plants.
Materials and Methods
Six carbon dioxide consuming plants were purchased from Lowe’s Hardware
Store in San Clemente, CA; three of which were a drought-tolerant species of succulent
Graptosedum, and three that were of the non-drought-tolerant species Epipremnum. Each
of the plants was enclosed in an airtight terrarium made of a clear plastic Tupperware,
then covered with clear saran wrap, and taped airtight. Each container was then
positioned outside in direct sunlight. Using the Xplorer GLX Pasport, the carbon dioxide
levels of each container was immediately analyzed upon their placement into the
sunlight. For the next twenty-four hours the Xplorer GLX Pasport recorded carbon
dioxide levels hourly to determine the change in carbon dioxide concentration. This
experiment was conducted at the researcher’s house in San Clemente, CA on three
occasions; 29 October, 2015, 2 November, 2015, and 5 November, 2015. A one-tailed,
unpaired t-test was run to assess the mean intake of carbon dioxide from each species, to
test the hypothesis that non-drought-tolerant plants consume more carbon dioxide than do
drought-tolerant plants.
Results
An overall significant difference in the rate of carbon dioxide consumption was
observed over a period of three trials between the drought-tolerant and the non-droughttolerant plants, with non-drought-tolerant plants proving to be more efficient at carbon
dioxide consumption (p= 5.54 x 10-5) (Figure 4).
Day one of recording carbon dioxide concentrations in the plants’ terrarium, nondrought-tolerant plants averaged a rate of consumption of 829.33 ± 55.35 SEM parts per
million (ppm) of CO2. Drought-tolerant plants averaged a rate of 406.33 ± 39.55 SEM of
CO2. All six plant environments reached the greatest level of CO2 around 2:00 in the
morning, when plant respiration was complete. CO2 levels begin dropping at this point
when all plants began to consume the CO2 for photosynthesis.
1200
Non-Drought-Tolerant 1
1000
Non-Drought-Tolerant 2
800
Non-Drought-Tolerant 3
600
Drought-Tolerant 1
400
Drought-Tolerant 2
Drought-Tolerant 3
200
2:00
Noon
10:00
8:00
6:00
4:00
2:00
Midnight
10:00
8:00
6:00
4:00
0
2:00
Concentration of Carbon Dioxide
(ppm)
Comparison of CO2 Uptake in Drought-Tolerant vs. NonDrought-Tolerant Plants Day 1
Time (Hour)
Figure 1. Comparison of three drought-tolerant vs. three non-drought-tolerant plants over
twenty-four hours. Non-drought tolerant plants are significantly better at consuming
carbon dioxide (one-tailed, unpaired t-test; p = 2.05 x 10-4).
Day two of recording carbon dioxide concentrations in the plants’ terrarium, nondrought-tolerant plants averaged a rate of consumption of 815. ± 65.09 SEM parts per
million (ppm) of CO2. Drought-tolerant plants averaged a rate of 346.3 ± 13.48 SEM of
CO2. All six plant environments reached the greatest level of CO2 around 2:00 in the
morning, when plant respiration was complete. CO2 levels begin dropping at this point
when all plants began to consume the CO2 for photosynthesis.
COncentration of Carbon Dioxide (ppm)
Comparison of CO2 Uptake in Drought-Tolerant vs. NonDrought-Tolerant Plants Day 2
1200
Non-Drought-Tolerant 1
1000
Non-Drought-Tolerant 2
Non-Drought-Tolerant 3
800
Drought-Tolerant 1
600
Drought-Tolerant 2
Drought-Tolerant 3
400
200
0
Time (Hour)
Figure 2. Comparison of three drought-tolerant vs. three non-drought-tolerant plants over
twenty-four hours. Non-drought tolerant plants are significantly better at consuming
carbon dioxide (one-tailed, unpaired t-test; p = 1.07 x 10-3).
Day three of recording carbon dioxide concentrations in the plants’ terrarium,
non-drought-tolerant plants averaged a rate of consumption of 770. ± 31.48 SEM parts
per million (ppm) of CO2. Drought-tolerant plants averaged a rate of 330.67 ± 25.51
SEM of CO2. All six plant environments reached the greatest level of CO2 around 2:00 in
the morning, when plant respiration was complete. CO2 levels begin dropping at this
point when all plants began to consume the CO2 for photosynthesis.
Concentration of Carbon Dioxide (ppm)
1200
Comparison of Average CO2 Uptake in Drought-Tolerant vs.
Non-Drought-Tolerant Plants Day 3
1000
Non-Drought-Tolerant 1
Non-Drought-Tolerant 2
Non-Drought-Tolerant 3
Drought-Tolerant 1
Drought-Tolerant 2
Drought-Tolerant 3
800
600
400
200
0
Time (Hour)
Figure 3. Comparison of three drought-tolerant vs. three non-drought-tolerant plants over
twenty-four hours. Non-drought tolerant plants are significantly better at consuming
carbon dioxide (one-tailed, unpaired t-test; p = 2.05 x 10-3).
Figure 4 compares the average of each group of plants for each of the three days
shows there non-drought-tolerant plants are significantly better at absorbing CO2 from the
environment (p = 5.54 x 10-5). On average, non-drought-tolerant plants consumed 804.78
± 17.87 SEM ppm of CO2, while drought-tolerant plants consumed 361.1 ± 23.06 SEM
ppm CO2.
Concentration of Carbon Dioxide
(ppm)
Comparison of Average CO2 Uptake in Drought-Tolerant vs.
Non-Drought-Tolerant Plants
1000
800
600
Non-Drought-Tolerant
400
Drought-Tolerant
200
0
1
2
Trial (Day)
3
Figure 4. Comparison of carbon dioxide uptake (ppm) in non-drought-tolerant plants vs.
drought-tolerant plants. Non-drought-tolerant plants are significantly better at consuming
carbon dioxide from the environment, averaging 804.78 ± 17.87 SEM ppm, while the
drought-tolerant plants averaged a CO2 intake of 361.1 ± 23.06 SEM ppm (one-tailed,
unpaired t-test; p = 5.54 x 10-5).
Discussion
Expectations that the drought-tolerant plants would be found less effective at
absorbing carbon dioxide than the non-drought-tolerant plants were consistent with the
results of the experiment, as shown in Figure 4 (p = 5.54 x 10-5). CO2 levels peaked in all
terrariums around 2:00 in the morning (Fig. 1; Fig. 2; Fig. 3), after the plants completed
respiration during periods of low light levels. The difference in each of the terrarium’s
greatest level of CO2 concentration and lowest level of CO2 concentration shows the
amount CO2 each plant is able to consume in parts per million (ppm) for photosynthesis.
The differences in ability to consume CO2 are due to the varying about of stomata each
type of plant holds.
Nearly every terrestrial plant (one that grows on land) contains stoma, the pores
through which carbon dioxide is consumed and converted to oxygen during
photosynthesis, producing sugars necessary for plant survival and production (Kayvon,
2008). The commonly accepted equation for this chemical reaction is 6CO2 + 6H2O (+
light energy) → C6H12O6 + 6O2. Drought-tolerant and non-drought-tolerant plants have
been found to contain vast differences in the number of stomata present on their leaves
(Eckerson, 1920). These differences occur to genetic modifications made by the plants in
response to the availability of water, a necessary component to partake in photosynthesis
(Eckerson, 1920).
Non-drought-tolerant plants respond to drought by altering their physical makeup,
such as a reduction of leaf area, leaf wilting, as well as increased root growth and closure
of stomata through the release of abscisic acid (ABA), a plant stress hormone (Kayvon,
2008). While these measures result in short term water use efficiency, there is also a
reduction in carbon dioxide consumption for photosynthesis (Assmann, 2013).
Drought-tolerant plants grow naturally in regions with limited water availability
(Kayvon, 2008). These plants have evolutionarily modified their genetic makeup to
improve their ability to survive in drought conditions (Assmann, 2013). Many develop
leaves with a waxy coating to prevent water loss; others have lighter leaf color to reflect
sunlight, or tiny hairs covering their exterior to reduce water transpiration (Assmann,
2013). Many of these plants also grow their roots laterally to increase water and nutrient
uptake (Assmann, 2013).
The greatest factor contributing to lower photosynthetic capability is the reduced
number of stomata drought-tolerant plants develop (Edwards and Ogburn, 2012)..
Stomata (stoma; plural) are the sites in plants where carbon dioxide is absorbed, and
oxygen and water is released back into the environment (Edwards and Ogburn, 2012).
Although the reduced carbon uptake of these drought-tolerant plants is the main
focus of this experiment, the water cycle resulting from water released by the plant
during the photosynthetic process is also an important factor. Lower quantities of water
released from the plants leads to less moisture in the atmosphere for rainfall, leading to
less moisture in the soil available for plant growth. When conditions are dry, stomata
condense to prevent loss of water needed for survival (Lampard, et al., 2008). The
smaller stoma in effect also reduces carbon consumption due to smaller surface area of
the stoma opening for carbon consumption.
According to the Unites States Environmental Protection Agency, carbon dioxide
is the most emitted greenhouse gas due to human activity, claiming 82% of all the
greenhouse gases individuals produce (NRC, 2010). Since the start of the industrial
revolution, human contribution to carbon dioxide released emissions have risen
drastically, roughly 80% between 1970 and 2004 (Wild, 2010). This is due primarily to
fossil fuel use and land desiccation (Wild, 2010). When gases such as carbon dioxide
enter the atmosphere, they are trapped in the environment and prevent escape of heat
radiating towards space, a phenomena known as the “greenhouse effect” (Gardiner,
2004). Carbon dioxide is believed to contribute to climate change as it resides in the
atmosphere and does not respond to fluctuations in temperature (Gardiner, 2004). This is
where plants play a crucial role in the regulation of our environment, as they are needed
to adjust the levels of carbon dioxide fixed in the atmosphere. An abundance of droughttolerant plants means lowered photosynthetic activity, resulting in higher levels of carbon
dioxide in the atmosphere, decreased water availability, and fewer plants rich with
stomata. As a plant is growing, environmental conditions affect “stomatal-development”
pathways, which determines the number of stoma the plant will grow (Lampard et al.,
2008). In an environment containing high levels of carbon dioxide and low levels of
water, a plant will develop fewer stoma than it would in an environment rich in water
availability and low in carbon dioxide (Edwards and Ogburn, 2012).
Upon finding there to be a significant difference between drought-tolerant
Graptosedum and non-drought-tolerant species Epipremnum in the consumption of
carbon dioxide, further experiments should be conducted using a greater variety of
drought-tolerant and non-drought-tolerant plants to obtain more comprehensive results.
Studies should also be completed to find the total overall increasing rate of net CO2 in
the atmosphere, after accounting drought levels and percentage of non-drought-tolerant
plants being replaced with drought-tolerant plants.
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