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Stoma are microscopic pores on the underneath of plant leaves that open and close. They maintain the two basic functions of water regulation and gas exchange for respiration and photosynthesis (Petrova 2012). Although all plants have stoma, one must consider how and why they respond to their environment in order to make a plant’s stomatal density different from that growing in a contrasting environment. Various environmental factors such as soil moisture, elevation, temperature, etc. all potentially affect the stoma density of plant leaves, and among this list of influencers is sunlight exposure.
For as long as plants have thrived in environments globally, an evolutionary process has been set in place to support the exchange of gases CO2 and O2 to promote photosynthesis and respiration.
Nearly all plants worldwide are covered by leaves or other stoma covered mechanisms, which lessens water evaporation and regulates the exchange of CO2 and O2. In most plant leaves, the upward-facing side is exposed to the most sunlight, while the underside of leaves typically experiences the most CO2 uptake where nearly all of the leaves’ stoma are found (Smith 1997).
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This is because the opening of stomatal pores allows this uptake of CO2 needed for photosynthesis (Boccalandro et al. 2009). The opening of stomatal pores CO2, combined with sunlight and water, are the three components required for photosynthesis to occur. For example, the presence of stoma on both sides of a leaf’s surface significantly increases CO2 supply to the rest of the plant, which in turn supports the photosynthetic rates per unit on a leaf’s surface area (Smith 1997).
Therefore, gas exchange overall is administered through stoma and contribute to the higher rates of photosynthesis (Lendzian 2006).
All plants have a different stomatal density to best suit their environmental conditions. A plant’s stomata pores will open and close in accordance with how much water it is receiving, and about 98% to 99% of all water absorbed by a plant’s roots travels up the plant and is eventually released through the plant’s stoma (Thompson 2014). As the water travels upwards, it cools the plant in a process called evaporative cooling. In addition, stoma act as a protective measure when a plant is not receiving enough water. In the case of water deficiency, a plant’s stoma will close in order to preserve its water to continue basic functions. When a plant has more than enough water at its disposal, the stoma will open to allow excess water to leak out through its pores (Georgieva et al. 2012). This surplus of water, or lack thereof, potentially affects stomatal density. However, how much water a plant is receiving can also be altered by the presence of sunlight. This is due to evaporation and plants becoming warmer and needing to rely on the evaporative cooling process.
All plants have stoma, and all plants use their stoma for water regulation and gas exchange, but one must consider what makes a plants stomatal density differs from another. To test if sunlight conditions may affect stomatal density, I sampled 20 stems of Liriope muscari (commonly known as Monkey Grass) to observe. Typically, the exchange of gases hits its maximum for a plant in July and its minimum during autumn and winter (Lendzian 2006). This is potentially due to the amount of sunlight exposure. Overall, I hypothesize that Monkey Grass planted by a building with direct sunlight will have a higher stomatal density than the grass planted by a building with no direct sunlight because plants produce more stoma in warmer areas to maintain temperature through evaporative cooling.
I collected 10 stems each of Liriope muscari (commonly known as Monkey Grass) on January 30, 2019 from the sides of two different buildings on a college campus in Georgia: an area always exposed to direct sunlight and an area that is constantly shaded. The stems were picked at 9:30 AM and the weather was mostly sunny, sitting at 29℉ and 39% humidity. I gathered the first 10 stems from the sunny side of a building’s walkway. Its habitat was urban, consisting of a small tree, a different tall grass, and mulch. The second set of 10 stems were taken from the always-shaded side of a different building. These plants grew in an urban habitat as well, consisting of multiple shrubs and mulch.
To get the imprints of the Monkey Grass, I applied two coats of nail polish to about an inch of each stem. While the polish dried, I cut 20 appropriately sized strips of tape. I then firmly laid the tape onto the grass’ area of applied nail polish and pushed down on it to create an imprint. Next, I carefully removed the tape from the stems and placed the tape pieces onto 3 clean microscope slides. I then put a slide under a compound microscope and set its magnification to 400x to count the number of stomata present in the single field of view. Another person counted the stoma as well, and I collected the average of both. I repeated this process for all 3 slides.
After observing and collecting the data from my stomata, I created a bar graph to clearly show the difference in stomatal density between the Monkey Grass collected in the sun and the Monkey Grass collected in the shade. I paired the graph with a t-test I conducted a t-test using Microsoft Excel to provide further detail in the discoveries of the experiment. All of the methods used to conduct this experiment are credited to Hyslop et al. 2017.
The average stoma density of Monkey Grass growing in the sun was 226 stoma per mm2 with a standard deviation of 57 per mm2 (figure 1). The average stoma density within the Monkey Grass growing in the shade was 144 per mm2 with a standard deviation of 53 per mm2 (figure 1). The result of the t-test was p=0.003, meaning my samples were statistically different. Overall, the stoma per mm2 in Monkey Grass growing in the sun were higher than that growing in the shade. Therefore, the data received from my conducted experiment directly supported my hypothesis.
Initially, I hypothesized that Liriope muscari (commonly known as Monkey Grass) planted by a building with direct sunlight will have a higher stomatal density than the grass planted by a building with no direct sunlight because plants produce more stoma in warmer areas to maintain temperature through evaporative cooling. In my conducted experiment, the average stomatal density found in Monkey Grass planted in direct sunlight was statistically different and significantly higher than the grass planted in a more shaded area. Thus, the data directly supported my hypothesis.
In a similar scientific study, Georgieva and other researchers found apparent differences in stomatal densities of H. rhodopensis (commonly known as Haberlea) growing in direct sunlight and H. rhodopensis growing in shade (Georgieva et al. 2012). Using multiple calculations, they ensured both of their samples taken were growing in fresh and dry areas with very similar water content. Georgieva’s study found that the stomatal density of the Haberla growing in direct sunlight was noticeably higher than that growing in nearly consistent shade. The same result was also found in my experiment. Therefore, this study supported my hypothesis.
Although my hypothesis was supported through my sampling and comparatively through a similar scientific study, there were still openings for biological error out of my control as well as room for improvement. Both samples were taken on a college campus close to walkways, so passerbyers may have stepped on any plants to be sampled from. Conducting this experiment in a different season may also lead to differing results, as well as weather conditions. Generally, my study could have been improved through utilizing a larger sampling size and choosing to sample plants more isolated from foot traffic.
Differences in Plants’ Stomatal Density Due to Sunlight Exposure. (2021, Dec 20). Retrieved from https://studymoose.com/differences-in-plants-stomatal-density-due-to-sunlight-exposure-essay
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