Effect of Environmental Moisture Levels on Stomata Density in Privet Hedges

Stomata are microscopic pores found on the epidermis of leaves. These allow material to pass in and out of the lead. The stomata are surrounded on both sides by guard cells. These guard cells control the opening and closing of the stomata by swelling or contracting. The guard cells close the stomata when dehydrated, allowing the plant to conserve water. Most stomata are found on the bottom surface of leaves. The number of stomata on a leaf’s surface can tell you a lot about the plant itself. Most often, a higher stomata density indicates a high amount of sun exposure and an abundance of moisture available to the roots. A lower stomata density indicates higher amounts of carbon dioxide. In other words, stomata density is determined by the conditions that the plant experiences while the leaf is developing.

During photosynthesis, stomata take in carbon dioxide (CO2) and release oxygen (O2) and some water (H2O) vapor. During respiration, the stomata take in O2 and release CO2 as well as some water vapor. During transpiration, a plant cools itself by opening its stomata and allowing water to evaporate. Energy needed to convert to convert liquid water to water vapor is drawn from the surrounding leaf surface, which is cooled in the process. Stomata are located on the lower surface of leaves to reduce water loss due to minimized solar radiation. The moist air in these spaces has a higher water potential than the outside air, and water tends to evaporate from the leaf surface. The stomata act as pumps that pull water and nutrients from the roots through the rest of the plant to the leaves in a phenomenon known as transpirational pull. Transpirational pull is one of the forces that drives water flow in the plant.

Water is absorbed by the root hairs of a plant and, due to osmotic pressure, is passed through vascular tissues into the xylem where it is transported to the leaves and stomata. Vascular tissue is made up of more than one cell type and in plants consists of xylem and phloem. The xylem carries inorganic nutrients, like phosphorus and nitrates, and the phloem transports sugar—an organic nutrient—throughout the plant along vascular bundles of cells arranged from end to end to form long, narrow conduits. In the vascular tissue, water molecules form a column. These columns are the product of cohesion and adhesion. The uppermost molecule turns to water vapor and is transpired through the stomata. As a water vapor droplet evaporates, the high surface tension of water pulls the hollow formation outwards, generating force (Ogbonnaya, 2012). The force provides enough pull to lift water through the vascular tissue of the plant to the leaf surface.

All else being equal, we hypothesize that within a species, a leaf should develop so that its stomata density is greater in plants grown in moist conditions—stomata density should be significantly higher in leaves of plants found in a wet location (growing under moist conditions) when compared to similar leaves of plants found in a dry location (growing under dry conditions). Methods

Leaves were collected from a moist environment and from a relatively dry environment. On the bank of a small creek we collected 10 leaves from what we later identified as a privet hedge. From a thickly wooded area, without any water source nearby, we located another privet hedge from which collected an additional 10 leaves. We placed the leaves into plastic bags along with a small, dampened paper towel, to make sure that the samples would be kept fresh until examination.

We prepared the leaves by cleaning their undersides; we gently rubbed away any dust or fuzz. Then, using clear nail polish, we painted a small dime-sized area on the bottom of each leaf. We allowed the polish to completely dry and added a second coat to each leaf—and once again allowed the polish to completely dry. We pressed strips of clear tape directly onto the polished leaf print area and carefully lifted the tape to peel off an impression of the leaf surface. We placed the prints onto microscope slides.

Using a compound microscope, the prints were examined at 400X total magnification to identify the stomata on the surface of each leaf. For each print, two counts were taken, in which all of the stomata seen in the field of view were counted. The two counts for each leaf print were then averaged together. The stomata density/mm2 for each leaf print was calculated by dividing each average print count by 0.12. We calculated the mean (or average stomata density) and the standard deviation for each leaf set. In order to statistically evaluate our results, we conducted a t-test. (Hysop & Hoekstra, 2011). Results

The data was placed in a bar graph to interpret more clearly. As Figure 1 shows, the stomata density mean calculated for condition A (our moist environment), was 253, which was much higher than that for condition B (our dry environment), which was 27. The standard deviation for Condition A, which was 27, was lower than the standard deviation for Condition B, which was 38. A larger standard deviation, like that of our condition B, means that the stomata densities in this group were more different from each other while a smaller standard deviation, like that of our condition A, means that the stomata densities in this group were more similar to one another. Our calculated P-value was 0.000000001.

Figure 1: A comparison of average stomata density between privet hedge leaves collected from moist (N=10) and dry environments (N=10), Lumpkin County, Georgia, August 2012.

Discussion
As previously stated, our calculated p-value was 0.000000001. When comparing our calculated p-value to the critical p-value of 0.05, it was observed that p < 0.05; we were then able to conclude that not only did our results support our hypothesis, but that the difference in means was significant.

One of the circumstances (or unknown variables) that might have influenced our results is that conditions in environments can change over time, so the plant may not have been under the same condition when the leaves developed as when they were collected. Another is the fact that the privet leaf is adapted to a moderate habitat, one that is neither very wet nor very dry. Also, privet leaves are very large; in fact, when we collected our samples, we observed that in both environments the leaves of the privet hedges that we had chosen to sample from were larger than leaves on most of the other plants in the area. The larger a leaf’s surface is, the larger the number of stomata will be. The concentration of stomata is in direct correspondence with the stomata density—leaves that have a larger stomata density have a higher concentration of stomata; leaves that have a smaller stomata density have a lesser concentration of stomata.

Privet leaves are characteristically large in size, and when compared to other species, have a higher stomata density and a greater concentration of stomata. In the field of vision, a high stomata concentration means that stomata are clustered very close together. This can make it extremely difficult for observers to make an accurate count of stomata. If we choose to repeat this experiment, we probably would not choose the same methods. We would perhaps choose to start with seeds, to place them in greenhouses where conditions such as CO2 levels, amounts of water and other nutrients, and sun exposure could be manipulated and controlled. If we started from seeds, we could observe stomata develop on the unfurling leaves during each stage of the plant’s lifecycle.

From this experiment I learned that the conditions under which a leaf develops determines its stomata density. Leaves with high stomata densities are likely to have formed in a wet environment and that leaves with low stomata densities were likely to have formed in a dry environment. I learned that more stomata are made on plant surfaces under higher light, lower atmospheric CO2 concentrations, as well as moist environments. (Swarthout, Hogan, & Taub, 2010).

Works Cited

Hysop, N., & Hoekstra, J. (2011). Biology 1102 Laboratory Handbook. Gainesville, GA, USA: Self-published document, Gainesville State College Division of Natural Sciences, Engineering, and Technology. Ogbonnaya, D. (. (2012, September 14). SCT 112 Introduction to Environmental and Science Technology II. Retrieved from Lecture 3: Transpiration: http://water.me.vccs.edu/courses/SCT112/lecture3.htm Swarthout, D. (., Hogan, C. P., & Taub, D. (. (August, 2010 3). Stomata. (C. J. Cleveland, Ed.) Encyclopedia of Earth. Retrieved October 6, 2012, from Encyclopedia of Earth: http://www.eoearth.org/article/Stomata