The Effect of Acidic pH on Water Transport in Plectranthus scutellarioides
by Kelsey Nocilla
Pollution within the environment has become a popular topic of discussion in contemporary society, but the environmental effects of byproducts created by various types of pollution can be easily overlooked. For example, acid rain is produced when water vapor reacts with air pollution and harmful emissions within the atmosphere. Limited research has been done on the subject, but acid rain has the potential to affect a plant’s water transport system, which in turn hinders the plant’s growth and development. The goal of this study was to examine the effects of different pH levels on the water transport system of Plectranthus scutellarioides, a plant possessing medicinal properties. In two trials, plant stems from P. scutellarioides were placed in solutions of varying pH levels containing dye for ten minutes, after which the distance traveled by the dye through each plant stem was measured and recorded. The results from this study show that there is a specific, optimal pH level for the plant’s water transport system and for the plant’s growth. At a pH level of 4.6, the dye was able to travel the farthest average distance through the stems which allowed for the water transport system to function most efficiently. In addition, the results also indicate that the water transport system is negatively affected by water with pH levels that range higher or lower than this optimal level.
Keywords: Coleus plant, acid rain, water transport, pH, cross section
Though pollution is not a new problem, the past few decades of rapid technological and socioeconomic changes have caused a dramatic increase in the production of pollution and the negative affects it has on the environment (Lal 2016). Pollution can appear in many forms, but one of the most common forms is air pollution. Air pollution includes emissions such as carbon dioxide, sulfur dioxide, and nitrogen oxide. While these gases can be harmful to the environment directly, they can also affect the environment indirectly through the formation of acid rain.
Acid rain typically has a pH level of 5.6 and forms when sulfur dioxide and nitrogen oxide react with water vapor in the atmosphere. The hydrogen ion concentration associated with this pH has the potential to affect biological membranes, electron transport systems, and pH specific biochemical reactions (Lal 2016). Acid rain can affect terrestrial environments in a number of other ways, including acidification of soil and the alteration of nutrient supply (Lal 2016). When acid rain alters the composition of soil, it changes the main source of nutrients for the varying plant species in that particular ecosystem. Though there is limited research on the relationship between acid rain and the growth and development of plants (Evans 1982), it has been seen that abnormalities in the metabolisms of plants (photosynthesis, transpiration, etc.) can result from exposure to these acidic conditions (Lal 2016).
Acid rain has the potential to dramatically affect a plant’s growth because of one important component: the plant’s water transport system. A properly functioning water transport system is essential for a plant to survive. Water movement is a shared feature of almost all plant development processes, such as transpiration, photosynthesis, and the distribution of organic or inorganic molecules throughout the plant’s xylem, the vascular tissue that conducts water upward from the root (Kim et al. 2014). When acid rain falls, the water transport system of the plant is responsible for moving this water to each part of the organism in order to perform all of the functions and processes previously mentioned. If the acidity of the rain negatively affects this system, the plant will not be able to grow or function properly.
Different species of plants respond uniquely to varying levels of pH in their environment. For example, Odiyi and Eniola (2015) found that exposing V. unguiculate to different low pH levels caused the plants to exhibit a lower growth rate, chlorophyll content, and harvest index. By contrast, Zhang and Zwiazek (2016) found that a red-osier dogwood had greater growth, gas exchange, and root hydraulic conductivity at a higher pH level than a paper birch, which appeared to be more sensitive to high pH levels than to low pH levels. It was concluded that a high rate of water transport at the higher pH level for the red-osier dogwood was responsible for the increased growth rates.
Sen et al. (1993) found in a study of factors affecting growth and forskolin production in root culture systems of Coleus forskohlii, now recognized in the scientific community as Plectranthus barbatus, that pH affected the growth of the plant. The plants were exposed to pH levels of 5, 5.3, and 5.6 for forty days and the resulting data showed that P. barbatus had the most growth in a pH of 5.6 and that its growth decreased in lower pH levels (Sen et al. 1993). The species, P. barbatus, was of particular interest as it is the only source of forskolin, a compound that has medicinal benefits and is effective against glaucoma, congestive cardiomyopathy, and asthma (Sen et al. 1993). Based on the conclusions drawn by Sen et al. (1993), where P. barbatus had reduced growth in lower pH levels, it is possible that the water transport system of the related species used in this paper’s study, P. scutellarioides, will also demonstrate changes in fitness from exposure to acidic pH levels.
The purpose and objective of this experiment is to determine the optimal pH level for the water transport system and overall growth of P. scutellarioides. In this experiment, P. scutellarioides stems of equal size will be exposed to a water and eosin dye mixture with varying pH levels. The distance traveled by the dye in the stems of P. scutellarioides during a given period of time will help determine the pH levels at which the plant most easily transports water. To construct a hypothesis for this experiment, the results of previous related studies were considered. Sen et al. (1993) concluded that P. barbatus grew the most at a pH level of 5.6; however, pH levels above this value were not tested. Goulding (2016) determined that the optimal pH level of soil for most plants was 6.0 to 6.5. After examining the data collected by Sen et al. (1993) and Goulding (2016), it was hypothesized that a pH level greater than 5.6 would allow for optimal water transport. This hypothesis would be supported by the greatest distance traveled by the dye occurring in a solution with the pH level greater than 5.6. Determining the optimal pH level for water transport will allow for conclusions to also be drawn for the optimal pH for P. scutellarioides growth, considering the two are directly related.
There are two important real-world applications and benefits to determining optimal pH level. First, the research could be applied to understanding the water transport system of related species, such as P. barbatus. Knowing its optimal pH will allow for a greater yield of the plant to be made available for medical research in addition to increasing the available supply of forskolin. Second, examining the effects of different pH levels on P. scutellarioides could allow for a greater understanding of how pollution and acid rain could affect other plant species, and by association the environment, on a more global scale.
The optimal pH for P. scutellarioides to transport water was determined by placing stems from the plant in test tubes containing dyed water with varying pH levels. The independent variable tested was the pH level of the water/dye solution and the dependent variable measured was the distance traveled by the dye in the stems. The pH levels were measured by a pH meter, rather than litmus paper, for a more precise reading.
In preparation for the experiment, six test tubes were filled with 35 milliliters of water. The initial pH of the water was tested and hydrochloric acid was added to each of the test tubes until the desired pH levels of 7, 6.4, 5.8, 5.2, 4.6, and 4.0 were reached. Only the effects of acidic pH levels on P. scutellarioides were examined, so the neutral control group with a pH of 7 had the highest pH tested. Including a control with a neutral pH was necessary for observations to be made as to whether any amount of acid could affect the plant’s water transport system. After adjusting the pH to the desired levels, 10 milliliters of eosin dye was added to each test tube to ensure a visual representation of the distance traveled by the solutions through the plant stems. The dye was added after adjusting the pH of the water in order to avoid damage to the pH meter, but any minor pH changes related to the addition of the dye were negligible.
Next, the P. scutellarioides stems were each cut to an equal length of 8 centimeters measuring from the leaves at the end. A total of 12 stems were cut and two were placed in each test tube to provide two trials of data for each pH level. The stems were placed in the test tubes within 30 seconds of being cut to minimize the impact of the exposure to air. The stems remained in the test tubes for 10 minutes before being removed and rinsed off to eliminate any excess external dye. Lengthwise cross sections were taken of each of the stems and examined. The distance traveled by the dye was measured for both halves of the cross sections for each stem, resulting in four measurements taken for each pH level. These four distances were recorded and an average distance was calculated. The pH level that allowed the dye to travel the farthest was considered the optimal pH level for the water transport system of P. scutellarioides, and in turn, the optimal level for the plant’s growth.
Photo 1: Coleus stems are cut and ready to be placed in the test tubes containing water with varying pH levels.
Photo 2: Two stems are placed in each test tube to account for two trials of data.
All of the data collected from this experiment has been compiled into the table in Figure 1. Averaging the values measured from each cross section allowed a clear pattern to emerge within the data. At a pH level of 4.6, the dye traveled the greatest distance of 2.875 cm. The pH levels above and below this value resulted in shorter distances traveled by the dye. As the pH became more acidic, the distances traveled shortened. The dye associated with pH levels of 6.4 and 7 traveled the same average distance of 1.325 cm, the shortest distance recorded A graphical representation of the data is shown in Figure 2. The data trend visualized by the graph shows a normal distribution. Interestingly, upon measurement, two halves of the same cross sections sometimes demonstrated significant variation in the distance traveled by the dye, even though the stems may have been in the same solution. The data was important to record as this could be a common natural phenomenon in the vascular tissue of plants.
Cross-Section Distances Measured for P. scutellarioides Stems
Table 1: Data collected for each cross-section
Figure 2: The graph shows the relationship between the distance traveled by the dye through the P. scutellarioides stems based on the pH of the solution.
If P. scutellarioides was easily able to transport the solution, the distance traveled by the dye was greater. By contrast, if the plant was not able to transport a solution well, the dye traveled a shorter distance. Comparing the data from the acidic solutions to the data from the control allowed comparisons of whether acid within the water available to P. scutellarioides was beneficial or harmful to the plant and at what point the acidity could become harmful to the function of the water transport system.
The initial prediction for this experiment was that the optimal pH level for P. scutellarioides would be greater than 5.6, considering the findings of Sen et al. (1993) and Goulding (2016). However, due to the limits of previous studies, it was unknown how the effect of higher pH levels would manifest. Though the effects of pH levels greater than 5.6 were unknown before this experiment, it was predicted that the distance traveled by the dye would be greater in these levels than the distance traveled in levels lower than 5.6. It was predicted that a more acidic solution with a lower pH level would be harmful to the plant and its water transport system.
The results of this experiment differed from the expectation and hypothesis. The dye from the solution with a pH of 4.6 traveled the farthest average distance through the P. scutellarioides stems, so it was determined that a pH of 4.6 allowed for the most successful water transport through the xylem. The second farthest distance traveled by the dye occurred at a pH of 5.2, and the third farthest distance traveled by the dye occurred in a pH of 4.0. These findings are represented in Figure 2, which illustrates a clear bell curve trend between the pH level and the distance traveled by the dye. According to Sen et al. (1993), P. barbatus experienced better water transport in a pH of 5.6 rather than pH values of 5.3 or 5.0, which suggests that the plant’s water transport system functioned more efficiently in higher, more alkaline pH levels. Goulding (2016) described the ideal soil pH level for plants to be between 6.0 and 6.5. The information and findings of these two studies do not support the results of this experiment. The trend of the data in Figure 2 underscore that the distances traveled by the dye at higher pH levels are some of the shortest distances traveled.
However, the results from Zhang and Zwiazek (2016) may help explain why more acidic values facilitated more efficient water transport than less acidic, more alkaline values. In the study conducted by Zhang and Zwiazek (2016), the plants sensitive to high pH levels did not experience successful water transport when exposed to high pH levels compared to other plant species better suited for higher pH levels. Comparing those results to the data collected in this experiment, it is likely that P. scutellarioides is a species better suited for an environment that is slightly acidic rather than neutral or alkaline.
With that being said, the data in the graph also demonstrates a decrease in the distance traveled by the dye at pH values lower than 4.6. Though P. scutellarioides cannot perform effective water transport in highly acidic environments, there appears to be a small range in which the acidity is beneficial before it becomes harmful to the plant. Therefore, at a pH level of 4.6, the water transport system of P. scutellarioides performs most efficiently and allows the plant to undergo the greatest growth over time.
By determining the optimal pH level for the water transport system and growth of P. scutellarioides, the data and procedure of this experiment will be beneficial to those who grow this and other related species, such as P. barbatus, for medicinal use. More research could be conducted to further verify the optimal pH for P. scutellarioides and P. barbatus. Considering that P. barbatus is the only source of forskolin (Sen et al., 1993), it would be ideal for the preferred pH of the plant to be established to facilitate more efficient growth. By having a greater abundance of P. barbatus and the unique compound it contains, more forskolin can be made readily available for the treatment of different diseases and additional, more extensive research can be conducted on the benefits of the compound.
The data from this experiment can also serve as a small example for future studies on the effects of acidic conditions on plant species and the environment, such as those caused by acid rain. Though P. scutellarioides can sustain exposure to slightly acidic environments, there comes a point where the acidity becomes harmful to the plant and its water transport system. The negative effect that the excess acidity had on P. scutellarioides is a prime example of what phenomena such as acid rain can do to species of plants within the environment. The typical pH for acid rain is 5.6 (Lal, 2016), which may be too acidic, or too basic in the case of P. scutellarioides, for plants within their natural environments to grow, survive, and reproduce successfully. Not only would ecosystems suffer bottom-up trophic cascades from these effects, but there would also be economic consequences associated with imperiled plant life; for example, the scarcity of sought-after plants and crops would lead to increased prices of these goods.
At first glance, the optimal pH level for the water transport and growth of Plectranthus scutellarioides may seem to be a minuscule or unimportant issue. However, in reality, the findings of this study can create a basis for further research on the effect of acidic pH on other plant species and the environment.
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Zhang, W., & Zwiazek, J. J. (2016). Effects of root medium pH on root water transport and apoplastic pH in red-osier dogwood (Cornus sericea) and paper birch (Betula papyrifera) seedlings. Plant Biol, 18(6), 1001–1007.
The information gathered from this study and the investigation itself would not have been possible without help and support from a few different people. I’d first like to thank my group members who each contributed to designing and conducting the experiment: Sydney Sileno, Acacia Worley, and Micayla Civetta. I would also like to sincerely thank our GLA, Austin Heil. His constant encouragement and passion for biology enabled my group members and I to overcome the many challenges and setbacks that we experienced throughout this project.