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UTSC Photosynthesis Journal
BIO A01 2024- Fall; 1(1): 1- 1011238108
The effect of DCMU on the photosynthetic
rate of Elodea densa
Kotha Karmakar, Anjali Acharya, Thulaksigha Sayanthan Dept. of Biological Sciences, University of Toronto Scarborough, Toronto, Canada UTSC BIOA01 Lab PRA PRA0051 TA: Eric Wilson
Abstract: The experiment aimed to examine the impact of the herbicide DCMU on the photosynthetic rate of
Elodea densa , a fast-growing aquatic plant sensitive to environmental pollutants (e.g. herbicides, industrial chemicals). We investigated whether DCMU, known to inhibit electron transport at Photosystem II, would reduce the photosynthetic rate of Elodea densa. These aquatic plants were exposed to light under control (NaHCO₃) and experimental (NaHCO₃ + DCMU) conditions, and oxygen production was measured over 60 minutes to assess photosynthetic rate. The experimental group containing DCMU showed a significantly lower photosynthetic rate than the control, supporting our hypothesis that DCMU inhibits oxygen production, decreasing the photosynthetic rate. These findings emphasize the ecological risks of herbicide runoff in aquatic systems and highlight the need for further research on herbicide impacts on freshwater plant physiology.
Keywords: Photosynthesis, Elodea densa, Effects of DCMU, Photosynthetic rate, Aquatic ecosystems, Light-
dependent reactions, Calvin-Benson cycle, Electron transport chain, Herbicide.
Introduction
Photosynthesis is the process by which photoautotrophs, such as plants and certain microscopic organisms, convert light energy into chemical energy to produce glucose or carbohydrates (Lambers and Bassham 2019). This process is essential for life on Earth since photoautotrophs, which are also known as producers, use inorganic compounds to produce organic compounds themselves. They utilize carbon dioxide, water, and light to build energy-rich compounds and releases oxygen as a by-product. Eukaryotic photoautotrophs, such as Elodea densa, perform photosynthesis in chloroplasts through two main stages. First, the light-dependent reactions which occur in the thylakoid membranes transform chemical energy from sunlight into ATP and NADPH. During this stage, water molecules are split, resulting in the release of oxygen, electrons, and protons (Lambers and Bassham 2019). Photosystems II and I absorb light energy, exciting electrons in chlorophyll. These electrons move through a series of protein complexes and reducing agents, ultimately forming NADPH from NADP+ (Tohamo 2023). This process includes both noncyclic electron flow, where electrons move from water to ferredoxin, and cyclic electron flow, where electrons are recycled back to the electron carriers in order to produce more ATP when it is needed. (Lambers
and Bassham 2019) In the second stage, the Calvin-Benson cycle which takes place in the chloroplast stroma, utilizes ATP, NADPH, produced from the light-dependent reactions, and carbon dioxide from the air to synthesize carbohydrates. While ATP is produced in the chloroplasts, only carbohydrates are sent to the cytoplasm for cellular respiration (Lambers and Bassham 2019). Understanding these basic ideas is important for studying how environmental factors like light availability, temperature, and pollutants affect photosynthesis. Elodea densa, or waterweed, is a submerged aquatic plant native to North America and found in many freshwater ecosystems. (Pfingsten et al. 2016) Known for its rapid growth, Elodea is also highly sensitive to environmental changes, making it an ideal model for examining the impact of pollutants like herbicides on photosynthetic organisms (Pfingsten et al. 2016). In this experiment, we used the herbicide DCMU, also known as diuron, to study its effects on Elodea’s photosynthetic processes. Studies show that DCMU inhibits photosynthesis by blocking the electron transport chain at photosystem II, specifically at the plastoquinone binding site, which prevents the transfer of electrons (Trebst 2007). In this experiment our hypothesis or the question we asked was: How does the presence of the herbicide DCMU affect the photosynthetic rate of Elodea densa? We predicted that if there is a presence of DCMU, then oxygen production of Elodea densa is likely to be lower, so the rate of photosynthesis is expected to decrease since DCMU acts as an herbicide (Trebst 2007). To conclude, it is important to understand how herbicides like DCMU affect the photosynthesis of aquatic plants, especially since their use in agriculture is increasing. While DCMU is known to disrupt electron transport at Photosystem II, its effect on the overall photosynthetic rate in aquatic species like Elodea densa is less studied. This research, therefore, examines an important knowledge gap in environmental biology to clarify the effects of herbicides and their potential impact on aquatic ecosystems. Materials and Methods In this experiment, two Erlenmeyer flasks were prepared, each filled with 200 mL of water from the sink. Two labeled glass tubes were also used: one was the control (Tube 1) containing 75mL of 0.5% NaHCO 3 (sodium bicarbonate solution) which served as a baseline to measure the photosynthetic rate, and the other was the experimental tube (Tube 2) containing 75mL of a mixture of 0.5% NaHCO3 and herbicide DCMU. Therefore, the independent variable is the presence and absence of DCMU whereas the dependent variable is the photosynthetic rate of Elodea densa. After that, each tube was placed inside its corresponding Erlenmeyer flasks filled with water. Two aquatic plants, known as Elodea densa, were carefully collected using a white tray and forceps. They were kept in water inside the tray while being cut to a length of 15 cm from the growing tip. The first plant was then placed in Tube 1, with the cut end at the top and the growing tip submerged at the bottom. After that, 0.5 cm was cut off the stem while it was still underwater to prevent air bubbles. The same steps were followed for the second plant in Tube
- Each tube was then sealed with a rubber stopper connected to a glass pipette and syringe. After ensuring no air bubbles were trapped, the syringe adjusted the solution level in the pipette to 0.0 mL. Both flasks were positioned 15 cm from a lamp, measured with a ruler, to provide consistent light for photosynthesis and any extraneous light that might affect the results was turned off. After a 5-minute equilibration period, the pipettes were reset to 0.0 mL, and gas production was measured at 5-minute intervals for 60 minutes. If the solution level in the pipette reached 0. mL, it was reset, and 0.9 mL was added to the subsequent measurements. Finally, the total oxygen production was recorded, and the photosynthetic rate (in mL/min) was calculated based on the total volume of oxygen gas produced during the 60 minutes using the formula: total volume of oxygen gas produced in mL divided by 60 minutes. Altogether, there were 8 trials done in this experiment and in each trial the photosynthetic rates of both the control and the experimental group were recorded. After completing the previous steps, GraphPad software was used to determine the critical t-test value, the calculated t-test value, the actual p-value, degrees of freedom (df), the mean photosynthetic rates, standard deviation (SD), standard error of the mean (SEM) (GraphPad: t-test calculator 2018). After that, Excel was used to create a figure showing the mean photosynthetic rate for both the control and experimental groups, along with standard deviation error bars (SDE) to indicate how much the results varied. The t- test value and p-value were used to figure out if the experiment accepts the null hypothesis or accepts the alternative hypothesis. The null hypothesis states that DCMU will have no effect on the photosynthetic rate whereas the alternative hypothesis states that DCMU will have an effect on the photosynthetic rate.
0.00965) (See Figure 1). These findings provide clear evidence that DCMU inhibits oxygen production and thus supports the mechanism by which DCMU disrupts electron flow at photosystem II (Trebst 2007). When we compare our results with other studies, they are consistent with previous findings that document DCMU's role in photosynthetic inhibition. For example, a study on wheat plants demonstrated that applying DCMU to wheat plants made them more vulnerable to oxidative stress, because of a buildup of harmful reactive oxygen species (ROS) (Oliveira et al. 2021). The plants containing DCMU showed signs of oxidative damage, with increased levels of harmful molecules like superoxide anions and hydrogen peroxide (Oliveira et al. 2021). This ROS imbalance led to stress that damaged cell structures, and therefore reduced photosynthesis (Oliveira et al. 2021). Further supporting evidence can be drawn from another study which demonstrates that DCMU exposure can lead to chlorophyll and carotenoid photobleaching in plants, as observed in barley leaves (BARRY et al. 1990). This photobleaching reduces the effectiveness of photosynthetic pigments found in photosystems, further reducing the plant’s ability to capture and convert light energy. As a result, these studies confirm the effects of DCMU on photosynthetic processes and plant health. Specifically, this decreased photosynthetic rate can lead to reduced growth in aquatic plants like Elodea, which can lower oxygen levels in the water and negatively impact aquatic organisms that rely on dissolved oxygen. Given that Elodea densa plays a role as a primary producer in many freshwater systems, any disruptions to its ability to photosynthesize can lead to serious consequences for aquatic life and the overall health of the ecosystem. Furthermore, the use of herbicides in agriculture is also increasing. However, our study has limitations that should be noted. The observed decrease in photosynthetic rate was not directly measured with respect to ROS accumulation or chlorophyll degradation, both of which could provide a more comprehensive understanding of DCMU’s effects. Additionally, while our sample size was sufficient to detect a statistically significant effect, a larger sample could enhance the reliability of our results. Furthermore, Elodea densa was studied in a controlled laboratory environment, and DCMU's impact may vary in natural aquatic ecosystems where plants are exposed to multiple environmental factors. Given these findings, further research could investigate the chlorophyll degradation or specific oxidative responses of Elodea densa to DCMU, including the measurement of ROS levels. Examining DCMU’s impact on other aquatic species would also provide valuable insights into broader ecological effects, particularly in environments where herbicide runoff is prevalent. Additionally, future studies could explore alternative methods to mitigate DCMU’s effects on aquatic plants, such as enhancing antioxidant responses or using less environmentally harmful herbicides. In summary, this study contributes to the understanding of how DCMU negatively impacts photosynthesis in Elodea densa , aligning with previous research on its inhibitory effects. The findings emphasize the potential risks caused by herbicide pollution in aquatic ecosystems, especially as agricultural herbicide use is rising. Future research could involve examining more effects of these herbicides on Elodea densa or similar species, as well as exploring alternative methods to mitigate DCMU’s effects on aquatic plants. By expanding on our findings, further studies could improve our ecological understanding and establish conservation strategies for maintaining healthy aquatic ecosystems.
UTSC Photosynthesis Journal
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