Hannah Johnson
Dr. Franssen
BIO 121-50
9 October 2015
Sugar and Salt Effect on Photosynthesis
Introduction
Climate change has been affecting the rate of photosynthesis causing plants to underperform or go extinct. As the climate increases the availability of water becomes a pressing issue for many plant species. Water is one of the most influential factors when talking about photosynthesis because the energy the plants receive from the sun excite the electrons that come from splitting water molecules. The energy from these excited electrons produce ATP, which then uses more water and carbon dioxide to create carbohydrates to proceed with photosynthesis (Zheng).
Another concern is that soil salinity is also affecting how plants perform photosynthesis. While the temperature increases, the availability of water decreases causing the salinity levels to increase as well. Whether or not the plants will photosynthesize in conditions with high salinity is a growing question. Salt and droughts are affecting plants more now than they used to, making scientists experiment different variables that will aid or decrease the rate of photosynthesis. Studies have shown that high levels of salt stresses the plants to the point where it affects their photosynthetic metabolism and these plants respond to these factors rapidly changing their gene expression to adapt. The stress also causes plants to store water in their vacuoles due to drought and high salinity. A recent study has shown that both of these stressors, drought and salinity, led to the plant’s photosynthetic gene count to decrease. It also showed that the stress of salt dehydrated the plants and caused an osmotic stress across the membranes (Chaves).
The experiment we developed tested whether or not by adding salt to the water if it would increase or decrease the rate of photosynthesis. We also considered what could increase the rate of photosynthesis, so we decided to add sugar (glucose) to the water to see if it would make a difference or not. We chose to test sugar because it seemed like the opposite of salt, so we could truly compare the effects. By adding salt to the water it creates a pH around 8 making it alkaline or basic. When you add sugar to the water it does not affect the pH, so it stays at a neutral 7 due to the water, but since glucose is a product of photosynthesis will it increase the rate? Our experiment is designed to see what factors will help the rate of photosynthesis increase since the reduced availability of water, rising temperatures, and soil salinity are all causing the rate to decrease.
Methods
The question that began this experiment was: Does adding salt or sugar to the water solution affect the rate of photosynthesis in the Elodea plant? The hypothesis that was tested was if sugar is added to the tap water solution, then the rate of photosynthesis will increase. The dependent variable is the rate of photosynthesis measured by milliliter per minute of oxygen. The independent variable is the type of water solution that is given to the Elodea plant. Some controlled variables that were distinguished in this experiment were the use of tap water for all three solutions, the same amount of water used in each test tube, and the same amount of light. The control in this experiment is having a test tube of regular tap water, so the salt mixture and sugar mixture can be compared to the control. The materials needed are three test tubes, three stoppers fitted with bent glass tubing, three strands of the Elodea plant, sugar (glucose), salt (NaCl), two weigh boats, a scale, and a 60 watt lamp.
To begin this experiment one must use the scale and the two weigh boats to measure out 4 grams of the sugar and the salt. Then extract three strands of the Elodea plant that will fit in each test tube. When placing the plants in each tube make sure the cut side is facing up. Put the 4 grams of sugar in one test tube with the plant in it and add 30 milliliters of tap water to it. Next add the 4 grams of salt in another test tube with the plant strand already in it and add 30 milliliters of tap water to it. The third test tube is the control which has the plant strand and just 30 milliliters of tap water in it. Make sure to label each test tube. After you finish filling up the test tubes, you place a stopper fitted with a piece of bent glass tubing in each test tube. When that is done there should be enough solution that fills the tubing about one-fourth of the length of the stopper tube (Figure 3). Make sure to mark the starting meniscus point on each tube with a marker. Place the three test tubes in a rack and put it in front of the 60 watt lamp so each tube has an equal amount of light (Figure 4). Put a timer on for 20 minutes and wait to observe the new meniscus point. When the 20 minutes is over, turn the lamp off, and mark the new meniscus point with a marker (Figure 6). Then measure the distance each meniscus moved in millimeters.
Finally, calculate the net photosynthesis by dividing the millimeters by 20 minutes. The prediction used for this experiment was that the sugar solution will increase the rate, the salt solution will decrease the rate, and the tap water solution will have a normal rate of photosynthesis.
Results
When observing each test tube during the twenty minutes, it was seen that the sugar solution test tube began bubbling the most as shown in Figure 5. From this observation, the sugar solution’s bubbles were oxygen bubbles that were produced from photosynthesis. These bubbles began pushing the water up making the meniscus point go up as well. The tap water solution was seen to have bubbles, but definitely not as much as the sugar solution. The tap water meniscus point also increased, but not by a lot. The salt solution seemed to make the original meniscus point decrease. As shown in Table 1, the starting meniscus point, the end meniscus point, the difference between those two points, and the net photosynthesis rates were recorded. The difference in the two meniscus points were 2 millimeters for the salt solution, 11 millimeters for the sugar solution, and 5 millimeters for the tap water solution as shown in Figure 1. This information then led to the calculation of the net photosynthesis for each solution type. Figure 2 shows that the net photosynthesis for each solution was 0.1 mm/min for the salt solution, 0.55 for the sugar solution, and 0.25 for the tap water solution.
Discussion
In conclusion, our experiment showed that the sugar solution increased the rate of photosynthesis, the salt solution decreased the rate, and the tap water had a normal rate. The sugar solution made the rate increase because it produced oxygen bubbles faster than the other two solutions. A possible reason could be that since glucose is one of the products of photosynthesis it did not need to take up time producing more glucose since there was already some in the water. The salt solution made the meniscus point decrease, and a possible reason for that could be that the salt causes the plant to dehydrate making it take in more water. The results of our experiment support our hypothesis because we hypothesized that the sugar solution would increase the rate of photosynthesis.
A reason for developing this experiment is to understand how plants react to high levels of salinity, so scientists can find ways to protect plants from these conditions. One of the goals in protecting plants from underperforming is to stabilize the drought and salinity conditions, so they can adapt and thrive (Chaves). A study in the Mediterranean shows that with the increasing temperatures the soil salinity is increasing with it, so a goal for them is to figure out how to detoxify the salt-induced plants (Fini). It is essential to come up with a way to purify the water and soil from salt so that the plants can photosynthesize and survive because they are a primary food source and they create the oxygen we need to live.
Due to climate change, the carbon dioxide levels are rising which can be good and bad for plants. A study shows that the increased CO2 can help to improve the usage of water and energy in plants. By improving these prominent factors of photosynthesis, it improves the plants ability to prevent water loss. These factors then help the plants survival against rising salinity levels and it can aid in desalinizing soil and land (Geissler). The down side of the elevated CO2 levels is that it is a short term benefit for these plants because after a long exposure of it, the plants go through downregulation because they become accustomed to it (Zheng). So plants then change internally by altering the amounts of their cellular components, such as RNA and protein.
For future experiments, it would be informative to test different amounts of salt in the water mixture to see what the lowest amount of salt could be without it decreasing the rate of photosynthesis. It would also be helpful to know what the highest amount of salt would be to determine when the salinity level becomes detrimental to the plant. Another variable to test would be how different salinity levels affect photosynthesis when some plants are in the shade and others are exposed to light. It would be beneficial to also test other variables to see what would increase photosynthesis, so it could help plants survive with all these changes in temperature, water availability, and salinity. What comes next is more experiments and studies to determine how we can protect plants from dehydrating and going extinct.
References
Chaves, M. M., Flexas, J., & Pinheiro, C. (2009). Photosynthesis under drought and salt stress: regulation mechanisms from whole plant to cell. Annals Of Botany, 103(4), 551-560. doi:10.1093/aob/mcn125
Fini, A., Guidi, L., Giordano, C., Baratto, M. C., Ferrini, F., Brunetti, C., & ... Tattini, M. (2014). Salinity stress constrains photosynthesis in Fraxinusornus more when growing in partial shading than in full sunlight: consequences for the antioxidant defence system. Annals Of Botany, 114(3), 525-538.
Geissler, N., Hussin, S., & Koyro, H. W. (2009). Elevated atmospheric CO2 concentration ameliorates effects of NaCl salinity on photosynthesis and leaf structure of Aster tripolium L. Journal of Experimental Botany, 60(1), 137-151.
Zheng, M. Y. (2015). Photosynthesis. Salem Press Encyclopedia Of Science,
Table 1. Data collected throughout the experiment.
Starting meniscus point(mm) / End meniscus point
(mm) / Difference
(mm) / Net Photosynthesis (mm/min)
1– Salt solution / 17 mm / 15 mm / 2 mm / 0.1 mm/min
2– Sugar solution / 34 mm / 45 mm / 11 mm / 0.55 mm/min
W- Tap water solution / 35 mm / 40 mm / 5 mm / 0.25 mm/min
Figure 1. The differences between the starting meniscus points and the end meniscus points to show whether or not the rate of photosynthesis increased or decreased. From left to right: salt solution, sugar solution, and tap water solution.
Figure 2. The different rates of the net photosynthesis for each type of solution. From left to right: salt solution, sugar solution, and tap water solution.
Figure 3. The setup of the test tubes right before they are placed in front of the lamp. The test tube on the left is the tap water solution, it has a W labeled on it. The test tube in the middle is the sugar solution, and it is labeled with a 2. The right test tube is the salt solution, and it is labeled with a 1.
Figure 4. The three test tubes at the beginning of the twenty minutes with the starting meniscus points. From left to right: salt solution, sugar solution, tap water solution.
Figure 5. The test tubes half way through the twenty minutes and the middle tube, the sugar solution, is bubbling up a lot. From left to right: tap water solution, sugar solution, salt solution.
Figure 6. The three test tubes at the end of the twenty minutes and it shows the new meniscus point. From left to right: salt solution, sugar solution, tap water solution.