Effect of Different Light Intensity on the Rate of Photosynthesis of Aquatic Plant Leaves

The effect of different light intensity on the rate of photosynthesis of aquatic plant leaves through measurement of oxygen gas

Abstract: Photosynthesis is used by plants to convert carbon dioxide and water into a carbohydrate and oxygen. The oxygen produced serves as a vital source of oxygen for most living organism and removing carbon dioxide that is produced through human respiration. The experiment performed investigates the effect of varying light intensity on the amount of oxygen produced and therefore the rate of photosynthesis in the leaves of an aquatic plant. These plants were revealed to a variety of intensity light with a constant temperature and the amount of oxygen produced by each light intensity was recorded at five-minute intervals for as long as sixty minutes. The results was that the mean oxygen produced with light intensity #3 (2250 Lumens) is 0.168 mL, which was the lowest mean amount of oxygen gas produced while light intensity #2 (1800 Lumens) produced 1.034 mL, which is the largest mean amount of oxygen gas. Generally, there is a positive correlation between light intensity with oxygen gas production but light intensity #2 serves as an anomaly. Photosynthesis rate of aquatic plant leaves is tested using t-test and the mean of oxygen gas produced is shown by a bar graph.

Keywords: photosynthesis, light intensity, oxygen gas production, aquatic plants

Introduction

    Photosynthesis is important because it is needed in order to provide living organisms with oxygen. It is the process by which plants, algae and some species of bacteria convert carbon dioxide alongside with water into carbohydrate and oxygen gas; important component used in the cellular respiration of all living organisms. (OpenStax College, 2018). Photosynthesis is performed in nature using photon energy from sunlight to remove the electrons from water and transfer them to carbon dioxide which in turn produces glucose and oxygen gas. Two specific processes that happen during photosynthesis is the light dependent and the light-independent reactions which need light intensity to take place. (OpenStax College, 2018). The purpose of this experiment was to test the correlation of different light intensity with the rate of photosynthesis of aquatic plant leaves by measuring the production the oxygen gas (Keir et al. 2018). By varying light intensity against the controlled variable of no light and outdoor light, its effects on the rate of photosynthesis were examined by measuring the oxygen gas produced by each light intensity. The hypothesis is as values of the light intensity increases, the production of oxygen gas will also increase. There will be a positive correlation between light intensity and the amount of oxygen gas produced. The hypothesis can be tested using statistical methods including the t-test calculation, standard deviation, SEM, mean and p values. The null hypothesis states that there is no significant difference between the amount of light intensity and the amount of oxygen produced and therefore there is no direct effect on the aquatic plant’s photosynthetic rate. The prediction is if the light intensity of the treatment group increases, then the amount of oxygen gas produced will also increase but will then decrease at a certain value of light intensity because at that particular value enzyme that is present inside the aquatic plants will start to denature. This prediction can be supported by past experiments that states temperature indeed has an effect on oxygen gas produced as high intensities of solar radiation could cause photoinhibition or even death of plant cells due to the lack of ability of plants to adjust their structure or concentration of pigments at high irradiance so if light intensity increases too much, the amount of oxygen gas produced will start to decrease (Harb, 2018).

Materials and Methods

    In reference to the BIO A01H3 Fall 2018 lab manual, we obtained two 250 mL Erlenmeyer flasks and two 75 mL glass tubes that is available at the lab bench. The glass tubes were then labeled Tube #1 and Tube #2, and each Erlenmeyer flask was filled with approximately 200 mL of tap water. During the experiment, the tap water in the flask acts as heat absorbance to help control against change in temperature. The labeled glass tubes were filled with 75 mL of bicarbonate solution that can be found at the lab bench and placed into each Erlenmeyer flask. Then, we get 20 cm long samples of the aquatic plants along from the front bench and placed into a white plastic tray which can again be found on the lab bench. These samples were then cut with scissors to 15 cm from the growing tip while submerged in water present in the white plastic tray. A sample was placed in each test tube with the top of the plant at the bottom and plant fully submerged in the bicarbonate solution. With the plant in the solution, 0.5 cm was cut off each stem at a point between the leaves where the leaves are not attached to the stem of the plant. Stem must be kept below the surface of the bicarbonate solution while cutting it as it will prevent the freshly cut stem from being exposed to air, which will lead to an air-lock. A rubber stopper that is attached to 1 mL pipette together with 10 mL syringe and needle were placed on each test tube carefully to make sure there was no air bubbles in the tube. The syringe and needle were used to remove solution from the tube so the bicarbonate solution level was set at 0. Both Erlenmeyer flasks were then placed at approximately 15 cm in front of a desk light with a white light bulb and the lamp was then turned on. The plants were left to equilibrate for 5 minutes and the pipette was reset to its initial value before the lamp was turned on. Readings on the pipette were taken down every 5 minutes. After 60 minutes, lamp should be turned off and left to cool. Throughout the experiment, outside light was kept as a positive control and the purpose for this is to investigate the effect of maximum light intensity on the rate of photosynthesis and allows us to compare results with the experimental group while no light intensity act as a negative control and the purpose for this is to act as a dark treatment as we can expect the minimum amount of gas produce is when there is no light intensity. Light in the room was switched off to ensure that the light intensity of the surroundings will not affect the result of the experiment. Light intensity on each treatment was the manipulated variable while the amount of oxygen gas produced served as the responding variable. Using the data of the oxygen gas production, a bar graph was made with all the values of light intensity on each treatment using Microsoft Excel. The data was then combined with data of practical group #0024 and the standard deviation, calculated t-test, mean, SEM and p values, were analyzed using QuickCalc made by GraphPad Software. (Keir et al. 2018).

Results

    The calculated t-test values were also calculated with a sample size of 8, and the varying light intensity all had values that surpassed the critical t value of 2.145 (Table 1) except for comparison 2b where we compare outside light and light intensity # 2. Thus, the p values of all the comparison were also found to be less than 0.05 except for comparison 2b and as a result, the null hypothesis was rejected in almost all the cases. Since the sample size was constant, the degrees of freedom were constant at 14 within these calculations.

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    As seen in Figure 1, the mean total of oxygen produced by the controlled outdoor light intensity was much higher than the amount of oxygen produced by the rest of the different light intensity. Light intensity #2 (1800 Lumen) produced the largest mean amount of oxygen gas, followed by light intensity #4 (2600 Lumens) which produced the second largest mean amount. Light intensity #1 (600 Lumens) and #3 (2250 Lumens) appeared to produce a similar amount of oxygen with amounts smaller than that of all the other light intensity, but not less than controlled no light intensity. Comparing light intensity #1 with light intensity #2, there is a notable increase in the mean amount of oxygen gas produced of about 3 times. However, comparing light intensity #2 with light intensity #3, there is a significant drop in the mean amount of oxygen gas produced of almost about 5 times.

Table 1: Statistical analysis of comparison between control group of no light (0 Lumens) and outside light (>4000 Lumens) with experimental group of light intensity # 1 (600 Lumens), light intensity # 2 (1800 Lumens), light intensity # 3 (2250 Lumens), and light intensity # 4 (2600 Lumens).

Figure 1: The mean value of oxygen produced at different light intensity of no light (0 Lumens), light intensity # 1 (600 Lumens), light intensity # 2 (1800 Lumens), light intensity # 3 (2250 Lumens), light intensity # 4 (2600 Lumens) and outside light (>4000 Lumens). The error bars were computed using the standard deviation of the mean values of the sample size 8.

Discussion

    From the results acquired from this experiment, the null hypothesis is rejected, and the hypothesis is supported since most calculated p values were less than 0.05 (Table 1). Mean values of the oxygen gas amount produced for treatments with outdoor light were indeed higher than the mean values of the gas produced with other light intensities (Figure 1). Surprisingly, the light intensity # 2 was found to produce approximately three to five times more than that produced by light intensity # 1, #3 and #4; a difference that is quite noticeable. The oxygen gas produced by these three light intensities was roughly 0.3mL greater than the controlled value (no light). This pattern in photosynthetic rate corresponds with results found in previous studies on Anoectochilus roxburghii where rate of photosynthesis is highest at 30% (Shao Q et al. 2014) as well as conclusions from a study of Neochloris oleoabundans which stated that high light intensity level could result in loss of photosynthetic activity (Sousa C et al. 2013). From the result of the experiment we can conclude the prediction that was made earlier was right.

    While it was expected that the higher light intensity will produce a higher amount of oxygen gas, this was not actually what happened during the experiment. Although the highest light intensity of more than 4000 Lumens did produce the most amount of oxygen, the amount produced by light intensity # 3 and 4 is much lower than that produced by intensity # 2. Since the difference between the oxygen produced from the light intensity #2 and the light intensity #3 and #4 is large, the unexpected outcome could be due to photoinhibition due to high irradiance condition. Plants will absorb excessive light energy that will inactivate chlorophyll in chloroplasts and leads to a decrease in photosynthetic activity (Shao Q et al. 2014). Another possible reason is that light makes highly reactive oxygen by a process called photo-activation and this oxygen will cause damage to water-oxidizing center and deactivate electron transport chain and leads to a decrease in photosynthetic activity (Sousa C et al. 2013). However, since the difference between the oxygen produced from light intensity #1 and light intensity #3 isn’t large, the unexpected outcome could be due to the random error that occurs when two people read the same measured value. Each partner from the group may have looked at the pipette from a different angle or may have estimated the last digit differently which then can contributed to a different mean value. Allowing only one person to read the measurements for the entire experiment is a way to minimize this error. Eventhough light intensity had an obvious effect in the photosynthetic rate, photosynthesis also may be affected by different other factors such as pH level and the temperature of the bicarbonate solution or the surrounding.

    In conclusion, the prediction proposed was supported as the photosynthetic rate which was measured by the production of oxygen gas of the aquatic plant leaves decrease significantly as light intensity increases from 1800 Lumens to 2250 Lumens. While other factors likely contribute to this rate, it can be concluded that the light intensity does indeed play a major role in affecting the rate of photosynthesis. Further research can be done on the effects of light intensity around 1800 Lumens to estimate the maximum light intensity for a plant to have maximum photosynthetic rates.

References

Internet Resource

• Clark MA, Choi J, Douglas M. Overview of Photosynthesis online. c2018. Openstax College; [accessed 2018 October 25]. https://cnx.org/contents/dEDuZRSu@8/Overview-of-Photosynthesis

Scholarly Journal Article (primary source)

• Harb TB, Nardelli A, Chow F. 2018. Physiological responses of Pterocladiella capillacea (Rhodophyta, Gelidiales) under two light intensities. Photosynthetica. [accessed 2018 October 23];56(4):1093-1106. https://link-springer-com.myaccess.library.utoronto.ca/content/pdf/10.1007%2Fs11099-018-0805-9.pdf doi: 10.1007/s11099-018-0805-9

Biology Lab Manual

• Keir, K., E. Gladilina and C. Armstrong. 2018. BIOA01H3F – Life on Earth: Unifying Principles – Laboratory Manual for Fall 2018. Toronto, Ontario: University of Toronto Scarborough Printing Services.

Scholarly Journal Article (primary source)

• Shao Q, Wang H, Guo H, Zhou A, Huang Y, Sun Y, Li M, 2014. Effects of Shade Treatments on Photosynthetic Characteristics, Chloroplast Ultrastructure, and Physiology of Anoectochilus roxburghii. PLOS ONE. [accessed 2018 October 24];9(2):e85996.https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0085996#references doi: 10.1371/jourmal.pone.0085996

Scholarly Journal Article (primary source)

• Sousa C, Compadre A, Vermue MH, Wijffels RH. 2013. Effect of oxygen at low and high light intensities on the growth of Neochloris oleoabundans. Algal Research. [accessed 2018 October 24];2(2):122-126.

https://ac.els-cdn.com/S2211926413000246/1-s2.0-S2211926413000246-main.pdf?_tid=394bd9d8-3752-4418-b042-ebd5f3ac6554&acdnat=1540435045_40733c171e81874e85bbe76c80bc0a92

doi: 10.1016/j.algal.2013.01.007