Problem:
Which wavelength of light is the most optimal for promoting photosynthetic rate in spinach leaves within a 50-minute time period?
Hypothesis:
If shorter wavelengths of visible light have more energy, causing more energy to be absorbed by the electrons in the photosystems of chloroplasts, then the visible light with wavelengths between 380 nm and 450 nm, or violet light waves, will increase photosynthetic rate in relation to waves of visible light between 650 nm to 750 nm, or red light.
Materials:
Light sources (2 Flood Lamps)
5 clear cups
Distilled water
.02% Bicarbonate solution
Liquid Soap
5 heat shields (bowls with water)
Lego Robotics block structures
Green translucent cellophane wrap
Red translucent cellophane wrap
Violet translucent cellophane wrap
3 syringes
Tape
5 pipettes
Spinach leaves
Hole punch
Stopwatch
Labeling tape
Procedure:
Refer to the procedure of AP Biology Handout, Big Idea 2, Cellular Processes: Energy and Communication, Investigation 5, Photosynthesis, for procedure steps 1-5, with the following exceptions.
Teacher prepares 300 mL of 0.2% bicarbonate solution for each experiment.
Fill the remaining three cups with bicarbonate solution, labeling them as “violet light”, “red light”, and “green light”. Throughout the rest of the procedure you will be preparing material for both cups, so do everything for all five cups simultaneously.
Using a pipette, add two drops of a dilute liquid soap solution to the solution in each cup.
Using a hole punch, cut 10 or more identical leaf disks for each cup, being careful to avoid the veins. There should be fifty disks in all.
Draw the gases out of the spongy mesophyll tissue and permeate the leaves with the sodium bicarbonate solution, by using the syringes.
Construct three 6x6x6 inch structures from Lego Robotics pieces.
Cover one structure in green cellophane, secure with tape. Cover another structure in red cellophane, secure with tape. Cover the last structure in violet cellophane, secure with tape.
Place a glass bowl with water on top of each cup (glass bowls perform the function of, and are now referred to, as heat shields).
Hang two floodlights about three feet away from the flat surface on which the cups are placed.
Place the carbonated control (cup with ten spinach disks and carbonated water, with heat shield, no structure), and red group (cup with ten spinach disks and carbonated water, with heat shield, under red structure), under one of the lights so that the light is distributed evenly to both test groups.
Place the distilled water (cup with ten spinach disks, with heat shield, no structure), violet group (cup with ten spinach disks and carbonated water, with heat shield, under violet structure), and green group (cup with ten spinach disks and carbonated water, with heat shield, under green structure) under the other light so that the light is distributed evenly to all test groups.
Turn on the floodlights.
Immediately start the stopwatch; begin recording at minute 1, stop at minute 50.
Record all data for each cup every minute, on the minute.
Observations and Data:
Throughout the 50 minute recording period, in the carbonated cup, zero out of the ten disks rose. In the distilled control cup, zero of the ten disks rose. In the cup with carbonated water underneath the red cellophane covered structure, zero out of the ten disks rose. In the cup with carbonated water underneath the green cellophane covered structure, zero out of the ten disks rose. In the cup with carbonated water underneath the violet cellophane covered structure, zero out of the ten disks rose. In total, zero out of the fifty disks rose.
Analysis:
Autotrophic plants perform photosynthesis by absorbing light in order to convert carbon dioxide and water into glucose that they can use to carry out cellular processes. Certain wavelengths of light facilitate photosynthesis better than others. According to Campbell and Reece AP Edition Biology, Seventh Edition, the chlorophyll a pigment found in photosystem II (called P680) is best at absorbing light with a wavelength of 680 nm (in the red part of the spectrum). The chlorophyll a found in photosystem I (called P700) is most effective at absorbing light wavelengths of 700 nm (in the far red part of the spectrum). With regard to colors of light, the primary colors are red, blue, and green. Violet light is a secondary color made by combining red and blue. Therefore, it is one of the three light colors (red, blue, and violet) that contain no green light. Because green light is reflected by chlorophyll, these three colors that contain no green light are better absorbed by chlorophyll than light colors that do contain green light (such as green and yellow) (Campbell and Reece, 2005). Blue light and violet light have a similar wavelength of around 450 nm so they share some properties, such as how they are absorbed by chlorophyll in plants. Violet light is a mixture of the colors blue and red, which means it affects plants in a similar way to red light. Therefore, red, blue, and violet wavelengths of light are absorbed by chlorophyll better than any other wavelengths, providing more energy to plants for photosynthesis than other wavelengths.
In 2010, Giedrė Samuoliene, Aušra Brazaityte, Akvilė Urbonaviciute, Gintarė Sabajeviene, and Pavelas Duchovskis, from the Institute of Horticulture, Lithuanian Research Centre for Agriculture and Forestry, studied the effect of red and blue LEDs on frigo strawberries. One group of frigo sprouts were grown in a phytotron chamber under a red LED light (640 nm), and another was grown under a mixture of red and blue (455 nm) light. The lights were on for 16 hours a day, and the growth of the sprouts were tracked for a month. After a month of growth under these conditions, the plants were transported to a polythene greenhouse and grown without the use of colored LEDs. The results of the experiment were that the strawberries solely grown under red light were 30% taller than those grown under a mixture of red and blue light. Compared to the effects of the red light on plant growth, the blue light was not as efficient in the elongation process of the stems (Samuoliene, Brazaityte, Urbonaviciute, Sabajeviene, Duchovskis, 2010).
In another experiment in 2009 by Fánor Casierra-Posada and Javier F. Rojas B., from the Universidad Nacional de Colombia, the growth of broccoli seedlings and the effect of different wavelengths of light on plant growth were studied. Blue, yellow, orange, and transparent plastic films were placed on a seedbed and extended to 60 cm above the soil, with a section left uncovered as a control. After a 30 day germination period, under these conditions, the seedlings were transported to a field, where their dry matter content were measured, as well as the area of their leaves, the height of each plant, and the stem diameters. Of the seedlings, those grown under the red light had an increased growth rate compared to the blue, yellow, orange, transparent, and uncovered groups (Casierra-Posada and Rojas B., 2009). In the spinach lab conducted, spinach disks were also placed in red light and white light. If the spinach experiment supported Posada and Rojas’s, we too would have observed an increase in the rate of photosynthesis under the red light. However, from minute one to minute 50, no spinach disks rose.
In 2014, Heidi Wollaeger and Erik Runkle from Michigan State University Extension studied how the blue, red and green wavelengths from LEDs influenced growth of “tomato ‘Early Girl,’ salvia ‘Vista Red,’ petunia ‘Wave Pink,’ and impatiens ‘SuperElfin XP Red.’” They grew these four plant varieties in growth chambers for four weeks at 68 ° F, beginning after germination. The plants were provided with light for eighteen hours every day, and the light intensities for all light wavelengths were consistent at 160 µmol∙m-2∙s-1. Each group of each plant variety was also grown with different percentages of light wavelengths (such as 100% blue light; 100% red light; 50% red light and 50% green light; 50% blue light and 50% red light) from LEDs, and a control group was grown under white fluorescent lights. Factors including total leaf area, seedling height, stem length, number of flower buds (only in impatiens), and the fresh weight of the shoots were measured. They found that plants grown with 50% green light and 50% red light were about 25% shorter than those grown exclusively under red light, but about 50% taller than the plants exposed to more than 25% blue light. Additionally, plants grown under at least 25% blue light were 40-60% shorter than those grown under 100% red light (Wollaeger and Runkle, Michigan State University Extension, 2014). Seeing that green light is reflected from the chlorophyll and not absorbed, only half of the normal amount of red light is available to the plant, meaning that the overall plant growth was not as successful as those grown solely under red light. Since green light (495 nm- 570 nm) is reflected by the chlorophyll in plants, very little of the green light wavelength is actually absorbed by the plants, so its photons provide the plant with very little energy. Therefore, green light is less effective in photosynthesis than red light, which is more readily absorbed by the chlorophyll (Campbell and Reece, 2005). Wollaeger and Runkle’s research further supports the idea that the use of red light stimulates photosynthesis, which correlates to a higher photosynthetic rate. According to this data, if the spinach lab was performed under ideal conditions, the disks in the green cups should not have risen, since they were exposed only to green light, and not to any red light.
Since photosynthesis did not occur in the carbonated control group, it can be concluded that the experimental design was faulty and a factor other than the wavelength of light must have inhibited photosynthesis during the fifty minute recording period. It was hoped to have seen the spinach disks under the red and violet light perform the most photosynthesis, however this did not occur during the fifty minute recording period either.
The age of the bicarbonate solution used to carbonate the water on the cups is believed to be the cause of the spinach disks never rising. It was shown through a bromothymol blue test that the solutions were not properly carbonated. Since the carbon dioxide gas was missing from the bicarbonate solution used in the experiment, the spinach disks were not able to perform photosynthesis. Since no photosynthesis took place, no products (oxygen gas and glucose) were formed. Therefore, no oxygen was produced in the spongy mesophyll; as a result, the leaves did not rise within the solutions, as seen in the data, with zero disks rising during the fifty minute time period.
Conclusion:
No conclusion can be reached. In order to research the effects of colored light on photosynthetic rates further, the water used in the experiment needs to be properly carbonated to gain accurate results. In the future, the photosynthetic rate of a variety of plants could be tested to see if the results are consistent with different kinds and amounts of chlorophyll a and chlorophyll b, in different kinds of leaves. Additional wavelengths of light, such as orange (590–620 nm) and blue (450-495 nm) could also be tested to expand the range of conditions under which the plants are exposed. This way, no wavelengths that these plants might be naturally exposed to are excluded. Additionally, combinations of different wavelengths could be tested to see if there is an optimum light combination, or if there is more than one light wavelength that plants primarily rely on. One more factor that could be altered would be using the same wavelengths of light, but with different kinds of bulbs (such as fluorescent, LED, incandescent) to see if there is a difference in their effect on plants and if this difference might have affected the results of the experiment.