Observing the Effects of Different Wavelengths of Visible Light on

Photosynthesis in Spinach Leaves Using Leaf Disks

(or OTEODWOVLOPISLULD for short)

By Owen Cortet, Katharine Lee, and Christian Zapata-Sanin

Research Question

What is the effect of different light wavelengths (on the visible spectrum) on the photosynthetic rate in plants?

Predictions

● The green light will be the least efficient because the chlorophyll in plants reflects the green light and therefore does not absorb it.

● The red light will be more efficient because of the fact that chlorophyll can absorb red, blue and violet well.

● The blue light will be more efficient than the red light because of its wavelength and energy.

Hypothesis

The efficiency of light when used in photosynthesis is dependent on its energy (taken from its wavelength) its similarity to the dominant pigment, and on how much of each pigment is devoted to absorbing the color

Experimental Design

Control: The control was the first experiment done using only white light (normal light) on the leaves.

The way for interpreting the data correctly was by examining which light colors are absorbed by leaves, and matching it with the data from the lab.

Experimental Design (cont.)

Procedure: 12 leaf disks were cut using a hole puncher and had the air extracted using a syringe. The leaf disks were then placed in a solution with dissolved CO2 in a plastic cup with a 2” diameter. To control the light received by the leaf disks, a 6” tall cylinder of black paper was used to surround the cup. The cylinder had a 1” by ½ “ hole cut into it at about 2” height. The cup and cylinder were then placed under the light and the amount of leaves that rose above the water level were recorded by minutes.

Data

Minutes White light No light Blue Red Green

1 0 1 0 0 0

2 1 1 0 0 0

3 4 1 1 0 0

4 4 2 2 0 0

5 4 2 3 0 1

6 6 4 4 0 2

7 8 6 5 1 2

8 11 6 5 2 2

9 12 6 5 4 2

10 6 6 7 2

11 6 8 8 3

12 6 8 9 4

13 6 9 11 4

14 6 9 12 5

15 6 9 5

16 5 10 5

17 6 11 6

18 6 11 6

19 7 11 6

20 7 12 8

21 8 10

22 8 10

23 8 10

24 5 11

25 5 11

Leaf Disks vs. Different Colors of Visible Light

Caption

The above data table shows how many leaf disks (out of 12 each) had risen to the top of the solution every minute recorded when exposed to different wavelengths of visible light. Once all twelve leaf disks rose to the top, the data stopped being recorded.

Caption

The line graph displays the amount of leaf disks above the solution of soap, water, and baking soda cumulatively. The white light was the most efficient followed by the red light which is the most contrasting from green. The three different colored lights are in order based on their similarity to green despite their wavelengths.

DiscussionAfter testing the leaf disks with each type of light, surprising data was collected. The white light was most

efficiently used taking only 9 minutes for all leaf disks to rise to the surface of the solution then followed in order by red, blue, green, and lastly, no light. Although it was expected that the red light would be the second least efficient wavelength, it surpassed blue by 6 minutes. As expected, leaf disks exposed to either green or no light were the lowest performing. It was predicted that the blue light would be the most efficient, yet it was the red light that was the most efficiently used out of the non-white lights. This could be explained by the contrast of red to green, the color of the pigment chlorophyll. Since red is the opposite of green on the color wheel, it may be used more efficiently than blue when the same amount of each is given to the plant. In addition, it may be that different wavelengths of visible light trigger different processes within the plant and that depending on the plant, some processes may be triggered more than others such as growth which would be a property spinach would be bred for. The reason for the white light being most efficient seems to be due to evolution. The sun emits white light and since plants receive light from the sun by being outdoors, over time, they would have evolved to use white light most efficiently. In addition, white light contains the entire spectrum of visible light as evident when it is passed through a prism. The reason for the inefficiency of the green light is that since chlorophyll is the dominant pigment and it is green, it reflects green light which implies the opposite of absorption. This demonstrates that to determine the efficiency of light wavelengths, multiple factors should be considered including the efficiency of the light which are the wavelength of the light, the similarity to the dominant pigment in the plant, and how much of each pigment is devoted to absorbing the color.

Error

It is important to acknowledge the opportunities for error in performing this experiment, however. The amount of dish soap poured into each cup was not controlled which may have led to some of the solutions containing more or less CO2 than others. In addition, even in the no light condition, the leaf disks were able to conduct photosynthesis. Because of this, there may have been some ambient lighting influencing the results. Further ambient lighting may have come from nearby lamps when multiple wavelengths were being tested simultaneously. This extra light could have increased the rate of photosynthesis in some tests performed when very little to no photosynthesis should have taken place.

ConclusionThe light's wavelength energy, similarity to the dominant pigment, and how much each pigment is devoted to each color are all factors in determining how efficient light is in photosynthesis. Based on the data, certain colors of light produced a greater rate of photosynthesis. This can help to support the claim that some colors of visible light are not as efficient as others. A reason to accept the hypothesis would be because of the results of white light and green light. Out of all the lights tested with, white light had the highest rate of photosynthesis, and green had the lowest. This helps prove the fact that wavelength energy, similarity to dominant pigment, and how much pigment is devoted to each color determines the light’s efficiency in photosynthesis. Green is not absorbed by any pigments, explaining why it would have the lowest rate out of all the lights. Another reason to accept the hypothesis would be that red also had a higher rate than green. Red is absorbed by some pigments in the leaf, while green is not, which means the test helped support the hypothesis because green was least efficiently used. The use of red light being more efficient than the use of blue light despite the energy of blue light supports the third factor involved in the efficiency of the light as stated in the hypothesis because although the blue light had a shorter wavelength, the red light was more efficient probably because of how the spinach plant was engineered.

These are photos of the black construction paper tubes around the cups with the leaf disks in it. They are directly under the colored light, and there is a small hole in each that was used to check the amount of floating leaves each minute.

Conclusion

In conclusion, the different wavelengths (different colors) of light had different impacts on the rate of photosynthesis. White light made the leaf disks float up to the surface the fastest because all the pigments in the leaf could work to use its wavelengths to photosynthesize. Red and blue light came in second because they are the wavelengths absorbed by chlorophyll a and b. Green light came second to last because green light is not absorbed by the leaf and instead reflected back, giving leaves their green color. No light at all meant no photosynthesis, and therefore it came in last.