Abstract on the Rates of Photosynthesis Under Different Color Lights


Shannon Marie Stasi, Jason Scott, Alexa Tralla


CU Boulder, Fall 2007


Photosynthesis is, a process involving the presence of light energy in order to create chemical energy.  Plants contain specialized pigments which enable them to absorb energy from the sun to produce glucose and oxygen. These pigments absorb light at different wavelengths allowing plants to maximize photosynthesis efficiency. Chlorophyll b, carotenoids, and xanthophylls are accessory pigments that absorb energy that the primary pigment, chlorophyll a does not absorb. Absorbing most of its energy from violet-blue and reddish wavelengths and the least from green we can hypothesize that green light will be the worst concerning rate of photosynthesis.  Chlorophyll in plants reflects green light, while red light is absorbed the most and will have a high rate of photosynthesis.


To test this hypothesis we placed juniper needles inside a CO2 gas probe and measured photosynthetic rates. In each trial we exposed the plant to different colors of lighting attaching red, green, blue, colored coverings over the light. We used white light as the control. We exposed the plant to 7 minutes in the light and 7 minutes covered from the light. This allowed us to gather the rate of photosynthesis by subtracting the rates of respiration that took place in the dark from photosynthesis and respiration where the plant was exposed to light.


Our results demonstrated that the rate of photosynthesis was the greatest under the red light (mean = -9.29 ppm/min/g).  As predicted green light was a great deal lower (mean= -3.7 ppm/min/g).


Our results were consistent based on our hypothesis. Our hypothesis was adequately tested because we used the same time for each color. This gave us legitimate data for the rate of photosynthesis under different color lighting.


Potential problems in the lab could include, movement of the chamber anytime throughout the lab which may cause shifting of air within the chamber effecting data. Also if the chamber was not covered completely with aluminum foil it could allow light in altering the respiration stage of the plant. If the probe is not given enough calibration time it could throw the data off as well. A major factor in altering data could possibly be the surrounding overhead lights, because all light except for the light shining on the plant should be blocked out. We were unable to run a T test since we only ran one trial due to time constraints; however this would have allowed us to statistically determine results were significantly different.


Results from past student projects dealing with rates of photosynthesis also demonstrated the highest rates in red light.  Results of Redder Smith and Reich in their experiment on the CU website were consistent with ours in that their results demonstrated a higher rate of photosynthesis under the red light.  They used a different type of plant, their concept was the same as well as their conclusion.  Another experiment, Bir and Reynolds, also concluded that red light produced the highest rate of photosynthesis.


From the results of our experiment and the consistency with the other studies, we can propose a new hypothesis.  Since red light seems to produce the highest rate of photosynthesis, perhaps this is a result of the various pigments absorbing those different wavelengths of light.  So, the pigments that absorb the different wavelengths of light do not transfer that energy into the production of photosynthesis, resulting in reduced rates of photosynthesis.