Effects of Blue, Red, and Green Light on Photosynthesis

Craig Owens and Marsha No

CU Boulder, Fall 2008

For our experiment we tested the question, “What is the relative rate of photosynthesis under red, green, and blue light of Juniper needles?”  Our hypothesis is that the color of light is directly related to the rate of photosynthesis. A prior experiment has shown that plants absorb the most light in the blue wavelength range while reflecting the green wavelength range. Taking this into account, we predicted blue light to have the highest relative rate of photosynthesis.

To test our hypothesis, we measured the rate of CO2 change in a container with 0.65 grams of juniper needles under blue, red, and green light.  We performed 3 trials, designating 10 minutes, to observe the change in the light and dark.  We began our experiment with foil around the container to remove the light and eliminate any initial photosynthesis.  Knowing that respiration occurs at all times, we manipulated the data to retrieve the rate of photosynthesis by using the equation; (Photosynthesis+Respiration) - (Respiration) which gave us the amount of CO2 depletion and thus the net photosynthesis rate.

After gathering our data and comparing our results with previous experiments and primary sources, we discovered that red light actually had the highest rate of photosynthesis with an average rate of -0.1525 ppm/g/min while blue light had an average rate of -0.0805 ppm/g/min and green light had an average rate of -0.0192 ppm/g/min. It is important to note that the greater the negative value is, the greater the decrease in CO2 content, in turn increasing the rate of photosynthesis. A t-test comparing the rate of photosynthesis between the red and blue light gave us a P-value of 0.976 which is much greater than 0.05, meaning that there is no significant difference in photosynthesis rate between red and blue light.

Our results were not consistent with our hypothesis and prediction.  We can see from our results that in fact, red light has a higher rate of photosynthesis on juniper needles.  One source of error that could have been avoided was that we went directly from one light color to another without allowing the photosynthesis rate to stop before calculating it for the next wavelength of light. We could have wrapped the container with foil between each light color trial to allow photosynthesis to stop before beginning a new trial. According to the results of Javasankar et al. (2001), the rate of photosynthesis is lower in blue light because the high quantum yield of blue light reduces the content of chlorophyll in many species of Gracilaria.  Also, Korbee et al. (2005) believed that the rate of photosynthesis was slowed under blue light with alga Porphyra leucosticta due to the increased number of mycosporine-like amino acids (MAAs) under blue light which act as non-photosynthetic photoreceptors. From the results of these experiments, we believe that the rate of photosynthesis of juniper needles is highest under red light because the excess energy produced by blue light decreases the chlorophyll content in plant pigments, slowing the rate of photosynthesis.