Kelsey Schroeder

Ryan Provencher

Alex Ryder

Photosynthesis Rates under Different Wavelengths of Light

            Are rates of photosynthesis affected by different wavelengths of light?  Observations from previous lab experiments indicate that blue light gives off the most energy and red light gives off the least. In those experiments, Juniper plant pigments had the greatest absorbance in blue light, some in red light, and very little in green light. We hypothesized that the rate of photosynthesis would be influenced by different wavelengths of light in Juniper.

            We placed Juniper branches in a gas chamber with a CO2 probe to measure the rate of photosynthesis.  The Juniper branches were illuminated with red light until a consistent rate of photosynthesis was measured.  We then placed the Juniper branch in the dark until a consistent rate of respiration was measured. This was done to factor out respiration. We repeated this process for blue wavelengths of light on the same Juniper branch.  We ran five trials using different Juniper branches from the same plant and alternating between the two wavelengths of light.  Given our previous experiments that Juniper plant pigments absorbance is greatest under blue light, we predicted that the plants would have a higher rate of photosynthesis under blue light.

            The results from our experiment indicated that the rate of photosynthesis was greater under red light (avg. = 5.482 ppm CO2/min/g) than under blue light (avg. = 2.106 CO2/min/g; t=0.114, p<0.05)

            Our results do not support our hypothesis that the rate of photosynthesis would be influenced by different wavelengths of light. One potential problem with the experiment was that the Juniper branches sat without water throughout the experiment. We noticed in later trials that there was a significantly lower rate of photosynthesis. If the branches were dying and unable to function properly, then our results may not be accurate. We also used Juniper branches from the same bush that were in direct sunlight. This could cause the branches to contain certain pigments that absorb red light and reflect high-energy blue light. A good way to test this assumption would be to run trials with branches from sun and shade leaves. Rodionova et al. (2003) found that the rates of photosynthesis were higher in red wavelengths of light in comparison to blue wavelengths. This group did not state the type of plant they used or what area of growth the plant was from.

            Results of our experiment suggest that xanthophylls in the plant dispersed the energy from light absorbed under blue wavelengths into heat. This would cause the decreased rate of photosynthesis we observed under blue light. This would support a new hypothesis that the rate of photosynthesis is higher under red light then blue light when Juniper branches are in direct sunlight.