Effect of Different Colored Lights on Rates of Photosynthesis in C3 and C4 Plants

 

James Frederick, Hailey Hirschfeld, Matthew Lee

 

Cu Boulder, Fall 2008

 

We tested the rates of photosynthesis in a C3 and C4 plant sample under different light treatments to determine if C3 plants have a competitive advantage in photosynthetic rate under certain light conditions. Information presented in our lab manual demonstrated that the extra step required in carbon fixation in C4 plants requires more ATP energy than in C3 plants. We also know that the shorter the light wavelength is, the more energy it has, therefore, blue light has more energy than green light, which in turn has more energy than red light. Because C4 plants require more energy for photosynthesis, we hypothesized that a C3 plant can outperform a C4 plant in photosynthetic rate under low energy (red) light conditions.

            To test our hypothesis we placed 1.1g each of a C3 and C4 plant sample in separate gas chambers and measured the change in CO2 concentrations under different colored lights. For each trial we measured the change in CO2 concentration for a period of ten minutes per plant sample per light treatment-red, blue and green. We then placed both plant samples in the dark for a period of ten minutes and measured the change in CO2 concentration in order to determine the rate of cellular respiration. To calculate the rate of photosynthesis, we subtracted the rate of change in CO2 during the dark treatment from the rate of change recorded for each individual light treatment in each plant sample. Since photosynthesis requires more energy in C4 plants than in C3 plants, we predicted that a C4 plant would have a slower rate of photosynthesis than a C3 plant under low-energy, red light.

            Our findings were inconsistent with our predictions. The results indicated that the C4 sample had a faster rate of photosynthesis than the C3 sample under all light conditions, including the (red) light treatment (C4 rate of photosynthesis under red light treatment=11.45 ppm/min/g; C3 rate of photosynthesis under red light treatment= 2.3 ppm/min/g).

One potential problem with our experiment may have been the small size of the C3 and C4 plant samples used (1.1g). These samples may have been too small to accurately measure changes in CO2 concentrations. Future trials should use larger amounts of C3 and C4 plant samples and run multiple trials under each light treatment to test the significance of observed differences between treatments. In addition, future trials should test multiple species of both C3 and C4 plant samples in order to compensate for characteristics inherent to a particular species that could possibly affect data. Though the results of Donald et al. 2003 on the CABLE web site did not test C3 and C4 plant samples under individual light treatments, they did find that their C4 sample had a faster rate of photosynthesis than the C3 sample.