The Effects of Leaf Color on Rates of Photosynthesis

Courtney Marne, Erin Markley, Travis Pecha

CU Boulder, Fall 2004

For our experiment we tested how the color of leaves on a deciduous tree affects the rate of photosynthesis. We based our experiment on the fact that green leaves have a higher concentration of chloroplasts than red leaves. Since chloroplasts within a leaf are the sites of photosynthesis, we hypothesized that green leaves would have a higher rate of photosynthesis than red leaves.

For this experiment red and green leaves from the same deciduous tree were collected. Two trials were done, one trial where red and yellow leaves were tested and then a second trial where different red and yellow leaves were tested (all leaves still being from the same original leaf collection). First an experimental control was carried out using a sealed empty container containing a CO2 probe to factor out abnormal CO2 concentrations within the room and jar. Secondly a mass measurement of each sample of leaves was taken and then a sealed container containing a CO2 probe was used to record the concentration of CO2 within each sample. We measured the change in CO2 for the leaves for approximately 5-10 minutes in the presence of light and again in total darkness. To calculate the rate of photosynthesis, we subtracted the change in CO2 in the light from the change in CO2 from the dark, and then subtracted that from the control change in CO2 from the empty jar. We then divided that number by the mass of the leaves to get a rate of photosynthesis. We predicted that, if the concentration of chloroplasts has an effect on photosynthetic rates, then the rate of photosynthesis would be significantly higher in green leaves than in red leaves due to the higher concentration of chloroplasts in the green leaves.

Our results showed that green leaves did have a higher mean rate of photosynthesis (-.5855 ppm CO2/min/g) than red leaves (-0.200 ppm CO2/min/g). T-test results showed a P value of 0.066342, indicating the differences in our average rates of photosynthesis were not significantly different.

Our results supported our hypothesis, showing that green leaves do in fact have a higher rate of photosynthesis than red leaves. Despite the insignificant difference between our average rates of photosynthesis, we still accepted our hypothesis on the basis that the rate of photosynthesis was still much higher in green leaves than in red leaves. Our p-value was close to 0.05 and, if we would have allowed more time for the system to equilibrate, the means may have actually been significantly different. Results from other studies off the CABLE website show similar results (Krahn et al. CU, 2001, Kim et al. 2001, Nazarian et al. 2001).

Our results were consistent with our hypothesis, but we cannot say the rate of photosynthesis is based on the concentration of chlorophyll in the leaf. In order to determine if chlorophyll concentration is directly responsible for photosynthetic rates, we would have to do much more in depth research beyond the scope of our available materials and equipment. Based on this fact, we felt it would be a good area for future research. There were a few problems during the experiment. We didn’t allow enough time for the system to equilibrate, which may have directly affected the rates of photosynthesis. Too many leaves were placed in the jar, which inhibited rates of photosynthesis due to self-shading by the leaves. For extensions on our experiment we could run the tests with green and yellow leaves and determine the rates. A test could also be run on the rates of photosynthesis for different colored leaves from a different of tree than the tree used in this experiment.