Rate of Photosynthesis in Different Color Leaves

Cayla Kemper

CU Boulder, Fall 2008

Our experiment was designed to test whether yellow leaves have a different rate of photosynthesis than green leaves. Leaves change color during the fall because they lose chlorophyll. As leaves lose their green coloration, the yellow pigmentation caused by carotenoids becomes visible. By testing the rate of photosynthesis of two colors of leaves, we are indirectly testing the role of carotenoids in plant leaves. Many studies (Garc’a-Plazaola et al. 2003, Wang et al. 2003) have shown that carotenoids generally act as photoprotection and not as accessory pigments. Carotenoids therefore dissipate solar energy instead of relaying it into the photosynthetic apparatus. Since the predominant pigments in a yellow leaf are carotenoids the rate of photosynthesis should be lower in comparison to a green leaf due to this dissipation of solar radiation. From this information, we hypothesized that yellow leaves will have a lower rate of photosynthesis than green leaves.

            To test this hypothesis, we measured the rate of emission by green and yellow leaves in dark and light conditions. We placed a known mass of green leaves in a gas chamber and placed a probe in it. We then focused two lamps onto the leaves, with water temperature barriers in between the chamber and the lights. After five minutes of recording the emission, we covered the gas chamber with aluminum foil and continued recording emission for ten more minutes. We then found the rates of emission for the light portion of the trial and for the dark portion. These rates were divided by the mass of the leaves to find the rate per gram of leaf. This procedure was repeated for two green leaf trials and three yellow leaf trials. New sets of leaves were used for each trial. The rate of photosynthesis was found by subtracting the rate of emission in light conditions from the rate in dark conditions.

            The data supported the hypothesis that yellow leaves have a lower rate of photosynthesis than green leaves. Yellow leaves had an average rate of photosynthesis of 0.34 ppm/min/g while green leaves had an average rate of 11.61 ppm/min/g. These data are statistically significant with a p-value less than 0.05 (P=0.02). These results correlate with the results from Gar’a-Plazaola et al. 2003 and Wang et al. 2003. These experiments used different methods, but also showed that carotenoids primarily provide photoprotection.

            Some potential problems with this experiment became clear while running the experiment. Some of the yellow leaves used during the experiment had some green coloration. The green coloration signifies the presence of cholophyll, and this chlorophyll could have skewed the rates of photosynthesis between yellow trials. Also, the size of the leaves was not held constant. This oversight could have introduced an uncontrolled variable into the experiment. This experiment stimulated a need to generalize these results to other plant species. If the rate of photosynthesis in other plant samples that have predominantly carotenoid pigmentation is lower than normal samples, then the carotenoid photoprotection theory can be more confidently asserted.