Photosynthesis: Broad Leaves vs. Pine Needles


James Nixon, Katie McCarthy


CU Boulder, Fall 2008


         We tested if broad leaves have a higher rate of photosynthesis than needles in high light conditions? Large/broad leaves have large amounts of surface area on the top of the leaf that are exposed to collect light directly from the light source, whereas needles collect light from all angles. Deciduous trees thrive in the spring up until the fall because they have the sun as an overhead light source. When the sun disappears they shed their leaves, usually beginning towards fall and continuing throw winter, the seasons with little to no light. Our experiment aimed to show that deciduous plants thrive during high light conditions because of their high rate of photosynthesis during high light months of the year. We hypothesized that broad leaves have higher photosynthesis rates than needles in highlight conditions because of greater surface area exposed to sunlight. If broad leaves can utilize high light conditions, than their photosynthesis rates will be significantly higher than the needleŐs rates.

         To test this hypothesis, we measured the CO2 levels in light and dark environments.

We then filtered the high light through containers filled with water and sandwiched the chamber of blue spruce needles between the water containers. We attached the CO2 sensor to the chamber, and measured the change in CO2 levels for ten minutes. After ten minutes we wrapped the chamber in aluminum foil to simulate a dark environment and measured CO2 levels for another ten minutes. We repeated this process for the large/broad leaves. We ran this process twice. 

Are results show that the rate of photosynthesis was significantly higher in the broad leaf (-12.4363 CO2 ppm/ minĄg) than the needles (-0.003415 CO2 ppm/ minĄg). Both of our p-values were greater than 0.05. This means that there is no significant difference. The high p-value was due to the high variance

         Our results are consistent with our hypothesis. The broad leaves thrived under the direct light, but the needles did not.  A potential problem with our experiment would be that we did not have a light source that was from all angles. We could fix this by providing light from above and under our leaves and needles. This could possibly change our data because the needles would have more surface area exposed to light energy. Results of Lindroth et al. 2008 show that trees in the rainforest are mainly deciduous because of their high rates of photosynthesis. Results of Brodribb et al. 2008 show that even certain evergreen needles have changed to accommodate higher photosynthesis rates by flattening their needles. From the results of our experiment, we chose to provide an alternate hypothesis. Leaves have higher photosynthesis rates than needles in high overhead light conditions, such as the light from the sun.