The Effect of Deciduous and Coniferous Plants on the Rate of Photosynthesis


Emilie Kurtz, Janell Hendricks


CU Boulder Fall 2006



         For this experiment we tested whether the leaf shape of the deciduous trees and the needles of coniferous trees had an effect on the rate of photosynthesis. In an initial experiment, we looked at the rate of photosynthesis of a juniper plant and an unknown type of C4 plant. We observed that the C4 plant was broader and flatter than the juniper plant and seemed to have a faster rate of photosynthesis. Since the amount of light energy that is absorbed depends on the amount surface available to absorb it, we hypothesized that deciduous leaves are broader and more flat than needles, providing more surface area for light to be absorbed.

         To test this hypothesis, we collected samples from a deciduous tree and from a coniferous pine tree. We massed each sample and placed them into a jar attached to a CO2 probe. For each trial we used a different sample but from the same tree and all the samples were green and healthy. We ran three trials each for the deciduous leaves and coniferous needles respectively. Before each trial, we calibrated the CO2 probe until the rates measuring the CO2 concentration in the classroom leveled out. In each trial we placed the jar under the light and measured the CO2 concentration until we had a consistent slope and then covered the jar and measured the CO2 concentration in the dark until we had another consistent slope. We then subtracted the CO2 concentration in the dark from the CO2 concentration in the light to get the rate of photosynthesis. We predicted that deciduous plants would have a faster rate of photosynthesis than coniferous plants.

         Our results indicated that there was no significant difference between the rates of photosynthesis in deciduous and coniferous trees. Even though the mean value of the rate of photosynthesis in deciduous trees was -76.69 ppm CO2/min/g compared to the average value of -28.35 ppm CO2/min/g for the coniferous trees, the p-value of the t-test was 0.138, which does not indicate a statistically significant difference between the two plants.

         Our results are consistent with the predictions based on our hypothesis, so we fail to reject our hypothesis. However, since our p-value indicated there was no statistical significance, more work must be done to completely validate our hypothesis. We had some problems with the CO2 probe seeming to malfunction and showing increasing CO2 concentration initially rather than decreasing after the first two trials. Perhaps we were not calibrating the probe effectively; nevertheless, this could have influenced the data enough to affect the rate of photosynthesis and the p-value of the t-test. We also should have run trials in a consistent time trial to ensure that we were giving each sample an adequate amount of time for photosynthesis to occur. However, results from the CABLE website demonstrate the same trends as our experiment. Murray et al. 2004, and Peterson-Maiman et al. 2002 both received data that had a mean deciduous rate of photosynthesis that was greater than the mean rate of photosynthesis in coniferous plants, but a p-value that was greater than 0.05 indicating no statistical significance. Since the results of our experiment and previous experiments indicated that there was a faster rate of photosynthesis in deciduous trees, we sought an explanation as to why there was not a significant difference between the two rates. In nature, it is necessary for deciduous trees to have a faster rate of photosynthesis because of their short growing season. The leaves are too delicate to withstand the winter and so the tree needs to put forth a maximum effort in photosynthesis to account for the lack of food and sunlight energy in the winter. The coniferous trees on the other hand, have needles that are adapted to withstand cold temperatures and lack of sunlight and therefore can start performing photosynthesis earlier than deciduous trees. This eliminates the need for maximum photosynthesis and results in a slower rate of photosynthesis. It is evident that, through natural selection, deciduous plants should have a faster rate of photosynthesis in order to survive. Perhaps this discrepancy between our data and the evidence of nature, and the statistical significance is caused by flaws in the design of the experiment, and more trials should be done in order to completely validate our hypothesis.