A Comparison of the Photosynthesis Rates between Deciduous Leaves and Coniferous Needles

Matt Murray, Allison Boynick

CU Boulder, Fall 2004

For this experiment, the rates of photosynthesis of deciduous trees (leaf-bearing) and coniferous trees (needle-bearing) were compared. In temperate climates, deciduous trees tend to lose their leaves in the autumn months, and stay barren throughout winter until the spring. This is due to the fact that, because deciduous leaves are so broad, during the winter months when it snows the snow tends to settle and build up on the broad leaves which adds incredible weight to the tree and can lead to breaking of limbs and other damage to the tree. Coniferous trees, on the other hand, have fine leaves so snow slips through and does not build up, and therefore coniferous trees keep their needles for the entire year and do not experience leaf loss. Deciduous trees only hold leaves for a set period of time during the year (in temperate climates), and because photosynthesis occurs in the leaves of trees, and all trees cannot photosynthesize without their leaves, then deciduous trees must store up the glucose created from photosynthesis as quickly and as excessively as possible in the seasons where they do have leaves. This is in order to create a ready reserve of glucose so the supply will not run out in the winter and fall seasons when the leaves are absent. Photosynthesis occurs in the leaves of trees, where CO2 and H2O are taken up into the leaves, and through both the light reactions of photosynthesis and the Calvin cycle (the dark reactions), the leaves create oxygen (to be released into the atmosphere) and glucose (to be stored and used as a form of energy within the tree, just like heterotrophs eat food for energy). Because deciduous trees loose their leaves in the winter months, they cannot photosynthesize during fall and winter. To compensate for the lack of photosynthesis, deciduous trees must create glucose from the photosynthetic process as quickly and as excessively as possible. Therefore, we hypothesized that the photosynthetic rate of deciduous leaves (in temperate climates) will be greater than that of coniferous needles, of which have no need to compensate for periods without photosynthesis as they hold their leaves year-round.

To test this hypothesis equal masses of coniferous and deciduous leaves were collected and placed in separate gas chambers attached to a CO2 gas sensor which measures the concentrations of carbon-dioxide. But first, the CO2 gas sensor was placed in an empty chamber to ensure the CO2 output was zero (to eliminate any possible error before beginning the experiment). This was done until an equilibrium was reached. Each sample was then subjected to 10 minutes of light, and another 10 minutes of darkness, and the CO2 concentration was measured and graphed using the Logger Pro software contained on the General Biology Lab 1 CD-ROM. Using the slope of the first 10 minutes (Photosynthesis and Cellular Respiration), and the slope of the second 10 minutes (only Cellular Respiration), we were able to find the rate of photosynthesis by subtracting the second 10 minutes from the first 10 minutes. We then divided this answer by the mass (2.17g) in order to get the rate of photosynthesis in ppm/min/g. Because time and mass were held constant for each subject, then we know we obtained a valid CO2 concentration. A second trial was run with 1.35g of the same types of deciduous and coniferous leaves, but not the same sample leaves from the first trial in order to eliminate pseudoreplication of our data. The second trail was only run for 5 minutes of light and 5 minutes of darkness due to time constraints, but because time and mass were held constant for each subject, we know our results for the rate of photosynthesis were valid. In view of the fact that deciduous leaves can only photosynthesize for certain portions of the year, we predicted that the rate of photosynthesis would be greater in deciduous leaves than in coniferous leaves. If a tree only holds its leaves half the time it’s alive it is going to have to go through photosynthesis faster in order to get the nutrients it needs. With having those thoughts in mind we were able to make our prediction.

Our results indicated that there was a noticeable difference in the photosynthetic rate of deciduous trees (mean = -44.14 ppm CO2 /min/g) and coniferous trees (mean = -29.03 ppm CO2 /min/g). However, we received a p-value for this experiment of 0.087, which holds no significance in the results of our data, because a p-value of 0.05 and above statistically indicates that the individual values of the deciduous and coniferous rates in each of the trials were not far enough apart in their means to be considered significant data. The p-value being off simply means statistically more tests should have been done, but the actual data agrees with the hypothesis.

The mean results are consistent with our predictions based on our hypothesis that deciduous trees photosynthesize faster than coniferous trees, so we fail to reject our hypothesis. However, due to our p-value, more work will have to be done to correct this value in order to completely accept our hypothesis. First of all, more trials should be run in order to get a more accurate mean value for both subjects. For this level of significance, statistical tests take into account the sample size and the variance or variability in the data. It is more difficult to get a level of significance less than .05 with a greater degree of variability in the data set, so for each trial around the same amount of mass for each subject would be used (under 1.5g to ensure the maximum photosynthesis from each subject). There were numerous sources of error inherent in our experiment, the first of which being that only one type of deciduous and coniferous leaves were used. Because we only used one type of deciduous and coniferous leaf, this skewed the data because, had we had more variation and more trials run. If we would have used more than two kinds of leaves we would have had less room for any source of error, and would give a broader scope to our experiment, making it more relevant to our original problem. In our experiment, we were really only testing the rates of photosynthesis for two certain plants, of which were collected from the same area, which created a very narrow sample type. In order to get a more accurate experiment, we would have to run 10 or more trials on the current subjects, then collect other types of deciduous and coniferous leaves and run the same number of trials on each of those sample types, and then compile the data at the end. Another source of error in this experiment is the location of each subject. If some of the subjects were collected from shaded areas while others were collected from directly lit areas, then the rates of photosynthesis would be inherently different due to the varying degrees of sunlight received by each subject, so to rectify this subject samples would be collected from the same area where all receive approximately the same amount of sunlight. Results from the CABLE website demonstrated the same trends that our experiment demonstrated. Peterson-Maiman et al. 2002, Nguyen et al. 2001, and Rosebrock et al. 2001 all received deciduous photosynthesis means that were greater than the coniferous photosynthesis means, but each experiment had a p-value greater than .05 so the data was not significant, but was due to experimental error and a low number of trials, as seen with our experiment. Because our means are consistent with our predictions, we will keep our hypothesis, but will most likely modify it in order to account for these different errors. Perhaps deciduous trees in temperate climates photosynthesize faster than coniferous trees as a response to environmental stimuli and evolutionary traits to help compensate for loss of leaf mass, since evolution creates maximum efficiency and productivity (in this case, chloroplasts are extremely costly to make on the molecular level, so deciduous trees withdraw them from their leaves during the winter so as not to lose such a costly organelle, which reflects evolution’s main point of maximum efficiency).