Deciduous Versus Coniferous Leaves
We compared the rates of respiration in deciduous and coniferous leaves. Conifers keep their leaves in the winter months, so they have more time to respire. A deciduous tree loses its leaves, shortening the length of time it can perform respiration. Respiration cannot occur without the leaves, so the plant needs to produce all of its energy before it loses its leaves. Thus, we hypothesized that deciduous trees have a faster rate of respiration than non-deciduous trees.
We designed an experiment that measured the concentration of CO2 within a sealed container encasing the leaf sample. We conducted a chromatography test to verify that both leaf samples contained the same pigments. We created two samples from Conifer needles and deciduous leaves, using 50 micro liters of a petroleum ether/leaf extract solution, allowing the pure petroleum ether solution to saturate the paper and separate the pigments. We placed a sample from each type of leaf in a clear container, suspended on a test tube rack, surrounded by 3 lights and 3 water containers. The water displaced any heat from the lights that may affect the rate of respiration. We calibrated the CO2 probe until it reached approximately 500 CO2 ppm/min. The leaf samples collected sunlight before being covered with aluminum foil to produce the energy needed to carry out respiration. The rate of the covered container was tested for 10 minutes. We repeated this three times, weighing the leaves after each trial. The logger pro software graphed the data which we used to compare the results by adding a regression line to each trial run. By dividing the slope of the line by the weight of the sample we created a ratio of the respiration rate to the weight of the sample (ppm/min/g). We ran a t-test to determine whether the difference was significant or not.
The t-test showed a P-value of .028, supporting the hypothesis that deciduous and non-deciduous tree have significantly different rates of respiration. We fail to reject our hypothesis; conifers respire significantly slower than deciduous trees. Chromatography test indicated that both leaf samples contained chlorophyll A, chlorophyll B, xanthophylls, and carotenes. Therefore the type of leaf is responsible for the different rates. However, there were potential problems that could have affected our results: recalibrating the CO2 probe during a trial, a small sample size, no variety in type of plants. We used the same group of leaves for each trial, which may have led to data specific for that particular group of leaves. The small sample size increased variability between the trials. Only using one species of each type of leaf decreased the variance and relevance to a broader conclusion. To improve results, include multiple species of deciduous and non-deciduous leaves, run 15 or more trials, and collect leaf samples from similar environments.
Sources: Reich, P.B., et. al. (1995). Different photosynthesis-nitrogen relations in deciduous hardwood and evergreen coniferous tree species. Oecologia 104:24-30.