Exploring the relationship between the absorption indices of photosynthetic pigments and leaf color change in Cornus florida
Introduction
The leaf color change in deciduous trees, or trees that seasonally change color and lose their leaves, is visually observed during seasonal change. However, the chemical explanation behind this is less observable, as it requires an understanding of the photosynthetic pigments in a leaf and the changes in their absorption indices as autumn approaches and the availability of sunlight decreases. Careful observation of these absorption spectrums is required, as the spectrum shows the relative absorbance rate of each photosynthetic pigment at different wavelengths of light (Morgan and Carter, 2010), the comparisons of which can reveal the changes in these pigments during the seasonal transition.
In this study, we used chromatography and spectrophotometer readings to observe the photosynthetic pigment concentration in one species of these deciduous trees, the Dogwood, or Cornus florida. Five trees of the Dogwood species were chosen at random across the Oglethorpe University campus, from which we then took one green and one red leaf to measure the absorption index of five photosynthetic pigments from each in order to understand the reasoning behind leaf color change in this species of deciduous trees.
Discussion
We tested the hypothesis that deciduous trees leaves change color due to a change in the level of photosynthetic pigments found in the leaves. Flowering dogwood leaves turn red in the fall due to a decrease in Chlorophyll and an increase in Anthocyanin concentrations, while Carotenoid concentration remains constant. Given this hypothesis, we predicted that as the leaf color changes, the amount of Chlorophyll a and b will decrease in red leaves as compared to green, while the amount of Anthocyanin will increase, or be present only in red leaves. We also predicted that there would be no change in Carotene or Xanthophyll concentration from green leaves to red leaves.
Contrary to expectation, our results partially supported our hypothesis. According to our expectation the levels of Anthocyanin had in fact increased significantly in green than red leaves. Chlorophyll a, Xanthophyll, and Carotene were found in lower levels in red leaves than in green leaves. There was a single sample of red leaves in which Carotene was found. This suggested that our hypothesis did not fully account for how different the amounts of Xanthophyll and Carotene would be in red and green leaves. We conjectured that our chromatography technique may have blurred the lines so that different pigment concentration levels were near indistinguishable.
Furthermore, there was no significant decrease or increase in Chlorophyll b, contrary to what we predicted. This contradictory finding may be a result of imperfect chromatography technique as well, or error on the part of the spectrophotometer that made the resulting data insignificant. However, it is possible that only Chlorophyll a has a significant decrease in the transition from green to red this species of leaves, and not Chlorophyll b.
In conclusion, we find that our hypothesis was partially supported. As predicted there was a significant decrease in Chlorophyll a but not in Chlorophyll b, and the Carotenoid levels did not support our hypothesis. However, our prediction could have been modified along with our hypothesis to focus more on the pigments Xanthophyll and Carotene individually, as opposed to considering them as a monolith, since they were measured separately. Furthermore, the measurements of the absorption spectrums were done over two days, perhaps allowing for slight changes in method and attentiveness, which might account for discrepancies in the results. In the future, all readings and chromatography should be done within the same period of time to control for possible external factors that could cause changes in readings, such as spectrophotometer fluctuations, technique differences, etc.
