Jeff Harvey, Amber Johnson, Valerie Jackson

EBIO 1230 Student Project

In our experiment, we examined how the types of pigments and their relative concentrations affect the color of leaves during autumn. From a previous experiment, we found healthy green leaves have relatively high concentrations of chlorophyll a, chlorophyll b and carotenoids. Because healthy green leaves use these pigments for photosynthesis during warmer weather, and deciduous plants lose their leaves before winter, we hypothesized that the relative concentrations of these pigments change before the leaves fall off the tree.

If the relative concentrations of the pigments change during fall senescence, then a healthy green leaf should have a different concentration of pigments than an orange leaf that is preparing to fall off. To test this prediction, we used equal portions of orange and green leaves from Virginia Creeper. We used a mortar and pestle and equal amounts of acetone to isolate the pigments from the leaves. Paper chromatography was used to separate the pigments, which were then dissolved in methanol in preparation for the spectrophotometer. Special care was taken during these procedures to ensure that each vial contained equal proportions of each sample (orange leaf and green leaf). Because equal proportions were used, the percent absorbance (O.D) taken from the spectrophotometer can be used to infer information on the relative concentrations of the pigments. Therefore, the sample with a higher percent absorbance (O.D.) contains a higher concentration of the particular pigment.

Our results indicate that the concentrations of chlorophyll a, chlorophyll b, xanthophylls, and carotenes were all greater in the green leaves than in the orange leaves. Furthermore, we believe that the orange leaf contains no chlorophyll a, no chlorophyll b, no xanthophylls, and no carotenes. Instead, we determined the orange leaf contained only one pigment, which we identified as anthocyanin because it absorbed strongest in the 475-560 nm range and was relatively polar, as determined from the paper chromatography.

Our results are consistent with predictions based on our hypothesis: the relative concentrations of pigments are different in the orange leaf compared to the green leaf. From Introduction to Plant Physiology, 3^rd Edition, by Hopkins and Huner, leaves contain other groups of pigments, one of which is anthocyanins. One stimulant of the synthesis of anthocyanins is low temperature, which corresponds with leaf senescence. Because anthocyanins absorb strongly in the 475 — 560 nm spectrum, thus emitting red- orange light and explaining the orange color of the leaf, and the most common anthocyanins have three oxygen groups, making them relatively polar, we are confident that the orange leaf contained only anthocyanins and none of the other pigments. The presence of anthocyanins in the orange leaves would be consistent with their color, autumn temperatures and the results of our experiment.