Why do leaves change from green to yellow in the fall?


Nathan Kleist, Jonathon Hungerford


CU Boulder, Fall 2008



In the autumn, leaves change from green to yellow. This trademark of shorter and colder days is a sign that the plant is getting ready for the impending winter.  Plants prepare for this change by breaking down chlorophyll from the leaves for nutrients. With the green chlorophyll out of the way the carotenoids become the visible pigments and the leaves turn yellow. Why donŐt the plants dissolve the carotenoids? We hypothesize that leaves turn yellow in the fall because the green chlorophyll are absorbed while the yellow carotenoids stay in place for photoprotection. If carotenoids are photoprotectors, while under blue light, there should not be a high rate of photosynthesis in yellow leaves or a very high rate of photosynthesis in green leaves. Also if carotenoids are photoprotectors there should be low rates of photosynthesis from yellow leaves under red light and practically no photosynthesis from yellow leaves under blue light.


To test this hypothesis we needed to determine whether carotenoids in leaves act as an accessory pigment to chlorophyll, aiding in photosynthesis, or as a photoprotector, protecting the plant from harmful excess light. To do this we placed yellow and green samples of leaves from the same tree in a gas chamber with a probe measuring the rate of change in CO2 concentrations. Red light, blue light, and no light were then tested separately on the two types of leaves. The results are the photosynthetic rate of the leaves under each separate


Our experiment showed that under blue light yellow leaves have a very low photosynthetic rate as opposed to the high rate we would expect them to produce if carotenoids were a photosynthetic accessory pigment. Also while under blue light, green leaves do not photosynthesize at a very high rate. Comparing our results to a plant absorption spectra we conclude that carotenoids are not an accessory pigment that supports the plant with extra photosynthesis and therefore fail to reject our hypothesis that carotenoids absorb light for purposes of photoprotection.


Some potential problems with this experiment are human error, faulty equipment, and a lack of knowledge of the plant that we worked with. We failed to reject or hypothesis but had to quantitatively interpret the results of our experiment. We do not know what constitutes low, normal, or high levels of photosynthesis in this plant but our data does not reject our original hypothesis that yellow carotenoids remain in leaves for photoprotection.