Why Do Leaves Turn Red in Autumn?

M. P.

4^th Period Sustainable Agriculture

11-17-2013

Why Do Leaves Turn Red in Autumn?

             It is a well known fact that leaves are green because of the chlorophyll in the cells. Chlorophyll, found in the chloroplasts, is one of the main ingredients responsible for photosynthesis, the process where leaves convert water, carbon dioxide, and light into sugars that they can use to feed themselves (Photosynthesis, 2011). Right now, in the Madison-Avanti Giving Garden, we can see the leaves on the raspberry plants turning dark red. If chlorophyll is responsible for their green color in the spring and summer, then what is making the raspberry leaves change color?

            A common misconception about leaves is that as chlorophyll breaks down in preparation for winter dormancy, its absence reveals other pigments hidden in the leaf (Lee & Gould, 2002). Fairly recent research shows that the opposite is true. The chlorophyll is still present, but a higher concentration of a pigment called anthocyanin is what causes the red hue (Lee & Gould, 2002).

            What is the purpose of anthocyanin? Why does it appear so abundantly in the fall? We have yet to discover all of its various jobs within the cell, but there are a few purposes of the compound that have been hypothesized.

This November in Olympia has been bright and sunny, but very, very cold. In bright and cold conditions, a phenomenon called photoinhibition can occur (Lee & Gould, 2002). To put it simply, photoinhibition is the decline of photosynthetic processes (Lee & Gould, 2002). All this bright sunlight is made up of light waves. Different sizes of waves have different colors, some that are visible to humans, and some that aren’t. The visible light waves are red, orange, yellow, green, blue, indigo, and violet. Different materials reflect and absorb different colors of light. Chlorophyll appears green, which means that it absorbs red and blue light and reflects green light (Ray, 2013). Anthocyanin appears red, which means that is reflects red light, but absorbs green. This helps deter phtotoinhibition because anthocyanin absorbs the light that is not useful to the plant (Lee & Gould, 2002). The chloroplasts have no use for green light, so it is absorbed by the anthocyanin to prevent light overexposure (Lee & Gould, 2002). This absorbed light is released gradually in the form of heat (Lee & Gould, 2002).

On a similar note, light overexposure in an oxygen dense leaf can result in the production of highly reactive oxygen-based free radicals like superoxide (Lee & Gould, 2002). Free radicals often have unpaired electrons, which makes them highly reactive and unstable (Lee & Gould, 2002). Certain free radicals are beneficial to cell construction, but too many can destroy structures within the cells they inhabit (Lee & Gould, 2002). Anthocyanin discourages free radical production in the same way that it prevents photoinhibition, by “shielding” chloroplasts from non-beneficial light waves (Lee & Gould, 2002).

From an ecological standpoint, anthocyanins could appear in autumn to deter animals from eating the leaves (Lee & Gould, 2002). In nature, red is usually a color that means “back off.” This gives the tree time before the leaves drop off to reabsorb the nitrogen that is heavily present in chloroplasts (Lee & Gould, 2002). Nitrogen is very important for plants because it is an ingredient in ATP and nucleic acids, as well as part of the photosynthetic process. This “back off” theory has garnered criticism by some scholars because of observations of chimps eating young, red colored leaves in tropical forests (Lee & Gould, 2002). Others think that anthocyanin’s red pigment, and by default seasonal green to red leaf color changes, is a cue that caused three color vision to evolve in primates (Lee & Gould, 2002).

Intensive research into the properties of anthocyanin is still a fairly new concept, and as of now, we have yet to completely understand the full spectrum of its powers. In time, these kinks will hopefully be worked out, and a greater comprehension will be reached.  Happy autumn!

Works Cited:

Lee, D., & Gould, K. (2002). Why Leaves Turn Red. American Scientist, 90(6), 524. Retrieved from http://elibrary.bigchalk.com

Photosynthesis. In (2011). Hutchinson Encyclopedia. Helicon Publishing. Retrieved from http://elibrary.bigchalk.com

Ray, C. (2013, July 23). Seeing red. The New York Times. Retrieved from http://elibrary.bigchalk.com

Photo credit Madeline P.