Autumn: The Chemistry Behind the Colors

Every year during approximately three weeks of autumn, tens of thousands of people flock to the Adirondacks to see the beautiful colors of the changing trees. While people enjoy green forests, the reds, oranges, and yellows satisfy another level of love for the outdoors. I will talk about the chemistry behind how the leaves change color and try not to confuse you:

Chlorophyll is the primary molecule involved in energy-producing processes in plants and is responsible for plants' colors. It absorbs light of a certain wavelength - which matches up to a shade of red - to excite electrons to a higher energy state which cascade through other molecules to create energy. The interesting thing about this molecule is that it is not very stable (i.e. it doesn't like to exist in its environment in a leaf), so it decomposes shortly after being synthesized by a plant. Thus, plants must continually synthesize it to continue living and this requires nutrients. Plants also have what are called accessory absorbers that absorb different wavelengths of light and transfer this energy to chlorophyll to create more energy for the plant's processes. Some of these are also responsible for the beautiful fall colors we see every year; they are carotenoids, flavinoids, and anthocyanins. These are also unstable and continually decompose, but are not nearly as unstable as cholrophyll.

When the tree senses that winter is coming, it starts a process called abscission which essentially blocks off nutrients to leaves by blocking their transport from the leaf's stem. When this happens, the plant can no longer synthesize chlorophyll. However, the leaf does not immediately fall off because the accessory absorbers do not decompose as quickly as chlorophyll does because they are more stable than chlorophyll. This is where the color comes in. Since is chlorophyll no longer present to absorb red light - and reflecting green light - that red light is now being reflected. Carotenoids, flavinoids, and anthocyanins each absorb different wavelengths of light within the green spectrum and thus reflect red, orange, and yellow. Different species use these molecules to different extents and that is why different species appear different colors during the fall after chlorophyll has decomposed.
 

See these articles for more detail:

http://equipped.outdoors.org/2014/10/why-do-leaves-turn-different-colors.html

http://scifun.chem.wisc.edu/chemweek/fallcolr/fallcolr.html

Picture from: http://www.adirondack.net/fall/leaf-peeping-guide.cfm