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The Chemistry of Autumn Colors

| October 15, 2019
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Well, the summer has gone and now here in the Northern Hemisphere, the temperatures are falling and the evening is starting earlier than anyone feels like it should. The most obvious signal of the changing seasons are the leaves on the trees. Once bold greens, they're shifting every day into brighter colors, the warmer hues that mellow out the cooler weather. Some of the trees seem to be ready to give up early, which happens every year. There are always those trees that have a quick weekend of yellowish green and then the next time you drive by, there’s just naked branches and a trunk surrounded by the early losses. There are the trees that don’t seem to change color properly at all. The leaves dry to brown on the branches, first with crackling edges and then the whole leaf just sticks it out like crumpled chestnut tissue paper until they finally fall. After a brutally hot, rainless summer, that is certainly one way to be done with the season.

The autumn effect on the trees is unmistakable and iconic, but what’s going on? What makes them give up on the green?

As any school-age kid will tell you, the green pigment in the leaves is chlorophyll, which absorbs the sunlight needed for the endothermic process of photosynthesis. This takes the light energy and converts it into chemical energy the plant needs to grow, flower, and produce seeds to ensure there will be more plants. Almost ironically, it’s the sunlight that messes with the unstable compound of chlorophyll, meaning that plants need to continually make more with the help of sunlight and warm temperatures. So when we layer on sweaters and start eying up our hipster beanies, the leaves around us are stuck without the means to make more of the green stuff.

Leaves look green because the chlorophyll reflects green light. When the chlorophyll is gone, the other pigments in the leaves start to show up to the eye as the leaves reflect other wavelengths of light. Some of the brightest colors we see are the yellows and oranges of carotenoids. They tend to show up first, right when the chlorophyll has stopped being produced by the tree.

The favorite fall leaf colors are probably those really vivid reds which are the work of the anthocyanins, which absorb light in such a way as to make the light reflected by those leaves bright red. These pigments are also what make apple skin red. The reds develop with exposure to sun, which is why the trees that get the brightest autumn sunlight are the ones that also show off with the brightest scarlets.

After a while, with the lengthening of the nights and other physical changes to the tree to prepare for the colder, dormant season of winter, the leaves fall. Meaning raking, bagging, blowing around in the chilly breezes. The colors seen on all those leaves were there all spring and summer long. They just couldn’t be seen because of how much chlorophyll was in each leaf. Science!

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