Fall is arguably the best season, especially for the vibrant colors we see in leaves! What causes deciduous leaves to change color?
As temperatures drop at night in September and October here in northern Iowa, deciduous trees (trees that lose leaves) decrease the amount of chlorophyll produced in their leaves. Eventually, production stops entirely within plant cells. Chlorophyll breakdown occurs in shrubs and herbaceous plants, too, but this post will focus on deciduous trees, like maples and oaks. A layer of cells (callus) forms at the base of the leaves, cutting off nutrient and water supply, and the leaves fall off the trees. Evergreen plants like pines and cedars will keep photosynthesizing throughout winter with the help of a waxy coating, known as the cuticle, on their needles/scales/awns.
Chlorophyll a and b are the well-known pigments that give plants their green color. The green color results from the chlorophyll reflecting the green spectrum of light. Leaves also harbor other pigments like carotenoids and anthocyanins; combinations of these pigments are what we see during the fall season.
Once chlorophyll degrades, carotenoid pigment compounds are left behind, and a yellow-orange color is exposed. Carotenoids, specifically beta-carotene, are the same pigments that give carrots their orange color. Some tree species like sugar maples, cottonwoods, and hop hornbeam are examples of harboring carotenoids. The same light absorption concept applies to carotenoids in that all visible light is absorbed except yellow-orange waves. Xanthophylls are a subclass of carotenoids that appear yellow as they absorb more orange light than other carotenoid compounds. Oaks and elm trees are great examples of xanthophylls showing after chlorophyll fades.
Besides yellow and orange, the showiest would have to be red. Anthocyanins are pigments that give red color in hybrid maples, red oaks, and winged burning bush. Once the callus of the leaf stem stops the flow of water, the combination of high sugar content and anthocyanins produces a red color.
Once all the pigments have broken down and faded, tannins are tannish-brown acidic compounds left behind in the cells. So, the next time you are outside, observe the fallen leaves to discover what pigments you can see. For now, we look forward to seeing chlorophyll again in the spring.
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