The brilliant colors that leaves take on in fall is one of the most familiar and beloved aspects of the season. These colors emerge when the green chlorophyll (the chemical that allows leaves to turn sunlight and water into food for themselves) breaks down as the days grow shorter and colder; not ideal for conditions for continued plant growth.
What remains are the yellow, red, and orange hues that make fall trees so beautiful. But what causes these colors? Much like chlorophyll lends leaves their green color, these colors are caused by other chemical compounds in the leaves
YELLOW: Besides chlorophyll, other chemicals constantly present in leaves include flavonoids and carotenoids. These chemicals help with essential leaf functions and protect the chlorophyll from sun damage, but are hidden from view until the chlorophyll breaks down in fall. One type of carotenoid called lutein, is also responsible for the yellow color of egg yolks.
ORANGE: Carotenoids are also responsible for orange coloration, perhaps most famously in carrots; the word “carotenoid” actually comes from the Latin word for carrot. Carotenoids start breaking down around the same time as chlorophyll, but they do so much more slowly, which is why it’s still visible after the green from chlorophyll fades.
RED: Red to purple hues in fall leaves come from a group of chemicals called anthocyanins. Unlike chlorophyll, carotenoids, and flavonoids, anthocyanins are generally not present in leaves during the growing season. They start forming in late summer and early fall, as trees begin to draw nutrients from their leaves up into the tree itself. Their function isn’t well understood; one theory is that they protect leaves from sun damage as trees drain their leaves. Lycopene, a type of anthocyanin, helps give tomatoes their red color.
