Chlorophyll, Carotenoids, and Anthocyanins, Oh My (Why Leaves Fall in the Fall)

Well, it’s October, which means that here in New England, it must be leaf-peeper season. Drive up Interstate 93 anytime around now, and you’ll probably see hordes of license plates from more southerly states on cars packed in to see the leaves in the White Mountains. We residents refer to them fondly as “leaf-peepers” (and sometimes do some leaf-peeping ourselves).

Image result for white mountains fall
The White Mountains with fall colors.

 

So what does all this have to do with those three long words in the title of this post? Well, for my science post this week (which I normally do on a second Saturday, but I had a guest post  last week–check it out if you haven’t yet!), I thought it would be seasonally appropriate to talk about the biology behind leaf colors, the defining symbol of fall. And since I’m interested in plant biology, this is also right up my alley.

So during the spring and summer, leaves are green. This is because of Pigment #1 listed in the title: chlorophyll, the major photosynthetic pigment in plants. Chlorophyll is very important for exciting electrons and causing biochemical cascades and so forth, and all of that eventually leads to the plant producing its own glucose, which it can then use in respiration to essentially make energy for cellular mechanisms. So for most of the year, trees are green. Then why does it change in fall?

Well, in the fall, the weather starts getting colder, and the plant starts to go dormant in order to survive the winter. As the U.S. Forest Service explains, leaves are thin and contain a lot of water that could easily freeze in winter, so deciduous trees must get rid of them in order to survive each winter. And as nights get longer in the fall, the plant senses that it’s time to get rid of the chlorophyll, and Pigments #2 and #3, carotenoids and anthocyanins, show their colors, so to speak. Carotenoids, which are always present in leaves, cause yellow and orange colors. Anthocyanins, produced only in response to sugar buildup, cause reds and purples.

What affects leaf color? Well, you may have noticed it depends on the kind of tree. Oaks mainly have brown leaves (which, incidentally, don’t usually fall off until spring), beeches have lighter brown, and maples can be orange or red or other colors depending on the species.

Image result for autumn sugar maple
Maples in fall.

I found it interesting to learn that weather also affects the colors of leaves. Warm, sunny days cause buildups of sugars, and cool nights constrict the plant’s vessels, causing the sugars to stay in the leaves and the subsequent production of anthocyanins. Soil moisture can also affect leaf colors; if there’s a summer drought, for instance, color onset will be delayed a bit. The best colors occurs if there’s a warm, wet spring and good summer weather, according to the Forest Service.

One last question: what causes leaves to actually fall off? Starting early in the fall, xylem and phloem veins (veins that bring water and nutrients to leaves) start to close off, eventually leading the leaf to fall. The tree is left with only its winter-hardy tissue, giving it a better chance of surviving the winter. As an addendum, some trees actually have winter-hardy leaves that only fall due to old age. We know them as the evergreens: pines, spruces, hemlocks, and other trees with needle- or scale-like leaves. The waxy coatings on their leaves make them hardy enough to keep on photosynthesizing all winter long.

Image result for evergreens in winter
Which is why people use evergreens as Christmas trees: they’re still green.

 Are your leaves turning colors yet? Have you ever thought about why they turn colors in the fall? Are you going to go “leaf-peeping” this fall? Tell me in the comments!

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4 thoughts on “Chlorophyll, Carotenoids, and Anthocyanins, Oh My (Why Leaves Fall in the Fall)

  1. Fascinating! The leaves are beginning to change here. We’ve had a long hot summer here in the South so they’re taking a while, but the dogwoods are a lovely red at the moment!

    storitorigrace.blogspot.com

    Liked by 1 person

    1. That’s really interesting! It’s always neat to compare different regions of the country. I wouldn’t have thought that your leaves would only just be starting to change. Glad you enjoyed the post! 🙂

      Liked by 1 person

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