[Before I begin, I’d like to disclose that I am NOT a plant biologist. My training is related to fish. I do, however, have plenty of research-related experience so I know a thing or two about condensing a ton of info into something manageable. This blog post is intended to be informative and accessible to anyone no matter their education level.] To non-biology folk, the reasoning behind leaves turning colors in the fall is because Jack Frost makes a visit on the first frigid night and paints them that way. For the less creative, I suspect a popular response would be “magic”. And while I love the thought of both of these (there’s an air of mysteriousness associated with them), there’s actually a very scientific reason for it.
First, we need to understand that there’s a lot of yummy goodness in leaves such as starches (which are just lots and lots of sugar or glucose molecules strung together), proteins, and nutrients like nitrogen, phosphorus, and potassium. The plant requires these things to live.
“But wait. Umm. Don’t trees lose their leaves?” “Yes,” I say to you. “Then if they lose their leaves, and there’s a bunch of tasty stuff in them, isn’t the plant screwed?” Fortunately, no it’s not. The combination of shorter day lengths and lower temperatures triggers a change in trees, specifically the expression of certain hormones. The trees begin resorbing nutrients and breaking down large complicated macromolecules in the leaves like starch and proteins into their smaller subunits of sugars and amino acids. All of these things are stored within the plant and used throughout the winter and again in the spring when new leaf production occurs and trees begin growing again. Eventually, the cell walls are weakened by “Wall-Degrading Enzymes” and the petiole severs from the stem.
“And what about the colors?” I’ll address that question in a second, but let’s take a moment to reflect back on a random day in July. What color were the leaves at that time? Green, right? The green comes from chlorophyll (“More like borophyll!” You get extra credit if you can correctly name that movie in the comments) that reflects green wavelengths of light, absorbing everything else. As daylight shrinks and temperatures get cooler, less and less chlorophyll is produced, so the green fades away. In turn, other colors associated with different pigments become visible.
And that’s it in a nutshell.
So to summarize, 1) changing weather starts physiological changes that eventually lead to leaf loss and 2) less (green) chlorophyll is produced allowing other colored pigments to become visible. You know, I could have just written this one paragraph and it would have been much more efficient, but where’s the fun in that? 🙂 For more information, check out this link by a professor at my Alma Mater (University of Illinois at Urbana-Champaign), Dr. Jeff Dawson Autumn Leaf Article. And to see more info related to the differences between conifers and deciduous trees, check out this great blog post from The Roaming Naturalist, whose site I found while doing a little research for this post. And if I’ve missed something or you feel like you want to expand further, leave a comment!