Anthocyanin Crimson

I have been looking for words to express the red color of anthocyanins in fall leaves. The thesaurus gives me many delightful words for red such as the color of blood, bittersweet, bloodshot, blooming, blush, brick, burgundy, cardinal, carmine, cerise, cherry, chestnut, claret, copper, coral, crimson, dahlia, flaming, florid, flushed, fuchsia, garnet, geranium, glowing, healthy, inflamed, infrared, magenta, maroon, pink, puce, rose, roseate, rosy, rubicund, ruby, ruddy, rufescent, russet, rust, salmon, sanguine, scarlet, titian, vermilion, and wine - some of which (anyone heard of cerise?) are a bit obscure. Whatever you call it, red is definitely one of God's favorite colors, especially around fall.

Anthocyanins (an-though-SIGH-un-ins) are what makes fall leaves red. These pigments are produced in the leaves of deciduous trees as they transition to winter. The leaves will soon be lost, so the trees withdraw their nutrients and store them away as the leaves die off or senesce. Unlike carotenoids, which are present in the leaves throughout the year and are revealed by fading chlorophyll, anthocyanins are produced in the leaves as the leaves are dying.

But it seems illogical to use energy produce new compounds in a leaf that is already on its way out, right? This question has stumped scientists for a long while. They have proposed all kinds of interesting theories for why leaves produce anthocyanins in the fall. One theory that has recently stood out is that anthocyanins are protective compounds that keep the leaves on as long as possible to make sure the tree can suck all the nitrogen out before they fall. Anthocyanins, like carotenoids, are powerful antioxidants and could be protecting the leaves from oxidative stress. In addition, anthocyanins absorb UV and can act like a sunscreen to protect the leaf from light stress (and here I thought that trees couldn't get sunburned).

So the anthocyanin is like a shield detachment sent to cover the retreat of nutrients from the leaves. With anthocyanins, the leaves stay on longer and the tree can store away more nitrogen. But the anthocyanins still cost energy to produce, so trees that grow in nitrogen-rich conditions will be less red because the extra nitrogen that they get isn't worth the investment in the sunscreening anthocyanins. Trees that grow in nitrogen-poor conditions, on the other hand, will glow like red-hot embers as they try to protect as much of their nitrogen as they can.

Fall Colors

Fall is closing in on us rapidly. The past two days have been less sunny, more rainy, and the trees have just begun tinge yellow and red. My friend Katie texted me today and asked "What makes the leaves change colors in the fall?" I pondered how I would answer this in less than 160 characters, then wrote this text:

"The trees are withdrawing nutrients from the leaves before they shed them. When green chlorophyll is broken down you can see other colored pigments!"*

*yes, I am one of those obnoxious people who refuse to abbreviate in text messages. I have a keyboard and goshdarnit, I'm going to use it.

There's a little bit more to it than that, as you might imagine, but not much more. When the day length begins to shorten, the light to weaken, and the temperatures to drop, deciduous trees (trees that shed their leaves in the fall) pass through an important transition to survive the upcoming winter. In the warmer months, leaves are the solar collectors and energy factories of the trees, but in winter, the trees will retreat into a quiescent state of tightly clenched buds and bare stems, and their leaves will be unnecessary to their long winter sleep. Leaves will only make the trees more susceptible to damage in the coming snowfall anyway (see photo here of results of early October snowfall in central Pennsylvania last year). But the leaves represent a lot of energetic investment on the part of the plant, particularly nitrogen, which the tree will need to survive the winter ahead. So as the trees bundle up for winter they pull nutrients out of the leaves to store up for the winter.



Here's how it happens. In the beginning of the fall, trees will gradually shut off the production of chlorophyll, their green pigment and secret to their photosynthetic magic. The remaining chlorophyll and its accompanying photosystems in the leaf tissue will slowly be digested and the nitrogen-rich pieces sucked back into the trunk and roots of the tree. (Imagine the roots of a tree are a basement for sealing up canned goods to be eaten over the winter.) The vibrant green of chlorophyll fades from the leaf to reveal the bright yellows and oranges of carotenoid pigments, accessory pigments that harvest light much like chlorophyll but at a different wavelength. Carotenoids also protect the delicate chlorophyll from UV and oxidative damage and thus are really good for you too in things like carrots (something your mom always told you but you never really wanted to listen to).

Carotenoids are also highly attractive and earn sleepy little wooded hillsides seasonal fame for their spectacular colors every autumn. But now I really digress. Anyway, the bright carotenoids have been there in the leaf all along, helping with photosynthesis and protecting chlorophyll. These pigments are just more stable than chlorophyll, and will persist longer in the leaf after resources have been withdrawn in the fall.

Now, that's yellow and orange leaves explained. Check. But what about red? If you were anything like me in my first fall in the Northeast, you probably searched the forest floor for the brightest, most cheery red-colored leaf you could find, held it tightly in your little gloved hand and marveled at just how very red that leaf shined against the grey days ahead.

Red leaves are more complicated. Maybe we can talk about them tomorrow.

Photo by Flickr user mmwm, licensed for non-commercial use by Creative Commons License.