Oct 11

fall-leaf.jpgWith leaves beginning to turn in our neck of the woods (pardon the pun), we at HowDoWhy thought we’d answer your questions about why they bother to go through the trouble. Actually, it’s to prevent going through needless trouble that leaves bother to change at all. Confused? To understand why leaves change color, it helps to understand a little bit about what leaves do.

Color changing plants need three things to survive and grow. The first is water which they absorb through their roots. The next necessity for a plant to survive is carbon dioxide which they absorb from the air much like we breathe in oxygen from the air. The third and final component is sunlight which plants use as a source of energy to convert the water and carbon dioxide into Glucose, a kind of sugar, which plants need to grow. Ultimately, the glucose is the food and the water, carbon dioxide and sunlight are the materials the plant needs to make that food.

As you may have guessed, it’s the leaves of a tree that turn sunlight into the needed energy to power their glucose manufacturing plant in a process known as photosynthesis. One of the chemicals that aids in the process of photosynthesis is chlorophyll. It’s also responsible for the green color common to leaves.

Now, as the days grow shorter and the nights grow longer, there’s a net loss of efficiency in a plant’s ability to photosynthesize. Basically, there’s just not enough light to effectively power the factory to make the glucose. Ultimately, the energy required to feed and maintain leaves that aren’t doing the tree much good in the winter is simply not worth the effort. The leaves are also vulnerable to freezing temperatures. Rather than work hard and expend energy to maintain low producing element, the tree is better off shedding its leaves and feeding on stored energy sources until the days grow longer again.

So what to do with all those leaves? Well, remember we mentioned that chlorophyll is responsible for photosynthesis and for giving a leaf its green color. And, in the spring and summer, there’s a lot of chlorophyll. But as autumn comes, one or two things happen.

The first thing that happens is that production of chlorophyll dwindles and eventually stops entirely. As the chlorophyll vanishes, it not longer overpowers another chemical (a pigment that was there all along) called carotenoid. Read that word carefully again and you can guess what that implies for the leaf.

Carotenoid is responsible for yellow and orange coloring in many familiar plants such as corn and carrots. Now, remember, it was there all along in the leaf but simply overpowered by the abundant chlorophyll. With the chlorophyll diminished or gone, those brilliant yellows and reds begin to show.

The second thing that may happen in some leafy plants is the production of another chemical (pigment) called Anthocyanins. Unlike carotenoids, anthocyanin wasn’t in the leaf the whole time (unless we’re talking about a year around red-leafed plant) but is generally produced in autumn. Anthocyanin is responsible for those brilliant reds you see on some trees in fall. You may be thinking, “yeah, but some trees skip red altogether.” You’re right! Not all trees can make anthocyanins.

The theory behind why some trees produce anthocyanins and do so with more vigor in some seasons than in others is that it is believed that anthocyanins are responsible for delivering late-produced sugar sap (glucose) from the changing leaves to the tree before the leaves fall. In a mild fall season with no early freeze, the tree takes advantage of every last minute of photosynthesis, but the cold nights make sap transfer from the leaves to the tree particularly difficult. It is believed that anthocyanins help to facilitate the transfer.

And now you know why some trees have leaves that change color and why they do so. Can you think of a tree that stays green all year? And why are you sitting here reading this, anyway? Shouldn’t you be out there enjoying a great fall adventure full of color?

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