Q. Why do leaves change color?

Newton

A. Think of a leaf as a factory. It takes in raw materials, converts them to a finished product, and sends what it makes back to the tree.

Like any factory, it has operating costs, measured in how much energy it takes to do its work. And like any factory, the leaf wants to operate at a profit, an energy profit, sending more back to the tree than it takes to do its job. When it can't make an energy profit, it shuts itself down. As it downsizes, it changes color.

Let's branch out a little. The tree supplies the leaf with water and minerals. Those raw materials go to the leaf's cells, for use by little packets inside the cells called chloroplasts.

Chloroplasts make several products, including the green pigments chlorophyll A and B, the key machines in the factory of the leaf. Both act like little solar panels, absorbing energy from the sun.

Chlorophyll A can also use solar radiation to excite atoms in the cells of the leaf, setting off a reaction that converts the light energy into chemical energy. Then the leaf uses the chemical energy to manufacture sugars, the leaf's finished product. That process is photosynthesis.

The factory runs best when conditions are ideal. New England leaves operate at a substantial energy profit for most of the summer, when they have the maximum sunlight, warmest temperatures, and an adequate supply of water. But in the fall, conditions deteriorate. Sunlight dwindles, temperatures drop, and it gets harder and harder for the leaf to make as much energy for the tree as it takes just to keep the factory running. The biochemistry of the leaf senses that the leaf factory is about to start running at an energy loss, and genes that have been dormant all summer order up proteins that trigger a series of changes in leaf biochemistry, changes that give us the colors of early fall.

Here's what's going on at the intercellular level in all that glorious foliage. First, proteins shut down the manufacture of chlorophyll. But the chloroplasts keep on producing another pigment, carotenoids, which they've been making all along.

Carotenoids are yellow and orange. They are kind of the internal security force in the factory. Their job is to absorb damaging free radicals, which are created when the leaf is zapped by especially strong bursts of sunlight. Carotenoids soak up those free radicals to protect the all-important chloroplasts.

In the fall, the yellow and orange carotenoids are left behind as the green chlorophyll disappears.

But where does the color red come from? It comes from anthocyanins, the same chemicals that give us the red in roses, cranberries, and apples.

Contrary to popular belief, anthocyanins are NOT in the leaf all the time. They are manufactured as the leaf begins its genetically programmed shutdown. Everyone assumes they're unmasked, that they've been there all along, says David Lee, professor of botany at Florida International University.

But they're not. You can't find anthocyanins in leaves in the summer. They're only produced during senescence. Why? Lee says no one knows. The best guess is that they are a mechanism to help get rid of the remaining chlorophylls, which block the tree from claiming back any precious nitrogen left over in the leaf to store it for the winter.

Have you ever noticed that some trees go through this process sooner than others? Lee says the first trees to change are those under some kind of stress. Trees in wetlands, for example, often change earlier because their roots are under water, which isn't ideal. Trees near highways change early because their roots are being pounded by traffic, and possibly because pollution stresses the leaf factories.

When the tree has stopped sending raw materials to the leaf and sucked out anything it can use for energy over the winter, the abscission layer that glues the leaf stem to the tree breaks down. And what was yellow and orange and red and lovely a few weeks earlier floats down brown and lifeless to join the pile we have to rake up from the lawn.