First of all, let's understand why most leaves are green during the summer.
We know that the main function of most kinds of leaves is to convert sunlight energy into carbohydrate, which the plant uses in various ways. Elsewhere we've seen that this magical chemical process is known as photosynthesis.
In the cells of green leaves, photosynthesis takes place in molecules of a substance known as chlorophyll. Sunlight strikes the chlorophyll, then photosynthesis takes place.
Sunlight is made up of many colors. When sunlight falls onto a glass prism, the prism breaks the light into its rainbow of colors.
It happens that chlorophyll doesn't use all of sunlight's rainbow of colors. It absorbs only the red and blue part of the spectrum, so the light that's left -- the light that reflects back to our eyes -- is
Green leaves are green because chlorophyll absorbs all of sunlight's rainbow of colors except the green part, which is reflected.
You know that too much sunlight can damage your skin, fade colors in clothing and cause other problems. Therefore it shouldn't be surprising to find out that sunlight also causes chlorophyll molecules to break down. In fact, during the summer, green plants must continually create new chlorophyll to replace what has been destroyed. This creation, or synthesis, of chlorophyll requires not only sunlight but also warm temperatures. For this reason you can see why fall's cooler weather encourages our trees' leaves to begin showing colors other than green.
But, where do those other colors come from?
There's another substance in many leaves known as carotene. Carotene is a kind of "chlorophyll helper." That's because carotene absorbs sunlight energy like chlorophyll, but instead of keeping that energy and conducting photosynthesis with it, it passes its energy on to chlorophyll which then uses that energy to perform photosynthesis. Carotene is known technically as an "accessory absorber." Carotene holds up much better under sunlight than chlorophyll, so often in the fall when chlorophyll disappears from leaves, carotene is left behind. Since carotene absorbs blue-green and blue light in sunlight, the light it reflects back to our eyes from leaves in which the chlorophyll has disappeared is
Chlorophyll and carotene are both known as pigments. The dictionary says that in biology pigments are simply various kinds of "coloring matters" occurring in animals and plants, so the concept of "pigment" isn't a very technical one. In regular life you sometimes hear of "mixing pigments" in paint to create different colors.
Besides chlorophyll and carotene, there's yet another pigment, or class of pigments, occurring in many leaves, called anthocyanins. However, these pigments are not important because they capture sunlight energy. Anthocyanins are formed when sugars and certain proteins interact in the juice inside plant cells. Despite anthocyanins playing no part in photosynthesis, they are themselves not formed unless sunlight is present. Since anthocyanins absorb blue, blue-green, and green light, the color they reflect to our eyes is
Actually, it's a bit more complex than anthocyanins simply being red. The color produced by anthocyanins is sensitive to the pH, or degree of acidity, of the juice inside the plant cells -- the cell sap -- in which they find themselves. If the sap is very acidic, anthocyanin imparts a bright red color; if the sap is less acidic, its color is more purple.
There's a big debate going on right now about why leaves have anthocyanins and therefore turn red and purple in the fall. New research suggests that anthocyanins act as a sort of sun screen for leaves as they are preparing to be discarded onto the forest floor. The sun-screen protection shields delicate chemical reactions in the leaves enabling the tree to reabsorb nutrients from the leaves before the leaves are dropped. The tree doesn't "want" to throw away any more valuable nutrients than it has to. But another theory suggests that red pigments somehow protect a plant's water supply, since anthocyanins are water-soluble and as such change the freezing point and other qualities of water inside the leaves. Yet another theory says that red pigments either attract or repel animals. And on top of all this is the fact that many tropical plants in lands where fall doesn't exist also often possess anthocyanins and turn red. Well, the mystery of red leaves is just one of jillions of such mysteries in nature we haven't figured out yet.
You may be interested in joining The Foliage Network, which keeps a close watch on fall leaf colors in part of the US. Foliage watchers are sought in these states: Maine, New Hampshire, Vermont, New York, Massachusetts, Rhode Island, Connecticut, New Jersey, Pennsylvania, Maryland, Delaware, Virginia, West Virginia, Ohio, Michigan, Wisconsin, or Minnesota. If you decide to join, you will be asked to submit a report twice a week during September, October, and November.
The USDA Forest Service also has a page focusing on fall colors.