Teacher’s Corner

Dec 05, Jan,Feb06

Why some leaves change color in the fall

Leaf color comes from pigments – natural chemical compounds produced by the leaf cells that give the leaf color (we use the same word for the colors in paints.).

The pigments that give leaves their color are

· chlorophyll - green

· carotenoid and xanthophyll – orange, yellow

· anthocyanins – red, purple

· tannins – browns

Chlorophyll is the chemical in the leaves that uses energy from the sun to combine carbon dioxide (from the air) and water (drawn up through the roots) to form glucose (with leftover oxygen) in the process called photosynthesis. In the course of making glucose, the chlorophyll turns bright green. This gives the leaves their familiar hue.

In the fall, the days shorten and the nights grow longer. Trees respond to the decreasing day length by producing less and less chlorophyll. Eventually, a tree stops producing chlorophyll altogether. Then the carotenoid and xanthophyll already in the leaves can show through. Carotenoid and xanthophyll are the sources of bright yellow and orange coloring in fruits and vegetables, including corn, carrots and bananas and in egg yolks. They are in the leaves all the time, but we don’t really know why. Perhaps they play a part in photosynthesis. Different combinations of these chemicals give us a wide range of warm fall colors.

Anthocyanins (reds and purples) are a different story. They are not produced in the leaves during the spring and summer like the carotenoid. They are formed from the glucose left in the leaf after the chlorophyll disappears. Why would a plant use energy to make this chemical if the leaf is going to fall off soon anyway? Again, we don’t know for sure. Some scientists think that the anthocyanins help the leaf stay attached to the tree a little longer. That gives the tree more time to remove more of the sugars (glucose), nitrogen and other valuable chemicals from the leaf before it drops off. Other scientists think that the anthocyanins seep into the soil as the fallen leaves decay and prevent other plants from growing near the tree the following spring. Anthocyanins are important antioxidants found in lots of red and purple fruits and vegetables- beets, red apples, purple grapes (and red wine) and some flowers like hyacinths and violets.

Brown colors come from tannins, a waste product of photosynthesis that gives the bitter taste to black tea, and the brown color to some pond water.

While we always get yellows and oranges, some autumns include spectacular reds and purples and others appear much less bright. The year’s weather is the key. The best weather for fall color: a warm wet spring; a mild summer; and a fall with plenty of warm, sunny days and cool nights.

You can use fall leaf colors to help identify trees:

Fall leaf color key:

· Oaks red or brown

· Hickories golden bronze

· Pecans yellow

· Maples red to orange

Look at the trees in your area – what colors do their leaves turn?