Why Leaves Change Color
Chlorophyll goes, carotene stays
by Sue Stoessel
Submitted by Yew Kwong Hong
Submitted by Chris Royak
Green to Orange and Red:
The green we see in tree leaves is the pigment chlorophyll. Stored in chloroplasts in the watery cytoplasm of plant cells, it is responsible for creating food for the plants. Chlorophyll uses the sun's energy to split water molecules and recombine them with carbon dioxide in the air to make sugars. It's amazing to think that most of the mass of a big, heavy tree is created out of thin air!
The abundance of green chlorophyll hides other pigments that are found in the leaves at the same time. These hidden pigments may act as a kind of "sunblock" to protect the leaves from some dangerous types of ultraviolet light. Carotene (the orange in carrots) and Xanthophyll (yellows) are usually present in the leaves, but not revealed until the chlorophyll begins to break down in the Fall as the tree begins to get ready for the winter.
Other pigments are produced by some trees as the nights lengthen and temperatures become cooler. These environmental changes bring a surge in the tree's production of sugars. Stored sugars will help the tree survive the winter. Sugars also stimulate the production of pigments called Anthocyanins (red/purples found in rose petals & apple peels). These pigments produce the vivid red fall colors. These pigments can also be visible in very early spring leaves, before the chlorophyll is produced. Anthocyanins can be seen all year long in the leaves of trees such as Copper Beech and Japanese Maple.
What about brown oak leaves?
Other materials in plants can change the tint of the colors we see in the leaves. Tannins (the gold-brown color in your cup of tea) combine with the other pigments in oak leaves. Temperature, moisture content, soil composition, acidity and other environmental factors influence leaf colors as well.
These answers were developed by Sue Stoessel, senior education associate at the Museum of Science, Boston. One of the world's largest science centers, the Museum of Science takes a hands-on approach to science, attracting approximately 1.5 million visitors a year through its vibrant programs and over 700 interactive exhibits.
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