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What’s Behind Those Fall Colors?
My guess is most of us take this for
granted. As fall progresses to dreary
winter, we are fortunate to witness
the spectacular display of color in
fall. We live in one of the few areas of
the world where deciduous trees and
shrubs put on such a show. A few
areas of Europe, China, and Japan
have displays of color that rival those
that we see in Eastern North America.
Fall color is almost nonexistent in the
Southern Hemisphere.
The pace and characteristics of
autumnal color change are dictated mainly by plant genetics, which determine the chemical compounds
that the plant makes. Weather contributes to the timing and intensity of the color.
photo: Portland State University
By Gil Medeiros, Fairfax Master Garden
As everyone knows, chlorophyll is responsible for the green color of leaves. Chlorophyll, which uses light,
water, and carbon dioxide to produce the sugars that sustain the plant, is an unstable compound. That
means it falls apart easily. The plant must produce it continuously to maintain life-sustaining
photosynthesis. As days become shorter and temperatures turn cooler, the tree releases a hormone that
triggers the development of a membrane between each leaf stem and branch. This is the plant’s first step
to prepare for winter. The membranes act as valves that diminish the flow of water and nutrients to the
leaves.
As a result, the production of chlorophyll slows and stops. The remaining chlorophyll decomposes quickly.
In the absence of chlorophyll, other colorful pigments in the leaves such as carotenoids and anthocyanins,
already there, become visible to the eye. This is the magic of fall color change.
The carotenoids are highly visible in the yellow leaves of ginkgoes,
tulip poplars, birches, sycamores, eastern redbuds, and some
hickories before leaves drop in the fall.
Anthocyanins are responsible for red, blue, and purple colors in
leaves. When you are tired of talking politics at cocktail parties,
amaze your friends by telling them that anthocyanins are not
present in leaves during the entire growing season, as is claimed
by many textbooks. Anthocyanins in leaves are produced by
plants in late summer when sugars accumulate in the leaves.
Some plants convert the sugars in plant sap to anthocyanins —
again a function of genetics.
Fairfax County Master Gardeners Association, Inc. | © 2016
Gingko Leaves
photo: Joey Williamson, Clemson
Extension
Carotenoids are responsible for the yellow and orange colors in plant leaves. Carotenoids, which are very
stable, are in the leaves throughout the growing season. The function of this pigment is to help the
photosynthetic process by absorbing ultraviolet light and transferring energy to the chlorophyll. In this role
carotenoids make photosynthesis more efficient. Carotenoids also protect the chlorophyll from
decomposition.
photo: Joey Williamson, Clemson
Extension
Plants produce high concentrations of sugars in their leaves when weather conditions are just right.
Temperatures must be low but above freezing. Rainfall must be limited and sunshine abundant. These
conditions stimulate anthocyanin production; this results in more intense red and purple coloration.
The result is that leaves change color from the green of
chlorophyll to the yellow of carotenoids to the red and
purple of anthocyanins. The final step is evident in the
color changes of red maples, red oaks, white oaks,
black gums, sweet gums, sourwoods, and sassafras.
By the way, you can amaze your friends again by
connecting the anthocyanins of fall leaf color to the
dietary benefits of consuming anthocyanins. The
beneficial anthocyanins color flowers and fruits in
order to attract insects and animals. These compounds
are touted for their antioxidant and anti-inflammatory
Red Maple
properties, prevention of heart disease, and myriad
other claims. But make sure you do not counsel your friends to eat the red leaves of a maple tree in
autumn. They may be toxic. You get anthocyanins in your diet from fresh fruits, vegetables, and the
occasional swig of red wine, NOT by consuming fallen leaves of shade trees.
When the leaves turn brown, it is because of another class of chemicals called tannins. The leaf tannins
perform several functions during the growing season, one of which is to the make the leaf tissue
unpalatable for insects and animals. The highly water soluble tannins impart a phenolic smell to puddles
and shallow bodies of water where dead leaves have accumulated.
The same membrane in each leaf petiole that caused the demise of the chlorophyll eventually closes
completely. This seals off the circulatory system of the tree before winter sets in. It causes the leaves to
separate from the tree, drift to the earth, and return their nutrients to the soil. The cycle continues. Winter
is almost here.
Resources
“Chemical of the Week: the Chemistry of Autumn Colors”, SciFun.org
“Fall Color in the Carolinas”, Clemson Cooperative Extension
Fairfax County Master Gardeners Association, Inc. | © 2016