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THE
CHEMISTRY
OF AUTUMN
COLORS
Every autumn across the Northern Hemisphere,
diminishing daylight hours and falling temperatures induce trees to prepare for
winter. In these preparations, they shed billions of tons of leaves. In certain
regions, such as our own, the shedding of leaves is preceded by a spectacular
color show. Formerly green leaves turn to brilliant shades of yellow, orange,
and red. These color changes are the result of transformations in leaf pigments.
The green pigment in leaves is chlorophyll.
Chlorophyll absorbs red and blue light from the sunlight that falls on leaves.
Therefore, the light reflected by the leaves is diminished in red and blue and
appears green. The molecules of chlorophyll are large (C55H70MgN4O6).
They are not soluble in the aqueous solution that fills plant cells. Instead,
they are attached to the membranes of disc-like structures, called chloroplasts,
inside the cells. Chloroplasts are the site of photosynthesis, the process in
which light energy is converted to chemical energy. In chloroplasts, the light
absorbed by chlorophyll supplies the energy used by plants to transform carbon
dioxide and water into oxygen and carbohydrates, which have a general formula of
Cx(H2O)y.
| x CO2 +
y H2O |
light |
x O2 + Cx(H2O)y
|
|
|
|
chlorophyll |
In this endothermic transformation, the energy
of the light absorbed by chlorophyll is converted into chemical energy stored in
carbohydrates (sugars and starches). This chemical energy drives the biochemical
reactions that cause plants to grow, flower, and produce seed.
Chlorophyll is not a very stable compound;
bright sunlight causes it to decompose. To maintain the amount of chlorophyll in
their leaves, plants continuously synthesize it. The synthesis of chlorophyll in
plants requires sunlight and warm temperatures. Therefore, during summer
chlorophyll is continuously broken down and regenerated in the leaves of trees.
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| Paper birch |
Another pigment found in the leaves of many
plants is carotene. Carotene absorbs blue-green and blue light.
The light reflected from carotene appears yellow. Carotene is also a large
molecule (C40H36) contained in the chloroplasts of many
plants. When carotene and chlorophyll occur in the same leaf, together they
remove red, blue-green, and blue light from sunlight that falls on the leaf. The
light reflected by the leaf appears green. Carotene functions as an accessory
absorber. The energy of the light absorbed by carotene is transferred to
chlorophyll, which uses the energy in photosynthesis. Carotene is a much more
stable compound than chlorophyll. Carotene persists in leaves even when
chlorophyll has disappeared. When chlorophyll disappears from a leaf, the
remaining carotene causes the leaf to appear yellow.
A third pigment, or class of pigments, that
occur in leaves are the anthocyanins. Anthocyanins absorb blue, blue-green, and
green light. Therefore, the light reflected by leaves containing anthocyanins
appears red. Unlike chlorophyll and carotene, anthocyanins are not attached to
cell membranes, but are dissolved in the cell sap. The color produced by these
pigments is sensitive to the pH of the cell sap. If the sap is quite acidic, the
pigments impart a bright red color; if the sap is less acidic, its color is more
purple. Anthocyanin pigments are responsible for the red skin of ripe apples and
the purple of ripe grapes. Anthocyanins are formed by a reaction between sugars
and certain proteins in cell sap. This reaction does not occur until the
concentration of sugar in the sap is quite high. The reaction also requires
light. This is why apples often appear red on one side and green on the other;
the red side was in the sun and the green side was in shade.
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| Red Maple |
During summer, the leaves of trees are
factories producing sugar from carbon dioxide and water by the action of light
on chlorophyll. Chlorophyll causes the leaves to appear green. (The leaves of
some trees, such as birches and cottonwoods, also contain carotene; these leaves
appear brighter green, because carotene absorbs blue-green light.) Water and
nutrients flow from the roots, through the branches, and into the leaves. The
sugars produced by photosynthesis flow from the leaves to other parts of the
tree, where some of the chemical energy is used for growth and some is stored.
The shortening days and cool nights of autumn trigger changes in the tree. One
of these changes is the growth of a corky membrane between the branch and the
leaf stem. This membrane interferes with the flow of nutrients into the leaf.
Because the nutrient flow is interrupted, the production of chlorophyll in the
leaf declines, and the green color of the leaf fades. If the leaf contains
carotene, as do the leaves of birch and hickory, it will change from green to
bright yellow as the chlorophyll disappears. In some trees, as the concentration
of sugar in the leaf increases, the sugar reacts to form anthocyanins. These
pigments cause the yellowing leaves to turn red. Red maples, red oaks, and sumac
produce anthocyanins in abundance and display the brightest reds and purples in
the autumn landscape. (The purpose this red color may serve for the plant is the
subject of some scientific speculation, as reported by UW-Madison horticulture
Prof. William Hoch in a
recent article published in Tree Physiology
.)
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| Sugar Maple |
The range and intensity of autumn colors is
greatly influenced by the weather. Low temperatures destroy chlorophyll, and if
they stay above freezing, promote the formation of anthocyanins. Bright sunshine
also destroys chlorophyll and enhances anthocyanin production. Dry weather, by
increasing sugar concentration in sap, also increases the amount of anthocyanin.
So the brightest autumn colors are produced when dry, sunny days are followed by
cool, dry nights.
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| Red Oak |
In recent years, autumn colors have been
attracting more and more tourists and travelers to prime color regions: New
England, Michigan, and Wisconsin. (In Wisconsin, the fall-color tourists spend
over $1 billion.) The right combination of tree species and likely weather
conditions produce the most spectacular displays in these regions. States in
these regions maintain a fall foliage "hotline," keeping color watchers apprised
of the peak viewing locations and times. In Wisconsin, this information is
provided by the Division of Tourism at (800) 432-8747 and on the Web at
http://WWW.Travelwisconsin.com is the official tourism site for Wisconsin,
the Midwest's premier vacation destination..
The U.S. Forest Service also operates a Fall Foliage Hotline at (800) 354-4595.
A detailed report can be found on the Web at
Forest Service Fall Color Report.
.
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| Sumac |
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