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
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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.
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.
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
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.
In recent years, autumn colors have been attracting
more and more tourists and travelers to prime color regions: New
England, Michigan, Wisconsin and Colorado. (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. The U.S.D.A. Forest Service also operates a Fall Foliage
Hotline at (800) 354-4595. A detailed report can be found on the
Web at www.fs.fed.us/news/fallcolors/.
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