Just came upon this article. It reads better in the .pdf but some of you all using cell phones and the like might not be able to open it and who wants to go opening all kinds of documents anyways.
"Until recently, it was assumed by scientists is that the autumnal coloring
of leaves was caused by waste products accumulated in the leaves which only
become apparent with the fading of green chlorophyll pigments. However,
recent research indicates that fall color pigments are produced, or revealed, only
in living leaf cells during the seasonal process of leaf senescence and that they
serve to protect cellular functions during this critical time.
Senescence is triggered by a specific combination of shortening day length
and cooling temperatures in autumn at a given locale is typically “sensed” by
plant receptors resulting in the production of plant hormones that initiate leaf
senescence. In the living cells of senescing leaves, complex molecules, such as
starch and proteins, are broken down into smaller, soluble ones, such as sugars
and amino acids, and then exported to storage cells (resorbed). Living storage
cells are found in the inner bark of twigs, the outer sapwood of the main stem (in
and near wood rays) and in corresponding root tissues. Resorbing and storing
these compounds permits the tree to shed its leaves while avoiding loss of the
large percentage of their nutrients in leaves. This, in turn, allows the tree to avoid
having to compete with other plants and soil microbes for the resorbed nutrients
that would otherwise be cycled back into the soil system through leaf litter
decomposition. Resorbed nutrients including nitrogen, phosphorus, potassium,
sulfur, and carbohydrates, are mobilized from cells and stored within the tree.
The following spring the stored nutrients are remobilized and used to support
the intense flush of new leaves and spring growth burst in other tissues.
More energy is required for the biochemical breakdown of leaf substances by
enzymes, and for loading the soluble products into the leaf-veins for transport
out of the leaves, than that which is available as reserves in leaves. Hence it is
necessary to protect chlorophyll, at least during the earlier phases of senescence,
in order to prolong production of energy rich compounds that initiate the
enzymatic reactions necessary for leaf senescence. Additional important
biochemical processes supported by photosynthesis in senescing leaves include
the production of enzymes and their products that allow leaf cells to better
tolerate freezing and drying, that absorb energy from light bursts damaging to
the photosynthetic apparatus, that deter leaf predators, that prevent oxidative
damage to cell constituents, including membranes, proteins and DNA, caused
by free radicals produced during senescence, and that protect and transform the
cells of leaf tissue that form the abscission layer at the base of the leaf petiole. The
abscission layer allows the leaf to break
away cleanly from its branch without
forming an opening from which sap
could leak and through which disease
organisms could enter the tree.
Functions of specific pigments:
Carotenoids
Carotenoid pigments are found
abundantly in such vegetables as carrots
and tomatoes. The carotenoids include
lycopene and beta-carotene, known to
be powerful antioxidants and cancerfighting
substances in humans. Another
There’s a Method to Fall Color Madness
Continued on page 83
October 2010 83 Volume 16, Number 10
There’s a Method to Fall Color Madness continued from page 79
form of carotenoid found in senescing tree leaves is
xanthophyll. Carotenoids are responsible for the yellow
and orange colors of autumn leaves. The unmasking of the
carotenoids accounts for the yellow fall leaf color of Ohio
buckeye, yellow-poplar, sycamore, birches, hickories, ashes,
and many other tree species.
Carotenoid pigments and chlorophyll are attached
to membranes in intricate structures (organelles) called
chloroplasts. Chloroplasts give leaves their green color.
Carotenoid pigments assist chlorophyll in the capture of
sunlight for photosynthesis. These yellowish pigments
are always present in leaves, but are not visible for most
of the year because they are masked by larger amounts
of green chlorophyll. As chlorophyll degrades in the fall,
the carotenoid pigments degrade more slowly and persist,
revealing their yellowish colors. Spanish researchers found
that carotenoid substances actually increase during the
early stages of senescence of pistachio leaves and probably
provide both photo-protection and antioxidative protection
to the photosynthetic apparatus. Carotenoids dampen
damage, caused by high light intensity, to the susceptible
photosynthetic apparatus of senescing leaves.
Tannins
Tannins cause the brown hues in leaves of some oaks
and other trees in the autumn. The golden yellow or
copper colors produced in some leaves, such as those of
beech result from the presence of tannins along with the
yellow carotenoid pigments. Like the carotenoids, these
compounds are always present, but only become visible as
chlorophyll and carotenoids both disappear from leaves.
Often considered waste products, tannins actually act as a
defense mechanism in plants against pathogens, herbivores
and hostile environmental conditions. Oaks defoliated by
gypsy moths often produce a secondary flush of leaves higher
in protective tannins than the first set of leaves.
Anthocyanins
Anthocyanin pigments are responsible for the pink, red,
and purple leaves of sugar and red maple, sassafras, sumac,
white and scarlet oak, and many other woody plants. They
are formed in sap inside the vacuole, a storage compartment
within plant cells, when sugars accumulate and combine
with complex compounds called anthocyanidins. The variety
of pink to purple colors in leaves is due to many, slightly
different compounds that can be formed. Their color is also
influenced by cell pH. These pigments usually are red in tree
species with acidic sap, and are purplish to blue in alkaline
cell solution. Anthocyanins are not commonly present in
leaves until they are produced during autumn coloration.
Trees lacking the genes for production of anthocyanin
develop yellow and brown shades of autumn color.
With the formation of the abscission layer and with
higher viscosity of cell sap under cold conditions, the
phloem tissues of a tree’s vascular system, the pathway for
conduction of sugars out of leaves, become less efficient and
are eventually severed where the leaf petiole joins the tree
branch. However, the nonliving xylem vessels that transport
water and nutrients from the roots upward, remain intact.
This allows them to continue to carry water to the senescing
leaves while sugars derived from continued photosynthesis
and the conversion of stored starch to soluble sugars are
trapped by the impaired phloem of the abscission layer
and are available for anthocyanin production. Trees of the
same species growing together often differ in color because
of differences in amounts of soluble sugars in the leaves for
anthocyanin production. These differences are caused by
genetic and environmental factors. Leaves exposed to the
sun, such as those on the outside branches of the tree crown,
may continue photosynthesis and turn red while others in the
shade may be yellow. A single tree may even have branches
with different colored leaves due to differences in leaf shading.
It is common to see sugar maples with reddish leaves only
on exposed outer branches of the upper crown.
Fall weather conditions favoring formation of bright
red autumn leaf color are warm sunny days followed by
cool, but not freezing, nights. Rainy or cloudy days with
their reduced sunlight near the time of peak coloration
decrease the intensity of reddish autumn colors by limiting
photosynthesis and the sugars available for anthocyanin
production. Overcast conditions reduce light intensity,
and heavy rains and high winds can sweep the leaves off
trees prematurely.
Research by Gould in New Zealand indicates that senescing
leaves seem to need special protection against bright light
exposure, which overloads light-gathering chlorophyll and
slows it down (photoinhibition). Anthocyanins can offload
some of that excess energy, sustaining photosynthesis rates
necessary to provide energy for nutrient resorption and
other critical processes during senescence.
Recent research by Gould also indicates that anthocyanins
function as protective antioxidants in plant leaves.
Anthocyanins may also protect physiological processes
in leaves from cold temperatures. William Hoch of the
University of Wisconsin-Madison ranked the intensity of
red coloration in autumn of species in nine genera of woody
plants either from a cold zone in Canada and the northern
U.S. or from a milder maritime climate in Europe. The
species that produced the most intense red coloration came
exclusively from the North American cold zone.
Linda Chalker-Scott of the University of Washington
proposes that anthocyanins help leaves retain water.
Anthocyanins dissolve in water, whereas chlorophyll and
Continued on page 85
There’s a Method to Fall Color Madness continued from page 83
many other cell pigments do not. Water loaded with any
dissolved substance has lower osmotic potential: a decreased
tendency for water to flow away. Many plants produce
soluble anthocyanins that may help leaves retain water when
subjected to osmotic stresses from drought, salt buildup
on leaf surfaces, and heat. Loading water with solutes also
lowers its freezing point, possibly affording added frost
protection to senescing leaves.
Leaf pigments behind the flashy autumn display of color
in temperate hardwood forests are much more than cellular
trash. Recognizing tree colors not only for their beauty, but
also for the complex and vital roles the underlying pigments
play in forest function and survival, might just bring new
awe and appreciation to the autumnal rite of leaf peeping.
Excerpted from: Why Tree Leaves Turn Color in Autumn
Jeffrey O. Dawson, Professor of Tree Physiology
Department of Natural Resources and Environmental Sciences
University of Illinois at Urbana-Champaign
http://web.extension.illinois.edu/forestry/fall_colors.html
Christopher Starbuck
Associate Professor
Division of Plant Sciences
StarbuckC@missouri.edu"
Here is a link that might be useful: Missouri Environment and Garden
alexander3_gw
brandon7 TN_zone7
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