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About Fall Color Chemistry Easy Read Article

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

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