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.
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 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 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.
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 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
REVISED: July 27, 2012