The following is the second post in a series on fall leaf color in deciduous plants.
There are popular and scientific myths about the causes of fall color in temperate deciduous forests. Jack Frost is alleged to paint tree leaves with his chilling touch, bringing on color change along with a frosty coating. Another twist on this myth is that Jack Frost brings reds and purples to autumn trees by pinching the leaves with his icy fingers. A less poetic explanation of fall color, favored by scientists for decades, is that the autumnal coloring of leaves was caused by waste products accumulated in the leaves and revealed to us with the fading of green chlorophyll pigments. As it turns out, the waste product theory now seems to be considered a bunch of, well, crud. The fall color pigments are produced, or revealed, only in living leaf cells of deciduous trees during the critical, seasonal process of leaf senescence. In fact, if Jack Frost did his thing too early, or, in other words, if there was an early killing frost, the leaf color display would be dulled, if not stopped altogether.
What triggers these fall changes if not Jack Frost? 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. The initiation and timing of the various processes of leaf senescence are genetically controlled for tree populations of a rather narrow climatic zone. How narrow is it? Seedlings originating from a local population in Illinois are usually planted within northern, central and southern seed zones each about 130 miles from south to north. This precise genetic programming, evolving through the impetus of natural selection, allows leaves to escape autumn frost damage in a specific climatic region during senescence. In the western mountains, it is possible to observe the wave of aspen coloration beginning at higher mountain elevations and progressing downwards to milder climates at lower altitudes.
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.
Information from "Why Tree Leaves Turn Color in Autumn" by Jeffrey O. Dawson, Professor of Tree Physiology, Department of Natural Resources and Environmental Sciences, University of Illinois at Urbana-Champaign. Go to http://web.extension.uiuc.edu/forestry/fall_colors.html for the full article.