Autumn leaf color is a phenomenon that affects the normally green leaves of many deciduous trees and shrubs by which they take on, during a few weeks in the autumn season, one or many colors that range from red to yellow. The phenomenon is commonly called fall colors and autumn colors, while the expression fall foliage usually connotes the viewing of a tree or forest whose leaves have undergone the change. In some areas in the United States and Canada, "leaf peeping" tourism between the beginning of color changes and the onset of leaf fall, or scheduled in hope of coinciding with that period, is a major contribution to economic activity.
A green leaf is green because of the presence of a pigment known as chlorophyll. When they are abundant in the leaf's cells, as they are during the growing season, the chlorophylls' green color dominates and masks out the colors of any other pigments that may be present in the leaf. Thus the leaves of summer are characteristically green.
Chlorophyll has a vital function: that of capturing solar rays and utilizing the resulting energy in the manufacture of the plant's food?simple sugars which are produced from water and carbon dioxide. These sugars are the basis of the plant's nourishment?the sole source of the carbohydrates needed for growth and development. In their food-manufacturing process, the chlorophylls themselves break down and thus are being continually "used up." During the growing season, however, the plant replenishes the chlorophyll so that the supply remains high and the leaves stay green.
In late summer, the veins that carry fluids into and out of the leaf are gradually closed off as a layer of special cork cells forms at the base of each leaf. As this cork layer develops, water and mineral intake into the leaf is reduced, slowly at first, and then more rapidly. It is during this time that the chlorophyll begins to decrease.
Often the veins will still be green after the tissues between them have almost completely changed color.
Autumn's parade of red, yellow and gold following the disappearance of green chlorophyll relies on a single protein, a protease, recent research has shown. FtsH6 belongs to the FtsH family of proteases which target the chloroplast thylakoid membrane protein light-harvesting complex of photosystem II (LHC II), the most abundant membrane protein on earth. As this structural protein degrades away, the hidden pigments of yellow and red are revealed. The released amino acids are stored all winter in the tree's roots, branches, stems and trunk until next spring when they are recycled to re-leaf the tree.