Autumn leaf color

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, various shades of yellow, orange, red, purple, and brown. The phenomenon is commonly called autumn colours or autumn foliage in British English and fall colors, fall foliage, or simply foliage in American English.

In some areas of Canada and the United States, "leaf peeping" tourism is a major contribution to economic activity. This tourist activity occurs between the beginning of color changes and the onset of leaf fall, usually around September to November in the Northern Hemisphere and March to May in the Southern Hemisphere.

A green leaf is green because of the presence of a pigment known as chlorophyll, which is inside an organelle called a chloroplast. When abundant in the leaf's cells, as during the growing season, the chlorophyll's 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: it captures solar rays and uses 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 break down, thus are continually "used up". During the growing season, however, the plant replenishes the chlorophyll so that the supply remains high and the leaves stay green.^{[citation needed]}

In late summer, with daylight hours shortening and temperatures cooling, 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. During this time, the amount of chlorophyll in the leaf begins to decrease. Often, the veins are still green after the tissues between them have almost completely changed color.

Chlorophyll is located in the thylakoid membrane of the chloroplast and it is composed of an apoprotein along with several ligands, the most important of which are chlorophylls a and b. In the autumn, this complex is broken down. Chlorophyll degradation is thought to occur first. Research suggests that the beginning of chlorophyll degradation is catalyzed by chlorophyll b reductase, which reduces chlorophyll b to 7‑hydroxymethyl chlorophyll a, which is then reduced to chlorophyll a. This is believed to destabilize the complex, at which point breakdown of the apoprotein occurs. An important enzyme in the breakdown of the apoprotein is FtsH6, which belongs to the FtsH family of proteases.

Chlorophylls degrade into colorless tetrapyrroles known as nonfluorescent chlorophyll catabolites. As the chlorophylls degrade, the hidden pigments of yellow xanthophylls and orange beta-carotene are revealed.

Carotenoids are present in the leaves throughout the year, but their orange-yellow colors are usually masked by green chlorophyll. As autumn approaches, certain influences both inside and outside the plant cause the chlorophylls to be replaced at a slower rate than they are being used up. During this period, with the total supply of chlorophylls gradually dwindling, the "masking" effect slowly fades away. Then other pigments present (along with the chlorophylls) in the leaf's cells begin to show through. These are carotenoids and they provide colorations of yellow, brown, orange, and the many hues in between.