Delphinidin

The leaves of many oaks and several other plants generally turn some shade of brown or tan when their cells break down and die, due to a reaction between leaf proteins and tannins stored in the cell vacuoles. This is similar to the formation of leather when tannins react with animal hides. Leaves of many other deciduous plants, however, exhibit a variety of colors and drop before turning brown.

The chloroplasts of mature leaves contain several groups of pigments, such as green chlorophylls and carotenoids, which include yellow carotenes and pale yellow xanthophylls. Each of these groups plays a role in photosynthesis. Usually, considerably more chlorophyll than other pigments is present, and the intense green color of the chlorophylls masks or hides the presence of the carotenes and xanthophylls.

In the fall, however, the chlorophylls break down, and other colors are revealed. The exact cause of the chlorophyll breakdown is not known, but it does appear to involve, among other factors, a gradual reduction in day length. Water-soluble anthocyanin and betacyanin pigments may also accumulate in the vacuoles of the leaf cells in the fall. Anthocyanins, the more common of the two groups, are red if the cell sap is slightly acidic, blue if it is slightly alkaline, and of intermediate shades if it is neutral. Betacyanins are usually red; they apparently are restricted to several plant families, such as the cacti (Cactaceae); the Goosefoot Family (Chenopodiaceae), to which beets belong; the Fouro’clock Family (Nyctaginaceae); and the Portulaca Family (Portulacaceae).

Some plants (e.g., birch trees) consistently exhibit a single shade of color in their fall leaves, but many (e.g., maple, ash, sumac) vary considerably from one locality to another or even from one leaf to another on the same tree, depending on the combinations of carotenes, xanthophylls, and other pigments present. Some of the most spectacular fall colors in North America occur in the Eastern Deciduous Forest, particularly in New England and the upper reaches of the Mississippi Valley.

In parts of Wisconsin and Minnesota, one can observe the brilliant reds, oranges, and golds of maples, the deep maroons (and also yellows) of ashes, the bright yellows of aspen, and the seemingly glowing reds of sumacs and wahoos, all in a single locality. Some fall coloration is found almost anywhere in temperate zones where deciduous trees and shrubs exist.