The tree's response to the decreasing day lengths in the fall is to form an abscission layer at the base of each leaf. As this layer forms, it slowly cuts off water and mineral supplies to the leaf and reduces the manufacture of chlorophyll. As chlorophyll supplies decrease, previously masked carotenoid pigments in the leaf become visible. Carotenoid pigments are split into two classes based on oxygen content: xanthophylls and carotenes. Xanthophylls, which contain oxygen, are yellow. Carotenes, which do not contain oxygen, are orange. Carotenoid pigments absorb light energy like chlorophyll and serve to protect chlorophyll molecules from photo damage. A carotene you may have heard of is β-carotene which is a precursor to vitamin A. In humans, vitamin A is a pigment essential for good vision.
Anthocyanin pigments are another important component of fall color, contributing reds and purples to the fall palate. These pigments are not present in the leaf during the active growing season, and form in the leaf in the fall. Anthocyanin pigment formation is a function of sunlight, which is why you may see leaves at the tops and southern facing parts of trees turning colors before the rest of the tree. When the first leaves of the season fall, the remaining leaves receive more light and develop more color. This phenomenon can also be observed in wooded areas where trees on the edge of woods or those that are taller develop color first. The smaller trees that have been shaded by taller trees will not develop color until sunlight reaches them.
Photo 1 (left): Maple turning fall colors. Note the outer most leaves exposed to the most sunlight are turning first. Karen Jeannette
Anthocyanin pigments are also present in many fruits. For example, if you see an apple that is highly pigmented on one side and not on the other, it may be because the colored side was exposed to light and the other side was shaded by leaves. Many of our favorite fruit species, such as blueberries, cranberries, raspberries, blackberries, cherries, grapes, and eggplant contain anthocyanin pigments.
Functions of superior fall color
One environmental component that does not add to fall color is frost. A severe frost that is premature will kill the leaf cells and not permit the colors to develop. The leaves will instead turn brown, and drop
Superior fall color displays are a function of weather and the condition of the trees. When healthy, pest free plants with sufficient nutrients and water experience bright, sunny, and cool autumn days, and cool but not freezing autumn nights, we are treated to a magnificent display of fall color. Varying weather conditions can influence the timing of fall color. To get an up-to-date fall color report go to the following website. http://www.dnr.state.mn.us/fall_colors/index.html.
The following figure shows the average time "peak" color is obtained in different parts of Minnesota.
Image courtesy Explore Minnesota @ http://www.exploreminnesota.com/