Carotenoids tint the leaves of hardwood species, such as hickory, ash, maple, tuliptree, birch, cottonwood and sassafras. A third group of pigments found in leaves are called anthocyanins. These pigments produce the reds and purples we see in fruits, such as apples, grapes, blueberries, cherries and plums. Anthocyanins are not present in the leaf during the growing season like the carotenoids. They develop in late summer in the sap of the cells, formed by chemical reactions between accumulating sugars and organic compounds called anthocyanidins.
Light is necessary for the formation of anthocyanins.
Warm, sunny days followed by cool nights are ideal for their production. Sugar is made in the leaves during the day, but cool nights prevent movement of the sugar from the leaves. The brighter the light during this period, the greater the production of anthocyanins, and the more intense the colors. Anthocyanins temporarily color the edges of some young leaves as they unfold in the early spring. In our autumn forests, they show up vividly in maples, oaks, sweetgum, dogwood and blackgum.