Chemical reactions paint fall leaves
Bright, multi-colored leaves herald the arrival of autumn. But what causes those leaves to change from green to red, yellow, brown or purple?
Fall coloration results from a series of chemical reactions, said Dennis Adams, Nebraska Forest Service Extension forester at the University of Nebraska-Lincoln. The presence of certain pigments in all leaves, along with the weather, creates the recognizable color palette of fall.
"Each tree or shrub develops its own fall coloration depending on the proportions of yellow and red pigments present," Adams said.
People often credit frost with the changing colors, but they are actually due to the presence of pigments in the leaves: chlorophyll A, chlorophyll B, carotene, xanthophyll and anthocyanin. The chlorophylls create the familiar green color. Summer photosynthesis replenishes chlorophyll. However, during the fall, chlorophyll is broken down faster than it is replenished, allowing other pigments to dominate.
"If yellow pigments are characteristic of the species, they were present all summer, but they only become visible when the chlorophyll dissipates," Adams said.
Reds and purples are produced by excess sugars in the leaf reacting with other substances to produce anthocyanins, or red pigments.
Weather also influences the coloration of fall leaves. Cool nights accelerate chlorophyll breakdown, resulting in yellow leaves. Sunny days increase the rate of photosynthesis, producing more sugars and therefore more red leaves. Warm, rainy weather creates dull colors. Freezing temperatures leave brown, dead leaves.
Other factors include plant species and soil, which create different colors based on the pigments and nutrients present.