As the days get shorter and cooler, the nights become longer, and deciduous trees and shrubs put on an autumn show with leaves displaying shades of red, yellow, orange, purple and brown. Many people think cool weather or frost cause the leaves to change color. Those factors do contribute to the color and intensity, but it is actually the length of the dark period each day (longer nights) that sets off the cascade of events resulting in the splendor of crisp fall days.
Molecules called pigments color leaves. The pigment that causes leaves to be green is chlorophyll. Chlorophyll is a critical part of the process of photosynthesis, which is how plants capture sunlight and use that energy to produce glucose. 6CO2+ 6H20 with sunlight & chlorophyll = C6H1206 + 602
As the amount of sunlight in a given day decreases, the plant uses a corky layer of cells to block transport of carbohydrates from leaf to branch. This also blocks the flow of minerals from roots into leaves. This process reduces the amount of energy available to make chlorophyll, and the chlorophyll starts to breaks down, revealing other pigments called xanthophylls and carotenoids that are visible as yellow and orange. Red and purple pigments are manufactured from the sugars trapped in the leaf and are only produced after the leaf is sealed in the fall. Voila! Amazing science equals glorious fall colors
By Patricia Gully, Highlands Center Naturalist