Why Leaves Change Color in the Fall – Chemistry

Why Leaves Change Color in the Fall
Leaves change color in the fall because they change the pigments in their cells in response to light and temperature.

Have you ever wondered why leaves change color in the fall? Leaves change the kind and amount of colored compounds (pigments) in response to their environment. They change colors because they produce less chlorophyll (green) and sometimes start making other pigments. This helps them move from making food to storing it, while protecting the plant from sun and cold damage until the leaves finally die and fall.

Here is a look at the pigment molecules that produce colors in leaves and their functions.

Chemistry of Leaf Colors

A single leaf actually contains a mixture of pigment molecules. The color of a leaf depends on the mixture of pigment molecules it contains. If chlorophyll is abundant, leaves are green. When there is less chlorophyll, underlying orange and yellow pigments show. Some plants start producing red and purple pigments as temperatures drop.

The main classes of molecules are porphyrins, carotenoids, and flavonoids. This table lists the colors associated with pigment molecules.

orange, yellow
orange, red
red, purple, blue

Functions of Plant Pigments

These pigments serve different functions within a plant and also degrade at different rates when the leaf stops making them. Chlorophyll (green) is the main photosynthetic pigment. It’s great at making glucose, which plants use for food, but it requires a lot of light and water. Carotenoids (yellow, orange, red) also function in photosynthesis, plus they are photoprotective. They extend the parts of the Sun’s spectrum plants use to make food. Flavonoids (yellow, red, purple, blue) regulate plant development, attract and repel insects, protect against ultraviolet radiation, and defend plants from herbivores, pests, and other plants.

Factors That Affect Leaf Color

The three factors affecting leaf color are light, temperature, and pH.

In the fall, days shorten, so there is less light. Plants cut back on the amount of chlorophyll they make. When chlorophyll is abundant, leaves are green. Chlorophyll breaks down at a constant rate, so the green color gradually fades away. When there is less chlorophyll, other pigments are visible.

Most leaves change to shades of yellow or orange because production of the molecules responsible for these colors (carotenoids) does not depend on temperature. Carotenoids degrade slowly once the plant stops producing them. So, even if leaves freeze and die, the colors remains for a while.

Some leaves make flavonoids. Anthocyanins are flavonoids which vary in color depending on pH. Anthocyanins are not usually present in leaves during the growing season. Instead, plants produce them as temperatures drop. They acts as a natural sunscreen and protect against cold damage. Anthocyanins also deter some insects that like to overwinter on plants and discourage new seedlings from sprouting too close to the parent plant. Plants need energy from light to make anthocyanins. So, vivid red and purple fall colors only appear if there are several sunny autumn days in a row.

When it gets cold enough or dry enough, leaves die and fall. Eventually, the colorful pigment molecules decompose. Dead leaves are brown because that is the color of their cell walls. Some leaves contain tannins, which are brown.

Why Some Leaves Do Not Change Color

Deciduous plants have leaves that change color and die before winter. However, evergreen plants have leaves that do not change color. Why? The leaves of plants like conifers, camellias, rhododendrons, and holly have waxy coatings that protect them against cold damage and dry air.


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