What Makes Leaves Change Color?
Ask A Specialist: What Makes Fall Leaves Change Color?
Answer by: JayDee Gunnell, Utah State University Extension horticulture agent,
The autumn season has transformed our mountains and hillsides into vibrant palettes of color that range from yellow to orange and from hot pink to scarlet red, including purple hues. Color pigments found in leaves vary among plant species and even from year to year.
It is commonly believed that cooler temperatures and frost bring about the changing colors in leaves. While it is true that the fall temperatures can influence the intensity of some colors, the main reason for the color change is shortened day length.
During the growing season, leaves are constantly producing sugars via photosynthesis. These sugars are shipped throughout the plant for growth and storage. When days become shorter and nights become longer, a process within the plant triggers the cells around the base of the leaf, or petiole, to divide rapidly but not elongate. This process forms an abscission layer where the leaf will eventually separate. This abscission layer blocks or prevents sugars from escaping the leaves.
As chlorophyll within the leaves breaks down, it gives way to the other “hidden” pigments. If leaves contain carotenoids or xanthophylls, they will take on hues of yellow and orange while anthocyanins will display reds, pinks and purples. Colors are most brilliant when plants have had adequate moisture throughout the season followed by sunny autumn days and accompanied by cool nighttime temperatures. As the season progresses and freezing temperatures occur, the pigments, including carotenoids, xanthophylls, and anthocyanins, will fade. Consider this information.
• Chlorophyll (the green pigment) is found in nearly all plants and is a key component in photosynthesis, the conversion of light to energy. It breaks down readily in sunlight and is replaced constantly throughout the growing season. However, when photosynthesis slows, other pigments start to become more apparent.
• Carotenoids and xanthophylls (the orange or yellow pigments) also aid in photosynthesis and are produced throughout the season but break down slower. Quaking aspen, ginkgo, Norway maple, ash, birch and honey locust are a few examples of trees containing these pigments.
• Anthocyanin (the pink, red or purple pigment) is produced primarily in the fall and is found in species such as certain maples (like our native Bigtooth maple), burning bush, sumac and dogwood.
• Tannin (the brown pigment) is the last pigment to break down in a leaf before it falls. Oaks are notorious for having leaves containing tannin.
Before all we are left with is boring browns, lace up those hiking boots, go spend some time in the hills and enjoy the beautiful colors while they last.