Why Do Leaves Change Color in the Fall?

Why do leaves change color in the fall? When leaves appear green, it is because they contain an abundance of chlorophyll. There is so much chlorophyll in an active leaf that the green masks other pigment colors. Light regulates chlorophyll production, so as autumn days grow shorter, less chlorophyll is produced. The decomposition rate of chlorophyll remains constant, so the green color starts to fade from leaves.

At the same time, surging sugar concentrations cause increased production of anthocyanin pigments. Leaves containing primarily anthocyanins will appear red. Carotenoids are another class of pigments found in some leaves. Carotenoid production is not dependent on light, so levels aren't diminished by shortened days. Carotenoids can be orange, yellow, or red, but most of these pigments found in leaves are yellow. Leaves with good amounts of both anthocyanins and carotenoids will appear orange.

Leaves with carotenoids but little or no anthocyanin will appear yellow. In the absence of these pigments, other plant chemicals also can affect leaf color. An example includes tannins, which are responsible for the brownish color of some oak leaves.

Temperature affects the rate of chemical reactions, including those in leaves, so it plays a part in leaf color. However, it's mainly light levels that are responsible for fall foliage colors. Sunny autumn days are needed for the brightest color displays, since anthocyanins require light. Overcast days will lead to more yellows and browns.

Leaf Pigments and Their Colors

Let's take a closer look at the structure and function of the leaf pigments. As I've said, the color of a leaf rarely results from a single pigment, but rather from an interaction of different pigments produced by the plant. The main pigment classes responsible for leaf color are porphyrins, carotenoids, and flavonoids. The color that we perceive depends on the amount and types of the pigments that are present. Chemical interactions within the plant, particularly in response to acidity (pH) also affect the leaf color.

Porphyrins have a ring structure. The primary porphyrin in leaves is a green pigment called chlorophyll. There are different chemical forms of chlorophyll (i.e., chlorophyll a and chlorophyll b), which are responsible for carbohydrate synthesis within a plant. Chlorophyll is produced in response to sunlight. As the seasons change and the amount of sunlight decreases, less chlorophyll is produced, and the leaves appear less green. Chlorophyll is broken down into simpler compounds at a constant rate, so green leaf color will gradually fade as chlorophyll production slows or stops.

Carotenoids are terpenes made of isoprene subunits. Examples of carotenoids found in leaves include lycopene, which is red, and xanthophyll, which is yellow. Light is not needed in order for a plant to produce carotenoids, therefore these pigments are always present in a living plant. Also, carotenoids decompose very slowly as compared to chlorophyll.

Flavonoids contain a diphenylpropene subunit. Examples of flavonoids include flavone and flavol, which are yellow, and the anthocyanins, which may be red, blue, or purple, depending on pH.

Anthocyanins, such as cyanidin, provide a natural sunscreen for plants. Because the molecular structure of an anthocyanin includes a sugar, production of this class of pigments is dependent on the availability of carbohydrates within a plant. Anthocyanin color changes with pH, so soil acidity affects leaf color. Anthocyanin is red at pH less than 3, violet at pH values around 7-8, and blue at pH greater than 11. Anthocyanin production also requires light, so several sunny days in a row are needed to develop bright red and purple tones.

Sources

• Archetti, Marco; Döring, Thomas F.; Hagen, Snorre B.; Hughes, Nicole M.; Leather, Simon R.; Lee, David W.; Lev-Yadun, Simcha; Manetas, Yiannis; Ougham, Helen J. (2011). "Unravelling the evolution of autumn colours: an interdisciplinary approach". Trends in Ecology & Evolution. 24 (3): 166–73. doi:10.1016/j.tree.2008.10.006
• Hortensteiner, S. (2006). "Chlorophyll degradation during senescence". Annual Review of Plant Biology. 57: 55–77. doi:10.1146/annurev.arplant.57.032905.105212
• Lee, D; Gould, K (2002). "Anthocyanins in leaves and other vegetative organs: An introduction." Advances in Botanical Research. 37: 1–16. doi:10.1016/S0065-2296(02)37040-X ISBN 978-0-12-005937-9.
• Thomas, H; Stoddart, J L (1980). "Leaf Senescence". Annual Review of Plant Physiology. 31: 83–111. doi:10.1146/annurev.pp.31.060180.000503