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.
Key Takeaways: Why Leaves Change Color
- Leaves change color in fall as chlorophyll breaks down and other pigments become visible.
- The main pigments are chlorophyll (green), carotenoids (yellow-orange), and anthocyanins (red-purple).
- Weather conditions like light, temperature, and pH affect pigment production.
- Brilliant red and purple hues only appear under specific sunny and cool conditions.
- The timing and intensity of fall colors vary by species and environment.
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.
| Class | Molecule | Colors |
|---|---|---|
| porphyrin | chlorophyll | green |
| carotenoid | carotene lycopene xanthophyll | orange, yellow orange, red yellow |
| flavonoid | flavone flavonol anthocyanin | yellow yellow 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.
Leaf Senescence
Leaf color change is part of senescence, the aging process where a plant reclaims valuable nutrients from leaves before they are shed. As the plant withdraws nitrogen, phosphorus, and other compounds, chlorophyll breaks down and disappears. This reveals existing pigments like carotenoids and prompts synthesis of anthocyanins in some species. Leaf senescence helps the plant conserve energy and survive winter dormancy.
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 act 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 Are Some Trees More Colorful in Fall?
Not all trees produce anthocyanins. Species like maples, sumacs, and oaks often show brilliant red and purple hues because they synthesize these pigments in response to environmental cues. The red-to-blue color of anthocyanins depends on cell vacuole pH: acidic conditions favor red hues, neutral conditions yield purple, and alkaline conditions lean blue. However, pH changes in leaf cells are subtle, so environmental conditions and genetics are more influential on fall coloration. Meanwhile, trees like birch and aspen mainly rely on carotenoids, so their fall colors tend to be yellow or gold. Genetics, climate, soil pH, and nutrient levels all influence how much pigment a tree produces.
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.
FAQs: Why Leaves Change Color
Q: Why do some leaves turn red and others yellow?
A: Yellow comes from carotenoids already present in leaves. Red or purple leaves contain anthocyanins, which are synthesized in fall by certain species under the right conditions.
Q: Does frost affect leaf color?
A: A light frost may intensify red coloration, but hard frosts can damage leaves and shorten the color display.
Q: Does soil pH change leaf color?
A: Yes, it may influence anthocyanin hue slightly, but temperature and sunlight are more important.
References
- 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
- Davies, Kevin M. (2004). Plant pigments and their manipulation. Wiley-Blackwell. ISBN 978-1-4051-1737-1.
- 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