We’re just about halfway through autumn in New York City and any foliage that intends on turning iridescent red or purple or yellow have pretty much done their thing. Moving forward, it’s all about desiccated brown. Still, while there’s more than a enough color dangling from branches or coloring the floor like splotches of paint, it’s a nice opportunity to have a look at how plant biology and chemistry act together to create swirls of warm colors late-stage Jackson Pollock would be proud to call his own. It all starts with a process called plant senescence.
The green color in leaves – spring, summer, or fall – comes from the chloroplasts packed into any given leaf’s cells. Chloroplasts, in turn, get their signature color from chlorophyll, specifically the green photosynthetic pigments chlorophyll a and chlorophyll b.
Each molecule plays the primary role in a plant’s ability to use sunlight to synthesize food from carbon dioxide and water while providing oxygen as a byproduct.
Contrary to common belief, chlorophyll doesn’t exactly get its green tint by reflecting all light that falls within that wavelength. According to a 2020 study,
“The data show that the green colour of leaves is caused by preferential absorption of blue and red light by chlorophyll, not by reflection of green light by chlorophyll. The data suggest that the cellulose of the cell walls is the main component that diffusely reflects visible light within plant leaves.”
In other words, it’s not so much about greens as it is about blues and reds. Sometimes, science can be counterintuitive which is why it’s so fascinating.
ORANGES AND YELLOWS
The orange and yellow colors in autumn leaves come from molecules known as carotenes and xanthophyll. Both are present in leaves even during the summer. However, they aren’t visible because they are dominated by the abundance of chlorophylls and their green tint. Carotene gives the characteristic green color to carrots, cantaloupes, and winter squash. Xanthophylls are responsible for the deep orange color of egg yolks.
Carotene molecules are characterized by a long carbon chain with rings on both ends.
Xanthophylls are oxygenated derivatives of carotenes and share a similar structure.
They are structural elements of the photosynthetic apparatus in leaves and contribute to the light-harvesting process while at the same time playing a photoprotective role. β-Carotene is present in core complexes and light-harvesting system during photosynthesis. The xanthophylls lutein and violaxanthin bind to its antenna moiety while zeaxanthin protects chloroplasts against photooxidative damage under conditions of excess light.
During the fall, chlorophyll begins to break down and allows the colors of the carotenes and xanthophylls to appear.
RED AND PURPLES
Not all fall foliage colors are present in leaves all year round. The really special hues – the reds and purples and everything in between – come from molecules known as anthocyanins.
They are responsible for giving fruits such as cranberries, red apples, blueberries, cherries, strawberries, and plums their red colors. They also create red cabbage and beauties like this cauliflower.
Leaves only produce anthocyanins toward the end of summer. They are produced in the sap of the cells of the leaf – the result of complex interactions of many influences – both inside and outside the plant. Their formation depends on the breakdown of sugars in the presence of bright light as the level of phosphate in the leaf is reduced.
The brown color that you see when leaves dry is the end of the senescence process. However, dead leaves and their roles in a tree’s life cycle is a story for another day. In the meantime, enjoy the colors while they last.
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