Plants survive by using photosynthesis, which is a fancy way of saying that they use light to make their own food. But light comes in all sorts of colors, meaning that plants have the entire rainbow at their disposal when they are in full sunlight. Which color of light do plants absorb? Hint: it’s not green.
Why Do Plants Appear Green?
You might be surprised to find out that plants don’t absorb green light. So why do plants look green? Think about it this way: When you look in a mirror, that mirror reflects light back at your eyes. That explains how you can see your reflection on its surface. You see green when you look at a leaf because it is reflecting that light while absorbing others. The color most associated with plants is the color they are turning away.
What Colors Do Plants Absorb?
Photosynthesis works by absorbing light and using the energy from that light to make sugars the plant can use. The light plants can use for photosynthesis is called photosynthetically active radiation, and it just happens to be the same spectrum of light that we can see. Plants absorb light by using pigments called chlorophylls, which are really good at harnessing the energy from light as they absorb it.
Of those chlorophyll pigments, the primary one is called chlorophyll a. It doesn’t absorb all wavelengths equally. In fact, it’s a little picky about what it picks up. Think about the colors of the rainbow (red, orange, yellow, green, blue, indigo and violet, in that order).
Chlorophyll a picks up some red and orange light while it reflects yellow, green and blue. Its favorite colors, though, are indigo and violet, as it absorbs those colors at nearly double the rate it picks up red and blue. There are other pigments in leaves as well, like chlorophyll b, which absorbs some slightly different wavelengths, and beta-carotene, which reflects red through green while absorbing a bunch of blue.
What Color Is Best For Plants?
To perform photosynthesis, violet light is the most important color, and it's from these wavelengths that plants get most of their energy. The reason for this is because out of the visible spectrum, red light is the longest wavelength light that the photosynthesis process can use, but it has the least energy. As we move through the spectrum from red, wavelengths become shorter and shorter while energy increases. Violet light is the shortest wavelength, and it has the highest energy.
Why Aren’t Plants Black?
Plants are typically green because they reflect green and yellow light, but if they absorbed all colors equally, they would appear black. While some plants do, in fact, have black leaves, this is definitely not the norm. If photosynthesis is all about harvesting energy from light, then why not use all of it?
Plants likely don’t absorb all wavelengths equally because it’s possible that would be harmful to them. Just like staying in the sun too long burns your skin, absorbing too much light could actually interfere with photosynthesis by heating up the leaf and damaging it. The solution? Absorb some and reflect the rest.
The reason why green light is reflected as opposed to red is largely unknown. Some scientists believe it evolved as a way to avoid oversaturation, while others believe that chlorophyll photosynthesis evolved in the deeper layers of the ocean at a time when photosynthesizing bacteria called halobacteria were dominant on the ocean’s surface.
Halobacteria use pigments that absorb green light and reflect purple, meaning that the only available light beneath the surface would be purple, not green. As halobacteria began to die off, chlorophyll photosynthesis took over.
- Astrophysics Data System: Evolutionary Competition Between Primitive Photosynthetic Systems: Existence of an Early Purple Earth?
- University of Arizona - The Biology Project: Introduction to Photosynthesis
- PLOS One: The Functional Significance of Black-Pigmented Leaves: Photosynthesis, Photoprotection and Productivity in Ophiopogon planiscapus ‘Nigrescens’
About the Author
Cameron is a writer and educator based out of Fort Collins, Colorado. His work has appeared in New Scientist, LiveScience, Discovery's Curiosity Daily podcast, and MinuteEarth. He teaches Ecology and Evolution at the University of Northern Colorado.