Fruits and flowers come in a wide range of colors, which can help plants attract beneficial animals like pollinators. The leaves are usually green, though, since that's the color of chlorophyll, the pigment plants use for photosynthesis.
But photosynthesizers don't necessarily have to be green. Many plants have reddish foliage, for example, due to the presence of other pigments in addition to chlorophyll, like carotenoids or anthocyanins. And before Earth had an oxygen atmosphere, the planet may have even gone through a "purple phase," led by violet-hued microbes that used a different light-sensitive molecule — retinal — instead of chlorophyll.
And now, thanks to a team of photonics researchers and biologists, we're learning about another odd twist on photosynthesis: bright blue begonias.
Tangled up in blue
Unlike the purple microbes, these begonias' blue leaves rely on chlorophyll just as green vegetation does. Yet unlike many red-leafed plants, they don't get their color from additional pigments, either. According to a new study published in the journal Nature Plants, their sapphire foliage comes from something even more bizarre: nanoscale crystals that help them survive in the darkness of a rain-forest understory.
Begonias are popular houseplants, partly because they can survive indoors without direct sunlight. That skill evolved among wild begonias on tropical and subtropical forest floors, where only slivers of sunlight trickle through the canopy above. For photosynthesis to work there, chloroplasts — the cell structures that contain chlorophyll — have to make the most out of what little light they get.
More than 1,500 begonia species are known to science, including a few that have long dazzled humans with a bluish sheen to their leaves. As the new study explains, however, the biological purpose of these blue leaves has been unclear, leading scientists to wonder if it deters predators or protects plants from too much light.
That mystery persisted until researchers from the U.K.'s University of Bristol and University of Essex noticed something about the peacock begonia (Begonia pavonina), a species native to montane forests in Malaysia. It's known for bright green leaves that sometimes, at certain light angles, glint iridescent blue. Yet it stays green when grown in bright light, they found, turning blue only in relative darkness.
The dark crystal
The blue morpho butterfly owes its iridescent wings to photonic crystals. (Photo: caspar s/Flickr)
Normally, chloroplasts contain flattened, membrane-bound sacs known as thylakoids, which are loosely organized into stacks. These stacks are where photosynthesis happens, both in green plants and in blue begonias. In the latter, however, thylakoids are arranged more precisely — so precisely, in fact, they form photonic crystals, a kind of nanostructure that affects the motion of photons.
"[U]nder the microscope, individual chloroplasts in these leaves reflected blue light brightly, almost like a mirror," says lead author Matthew Jacobs, a Ph.D. biology student at the University of Bristol, in a statement about the discovery.
"Looking in more detail by using a technique known as electron microscopy, we found a striking difference between the 'blue' chloroplasts found in the begonias, also known as 'iridoplasts' due to their brilliant blue iridescent coloration, and those found in other plants. The inner structure had arranged itself into extremely uniform layers just a few 100 nanometers in thickness, or a 1,000th the width of human hair."
Those layers are small enough to interfere with blue light waves, and since the begonia leaves are blue, Jacobs and his fellow biologists knew there must be a connection. So they teamed up with photonics researchers at the University of Bristol, who realized the natural structures look like man-made photonic crystals used in tiny lasers and other devices that control the flow of light.
With the same techniques used to measure those artificial crystals, the researchers began shedding light on the peacock begonia's version. Its iridoplasts reflect all blue light, making them appear blue without pigment, similar to iridescent blue animals like the blue morpho butterfly. They also absorb more green light than standard chloroplasts, the study found, offering a clue about why begonias turn blue.
Green plants look green because they mainly absorb other wavelengths of light, leaving green to be reflected to our eyes — and down through gaps in the canopy. So while a ceiling of trees hogs lots of blue light, green is less scarce on forest floors. And since iridoplasts concentrate green light, they may help begonias live in deep shade by using available light more efficiently. When the researchers measured photosynthesis rates in dim conditions, they found blue begonias were harvesting 5 to 10 percent more energy than normal chloroplasts in green plants.
That isn't a huge difference, but in hardscrabble rain forests, it might give begonias the boost they need. And learning more about their foliage might benefit humanity, too, the Bristol news release adds, providing blueprints we could use "in other plants to improve crop yields, or in artificial devices to make better electronics."
More research will be needed to investigate potential perks like those, the study's authors say, and to reveal how rare this phenomenon really is. The study found that peacock begonias contain a mix of iridoplasts and normal chloroplasts, suggesting the blue structures "function almost like a backup generator," co-author and Bristol biologist Heather Whitney tells Popular Mechanics. Plants may use traditional chloroplasts if there's enough light, then switch when light levels drop too low.
"It's just wonderful and logical to think that a plant has evolved an ability to physically manipulate the lighting around it in a variety of different ways," she says.
Even if this is widespread, it highlights an important point about people and plants. The plant kingdom is full of amazing adaptations that can help humans, from life-saving medications to light-bending crystals, but they tend to grow in forests — ecosystems that face mounting pressure globally from logging and agriculture.
Blue begonias may be safe, but they're just a hint of the treasures hidden in what's left of Earth's old-growth forests. As Whitney tells the Washington Post, living in a competitive ecosystem pushes plants to evolve or perish. "They've probably got loads of tricks we don't know about yet," she says, "because that's how they survive."
(Peacock begonia photos courtesy of Matthew Jacobs/University of Bristol)