Why Do These Plants Have Metallic Blue Leaves?

Ooh, shiny.

Matthew Jacobs

Roses are red but violets aren’t blue. They’re mostly violet. The peacock begonia, however, is blue—and not just a boring matte shade, but a shiny metallic one. Its leaves are typically dark green in color, but if you look at them from the right angle, they take on a metallic blue sheen. “It’s like green silk, shot through with a deep royal blue,” says Heather Whitney from the University of Bristol.

And she thinks she knows why.

Similar metallic colours are common in nature—you can find it in the wings of many butterflies, the bibs of pigeons, the feathers of peacocks, and the shells of jewel beetles. These body parts get their color not from pigments but from microscopic structures that are found in evenly spaced layers. As light hits each layer, some gets reflected and the rest pass through. Because of the regular gaps between the layers, the reflected beams amplify each other to produce exceptionally strong colors—at least, from certain viewing angles. This is called iridescence.

Iridescence is less obvious among plants, but there are some stunning exceptions. The beautiful peacock moraea—a type of iris—has iridescent blue patches at the base of its white petals. The marble berry produces fruit that look like Christmas baubles and are among the shiniest biological materials known. And many flowers, as Whitney discovered earlier this year, use subtle iridescence as a signal to bees and other pollinating insects.

It makes sense for flowers and fruits to be shiny, since their conspicuous colors might attract animals that spread pollen and seeds.  But the peacock begonia has iridescent leaves—and that’s much weirder. Leaves are all about collecting light, and iridescence is about reflecting it. And the peacock begonia doesn’t have a lot of light to spare—in the tropical Malaysian rainforests where it lives, it grows in the perpetual shadow of much taller trees. “It seemed absolutely barmy that these deep-shade plants would produce structures that reflect photosynthetically useful light,” says Whitney.

To understand why, you need to peer inside the leaves. In a typical leaf, you’d see green, bean-shaped structures called chloroplasts. And if you looked inside these, you’d see sets of stacked discs that resemble piles of coins. These stacks are where photosynthesis takes place. But the begonia has modified its chloroplasts so that their stacks are highly ordered and precisely spaced—far more than usual. These modified structures are called iridoplasts, and they’re the source of the leaves’ iridescence.

Whitney’s colleagues Matthew Jacobs and Martin Lopez-Garcia simulated what happens to light when it enters the iridoplasts. They showed that blue light reflects away, as you’d expect. But they also found that the regularly spaced stacks also slow the passage of green light, and so absorb it more effectively. Compared to standard chloroplasts, the iridoplasts can harvest light 5 to 10 percent more effectively. That might allow the shade-dwelling begonia to eke out every last scrap of available light, and grow better in the shade.

Whitney admits that her team hasn’t yet tested whether the iridoplasts actually improve photosynthesis, and notes that such improvements would probably be minor. Doekele Stavenga from the University of Groningen, who praises the study, agrees. “The essential message may be that in biology every possible minimal step forward can be advantageous for survival,” he says.

But if the iridoplasts are that much better at harvesting light, then why don’t all plants modify their chloroplasts in this way? In fact, why doesn’t the begonia turn all its chloroplasts into iridoplasts? Whitney suspects that it’s because these structures are only more efficient at low light, and may actually cause problems in brighter conditions.

Also, why are the iridoplasts blue? Is that just a coincidence—an inadvertent consequence of evolving stacks that better absorb green light? “Sort of,” says Whitney. It seems that way for the peacock begonia, but there are other shade-growing plants that produce blue iridescence with other microscopic structures, which have nothing to do with chloroplasts. “There seems to be another reason why these shade plants converge on blue iridescence,” says Whitney.

Perhaps, she ventures, the gentle shimmer makes the plants harder to spot, and so defends them from hungry animals. Hiding by shining seems counter-intuitive, but then again, so does gathering light by reflecting it.