Art Courtesy of Alondra Moreno Santana
For plants, color is not just a matter of aesthetics—it’s a strategic tool for survival. The green, yellow, and orange hues in a tree’s leaves maximize their ability to capture light and make food through photosynthesis. Other colorful features play different roles; attracting pollinators or repelling predators with color can ensure a species’ survival. While scientists have long known that pigments like those in a tree’s leaves give rise to a variety of colors in plants, a lesser-known player produces color in an entirely different way in some plants: wax coatings. In a recent study published in Science Advances, researchers demonstrated that blueberries and other blue fruits owe their characteristic color to how light interacts with their wax coatings.
For Rox Middleton, a research fellow at the University of Bristol School of Biological Sciences and the first author of the study, the journey towards discovering this phenomenon began with a different optical effect. While completing their PhD, they studied Pollia condensata—a small, shiny blue fruit with a metallic sheen found in African rainforests—as well as the teardrop-shaped, dark blue fruits of the laurestine plant (Viburnum tinus), which is better known for its white flowers that bloom in the winter. Both fruits shine with a vivid blue hue. “From a materials science and an optics perspective, they’re incredible,” Middleton said. The root cause of the blue color on the fruits is what chemists call ordered crystal multilayers—structures in which crystal particles stack into layers in a repeating pattern. The thickness and composition of these layers can change how light interacts with the crystals, producing what is known as “structural color.” Scientists have also identified ordered crystal multilayers that create vibrant hues in peacock feathers and other brightly colored birds.
After spending their PhD uncovering the mysteries buried in other blue fruits, Middleton saw blueberries as a logical next step. As an expert in optics and structural color, Middleton knew exactly what to look for.
Middleton expected to find the same ordered crystal structure in blueberries as they had for the other two fruits they had studied. Blueberries—and fruits like damsons and barberries—do not actually have blue pigments. The only pigment molecules present in blueberries are dark red or purple, which is why mashed blueberries (in a smoothie, for instance) are that color. Instead of blue pigments, blueberries have a waxy layer on their skin called epicuticular wax, which contains nanoparticles that scatter light. The nanoparticles consist of only a few hundred atoms arranged into organic molecules. To better understand this wax coating, the research team isolated it from the fruit and recrystallized it on a flat card. “When [the wax] is dissolved, it is completely clear, so we knew it’s not a pigment,” Middleton said. As expected, once reformed on the card, the coating regained its blue hue. But, unlike what Middleton had expected based on previous work, microscopic analysis of the recrystallized blueberry wax showed no ordered layers—only randomly arranged crystals.
All structural coloring is caused by light scattering, the same effect that makes the sky blue. Scattering refers to the phenomenon of light changing direction after interacting with particles. Incoming white light from the Sun is composed of a range of wavelengths that correspond to different colors. Upon scattering, each wavelength changes direction at a slightly different angle. When this angle is large enough, we can see individual colors in the scattered light beam, like a rainbow, rather than a uniform white beam. The simplest type of scattering occurs in gases, where light can interact with individual particles.
The blue of the sky is a result of this effect. As sunlight travels through the atmosphere, it is scattered by air particles. The relatively short wavelength of blue light causes it to change direction at a greater angle than other colors, so blue light is more easily scattered. This angle is just large enough for blue light to separate from other colors in its relatively short journey through the atmosphere. When the sun is closer to the horizon, sunlight travels a greater distance through our atmosphere, revealing the scattered reds and oranges.
Scattering in solids is more complicated. “When light goes into a piece of material, it can bounce one time and come back, or it can go in and can bounce lots of times and then come back,” Middleton said. Unlike in gases, where scattering occurs at the level of individual particles, solids exhibit multiple scattering events.
Describing collective scattering in ordered solids like crystals is relatively straightforward due to their regularity. However, the randomness of the blueberry wax introduces an additional challenge. Apart from the scattering caused by collections of particles, single particles seem to cause their own additional scattering. “It’s a halfway point between this single-particle scattering in gases and this multiple scattering that you get in materials,” Middleton said. At this point, it was still unclear to what extent the researchers could accurately describe the scattering of epicuticular wax with just one model of scattering. To dig deeper, Middleton’s team used computer simulations to model scattering in fruit wax coatings. Using these simulations, they discovered that including scattering contributions from single particles produces scattering similar to that observed in blueberries. However, further work is still needed to fully understand how multiple scattering contributes to blueberries’ color from the standpoint of fundamental physics.
As for the biological side of the blue in blueberries, evolution offers an answer. Many fruit-eating animals, such as birds, evolved to distinguish blue and UV light. Blue pigments are fairly rare in nature, so plants with blue fruits stand out more easily and thus have an advantage by attracting animals to eat their fruits and spread their seeds. Given this advantage, multiple fruit-bearing species evolved to have structurally constructed blue hues despite the complexity of structural coloration.
Beyond blueberries, the research team hopes their work will contribute to our broader understanding of the epicuticular waxes present in most plants, even when they are not responsible for color. Since the 1990s, studies have revealed the remarkable properties of these waxes. They can undergo self-assembly, meaning they spontaneously regenerate themselves after being perturbed or even completely removed. Reproducing this property in man-made waxes or coatings could make them easier and faster to fabricate. The waxes also possess powerful hydrophobic (water-repellent) properties, which could be used to develop more sustainable forms of packaging or even hydrophobic materials for medical applications.
Surprisingly few studies on the applications of these waxes have focused on their optical effects. “When I looked at these studies, I thought, ‘You didn’t look at the optical properties one time?’” Middleton said. Middleton’s team hopes that their work on understanding structural color can help create more sustainable colorants and coatings with hydrophobic and self-assembling properties. Because they are biologically derived, these hypothetical products would be non-toxic and usable in fields ranging from medicine and food packaging to cosmetics and textiles. While plants might not be able to perceive the aesthetic value of their vibrant colors, we humans can certainly appreciate both their beauty and utility.