The large, leafy honeysuckle bushes have spanned decades in the United States and Europe – their flower arches have no small appeal. But once the flowers of the Viburnum tin plant die, the shrub produces something out of the ordinary: the bright, shiny green fruit.
Scientists have found that the pigments related to those in blueberries exist in honeysuckles, and think that this must be the source of their exotic colors. Green fruit, after all, is rare. But the researchers reported last week in the journal Current Biology that the honeysuckle’s green color is actually produced by layers of molecules arranged under the skin̵7;s surface, a form scientists call structure color. Meaning that as yet unknown, the plant’s cells produce thin slabs of fat that fold into a pile, like pieces of a puff pastry, and the result is their strange glow.
Rox Middleton, a researcher at the University of Bristol in the UK and the author of the new paper, has studied African pollinators, which produce exotic green pods themselves. But honeysuckle berries were everywhere, and she realized that their green was not well studied. Together with Miranda Sinnott-Armstrong, a researcher at the University of Colorado, Boulder, and other colleagues, she began to take a closer look at the peel of the fruit.
The blue color of a pollinator is a structural color in which the light is emitted from an evenly spaced arrangement of microscopic structures such that certain wavelengths, usually those that look like blue. blue or green to us, reflected back into the viewer. In the pollinated fruit, the color coming from the light interacts with the folded cellulose thin sheets. At first, the team thought that there would be something similar in the honeysuckle tree. But they did not see cellulose.
Experimenting with different ways of looking at fruit, they used a transmission electron microscope to obtain a side view of a plant cell. The nucleus of the cell is actually excreted by pigment spots. But between it and the surface of the skin was a massive, thick body the nucleus itself. A closer look will see that it is made of neatly spaced layers.
It’s a strong indication that structural colors, which require microscopic materials, with regular spacing for light to emit, are involved. “Once you see a repeating structure, that’s exactly the size – yes, it should be,” says Dr. Middleton.
The layers appear to be made of a spherical, uneven substance. Mathematical models of the classes show that this bump creates the distinctive blue color of the honeysuckle. If the layers are smoother, the blue color will be purer, like the wings of a beetle, says Dr. Middleton. The pigments in the kernel, although related to those in blueberries, are in fact a very dark, dark red color, allowing structural blue to prevail.
To the researchers’ surprise, the layers were made of fat molecules.
“We have never seen anything like that before,” said Dr. Sinnott-Armstrong.
Wax and proteins are commonly found in plant cell walls. But fat is rare in the cell wall, and it is not clear how the layers form.
The seeds of the honeysuckle are dispersed by the birds that eat and digest them. Researchers suggest that the vibrant colors produced by the fats may provide a signal to birds that the fruit is nutritious and worth eating.
“We have found a few other species that look like they have a similar structure to this one,” said Dr. Sinnott-Armstrong. She is planning to investigate whether they are also made with fat.
Dr. Middleton reflected on unusually colored blue fruits, honeysuckle is certainly easier to pick up than pollinated fruit.
“That’s great,” she said, “there’s one that I can choose from in my backyard.”