Mysteries in Science: Why don’t glowing plants exist?
(Professor, Botanical Gardens)
It is known that various organisms such as animals and fungi (e.g., mushrooms) emit light, but this has not yet been discovered in plants. If plants had the power to glow, I believe that luminous flowers would bloom in forests at night. By glowing, these flowers could advertise the existence of honey, which would encourage pollinating hawkmoths to visit them. So then why are there no plants that can glow? Maybe there are less advantages for plants to glow than we think. Or perhaps there is a special reason why plants do not have the ability to glow.
Flowers that bloom at night and are pollinated by moths, such as gardenias, trumpet lilies or jasmine, mostly have white petals. This is because light from the Moon makes these flowers more salient. The scent of flowers also plays an important role in helping moths locate them in the forest at night. It is natural to think that if these flowers could emit light, they would stand out even more and become easier for insects to find. Furthermore, it is known that a number of plants, such as Mirabilis jalapa, have fluorescent petals. Fluorescence is light that is emitted when matter is excited by a short wavelength of light (such as ultraviolet rays) and returns to its original state, which differs from light emitted by organisms in the dark. However, similar to fluorescent clothing that is visible at dusk and on cloudy days, Mirabilis jalapa, which blooms at dusk, is noticeable even from a distance. What makes luminescence different from fluorescence is that it requires energy. Whether luminescence has any advantages needs to be explored, but when looking at flowers in the natural world that compete for the attention of pollinators, it is almost as if one can hear the voices of plants saying, “I want to glow if I can.”
On the other hand, I wonder if there is a reason for why plants cannot develop the mechanism to glow. To answer this, we need to study substances and proteins that are related to luminescence at the genetic and molecular level. However, many animals, such as fish and squid, glow by forming symbiosis with luminescent bacteria without having the mechanism to emit light by themselves. As it is normal for plants to form symbiosis with nitrogen-fixing bacteria and nutrient-absorbing fungi, which are in their roots and leaves, it is not impossible for them to also host bacteria and fungi that emit light. If you turn off all sources of light at night and allow your eyes to adjust to the pitch darkness in a warm-temperate Castanopsis forest, here and there you will see glowing branches and leaves that have fallen from trees (see image below). This is caused by the light that is emitted from the mycelium of plant-rotting fungi, but I cannot help but to think that if this could occur within living plants, those plants would also glow.
In our laboratory, we are doing research focusing on the diversity in the color, shape and scent of flowers and the shape of leaves, and have found new symbiotic relationships between plants and insects as well as new defense mechanisms in plants. We still overlook many phenomena that we can see and understand with our naked eyes in everyday plants, but we are aiming to expand the world of the plants before us by gaining an understanding of these kinds of unknown ecology. It may be considered a dream to find plants that glow, but we cannot find these plants to begin with unless we believe it is possible.
I believe that the first step is to take a walk in a pitch-black forest at night.
Image: If a dead branch that has fallen to the forest floor is decomposed by a bioluminescent fungus, it glows similar to this picture. The genus Armillaria is one fungus group whose mycelium (other than the mushroom part) has glowing potential. (Note that the fungus in this photo is unidentified.) This fungus can be typically seen after it rains in warm-temperate forests spanning from the main island of Japan to Okinawa, so if you have a chance, I highly recommend you see it. The light is faint but once your eyes become accustomed to the darkness, the dead branches will appear, resembling a starry sky.
This image was taken at Yonaguni using a high ISO with a 30-second exposure.
― This article is from the "Mysteries in Science" series in The Rigakubu News ―
Translated by Kristina Awatsu, Office of Communication
― Office of Communication ―