The secret of the blue leaves – iridescence and structural colors in plantsAlexander
1. How? What? Blue plants?
It must have been sometime in the fall of 2014 when I first saw a picture of a Begonia pavonina, the “peacock begonia”. It was in a listing on eBay for this plant and, according to the description, it was supposed to “glow at night.”
If you regularly search for plants on eBay, you surely know the numerous fraudulent offers with laughably badly edited pictures on which all kinds of rainbow-colored or blue plants can be seen. This begonia looked just as fake to me at the time, and then this plant should also serve as a night light? As if. So I ignored it and kept scrolling…
…and not five minutes later I was back. Somehow, this didn’t look fake. Were there really plants with blue leaves? Do they really glow?
And that’s exactly how my fascination with iridescent blue plants began.
These days I have a small but steadily growing collection of about 30 plant species with some form of blue iridescence or another (more on that later), and a bigger and much faster growing wish list of more specimens.
I also know by now that these plants do not glow at night, of course, and that it is very difficult to convey in pictures what this unique effect really looks like in real life.
In this blog article I present some specimens from this very diverse group of plants, explain (or at least attempt to explain) the background and possible function of their metallic appearance, and convey the current state of research on this phenomenon.
2. Are these plants really blue now?
First of all, we should clarify what this effect actually looks like in reality:
What is edited?
What do I really see when I stand in front of it?
And why do the pictures often look so fake?
To the first question, “What is edited?” the answer is very simple: Basically everything.
Even pro-level cameras have problems reproducing the effect faithfully. All of the images in this blog article, and most of the images online, are necessarily digitally processed, because no image from a camera really captures what you see with the naked eye.
Accordingly, the images in this article are all adjusted to give the most natural impression possible. Other pictures that you can find on the Internet are sometimes a bit too… optimistic, and show exaggerated colors and contrasts.
The fact that you “have” to edit the images is mainly because the effect is only clearly visible under certain conditions, and your own eyes are much better at seeing it than most cameras. The metallic iridescent effect mostly depends on the angle between the incident light and the viewer. The smaller the better.
Since this is a bit abstract, here is an explanatory graphic:
In direct light, for example in an unclouded sky or under a lamp, the effect is not visible at all from the vast majority of viewing angles. Only if the angle between the light source and the observer is small (usually 15° to the leaf surface or less) you can see the metallic shimmer.
However, where these plants grow in nature, namely in the undergrowth of rainforests, the light is not direct but very diffuse. It comes almost uniformly from all directions.
Here, the iridescence is clearly visible to the eye, but cameras have their problems with it because they have a lower dynamic range than human eyes in low light. What to my eye is a bright blue and beautifully shimmering plant on a cloudy day is greenish gray to most cameras.
The way to “solve” this problem is simple: if you take a photo with a camera with flash that outshines the ambient light, the angle is very small (often less than 5°) and the effect is correspondingly strong. Much stronger than when viewed under normal light!
This is also how most of the images of these plants are created, and also the misconception of their colors. You can actually see such a strong effect with the naked eye. To do this, you just need to imitate this light situation, for example, simply by looking at the plants at night under the light of a headlamp.
Begonia pavonina ‘Cyan Form’, filmed at night under the light of a headlamp. Here, the iridescence becomes particularly pronounced. You would never see this so intensely under natural light!
Begonia sizemoreae, filmed at night under the light of a headlamp. The iridescence shows strong angular dependence.
Not all iridescent plants are so dependent on the angle of light and observer. More on this later.
Also, not all blue sheen is iridescent. Some plants simply have very shiny and reflective leaves, which look very blue, especially in low light and under cloudy skies, as they simply reflect the bluish ambient light.
Piper sp. from Costa Rica with very glossy leaves, which mainly reflect the sky and make them appear blue.
In summary, yes, these plants really are blue. And not just slightly bluish, but, depending on the species, really rich turquoise, blue or purple. However, only from a small viewing angle and with the right light.
Unfortunately, trying to capture this effect with a camera is very frustrating, which leads to it often being exaggerated in pictures, and also attempts are made to show the absolute maximum of iridescence, mostly by shooting in the dark and with flash.
3. How does this effect work?
So, now we (finally) get to the core questions of this post:
1. Where does the color come from?
2. Why are these plants blue?
The answers to these two questions are indeed still (as of October 2021) not conclusively clarified, or at least they are still disputed.
But first, let’s get the established facts!
The iridescence does not come from pigments. There is no blue pigment anywhere in any of these leaves. Instead, it is a so-called structural color. These are colors created when light strikes nanostructures small enough to interact with the wavelengths of visible light.
These structures do not necessarily have to have a special shape, they can be grooves, holes, spirals or other. It is only important that they have the right size to disperse a big part of the wavelengths (destructive interference), and to focus only a few, specific wavelengths (constructive interference).
My choice of words here is intentionally rather vague, because the mechanism of iridescence is physically extremely complicated and for me as a non-physicist not possible to fully grasp.
A few examples of blue iridescence in different plants, which is caused by different structures:
Selaginella willdenowii and Selaginella uncinata
Selaginella willdenowii and Selaginella uncinata, spike mosses (Selaginellaceae). Their strong blue color is caused by interference at two thin layers (lamellae) in the epidermis that refract light.
Microscopic images of Selaginella leaves. From Hébant and Lee (1986), modified.
(1) Transverse section through the outer cell wall of the upper epidermis of a blue leaf of S. willdenowii in transmission electron microscope (TEM), the two lamellae are indicated by arrows.
(2) TEM image of S. uncinata; cross-section through the outer cell wall of the upper epidermis of a blue leaf; the two lamellae are indicated by arrows.
Image 1: “IMG_3620 Trichomanes elegans” by Clivid on Flickr.
Attribution-NoDerivs 2.0 Generic (CC BY-ND 2.0)
Image 2: “IMG_3544 Trichomanes elegans” by Clivid on Flickr.
Attribution-NoDerivs 2.0 Generic (CC BY-ND 2.0)
Image 3: “Helecho azul” by Corantioquia on Flickr.
Creative Commons Attribution-NonCommercial-ShareAlike 2.0
Trichomanes elegans, a filmy fern (Hymenophyllaceae). Its blue-green coloration is caused by the remarkably uniform thickness and arrangement of so called grana in specialized chloroplasts that lie against the underside of the epidermis.
Ultrastructure of a chloroplast of Trichomanes elegans in TEM. From Graham, Lee, and Norstog (1993), modified. Cross section of a leaf, with chloroplasts attached to the adaxial wall. Closely stacked thylakoid membranes are visible in the chloroplast.
Image: Danaea nodosa 1a by Scott Zona on Flickr.
Attribution-NonCommercial 2.0 Generic (CC BY-NC 2.0)
Danaea nodosa, a fern from the ancient family Marattiaceae. Its blue sheen comes from twisted cellulose fibers, which lie in dense layers on top of each other.
Adaxial epidermis wall with associated chloroplast, from young leaf of Danaea nodosa in TEM. From Graham, Lee, and Norstog (1993), modified.
(3), (4) Cross-section of a leaf, with chloroplasts attached to the adaxial wall. Tightly stacked thylakoid membranes are visible in the chloroplast.
Begonia pavonina, probably the most famous blue iridescent plant. Their blue sheen is caused by densely arranged stacks of so-called thylakoid membranes. These are densely packed in highly modified chloroplasts, christened “iridoplasts”.
Anatomy and ultrastructure of blue iridescent leaves of Begonia pavonina. From Gould and Lee (1996), modified.
(11) Light micrographs of transverse sections showing iridoplasts in the adaxial epidermis (arrows). Bar = 50 µm.
(12) Electron micrographs of sections through iridoplasts showing thylakoids. Bar = 0.5 µm
Phyllagathis rotundifolia, a Melastomataceae. My personal favorite! One of the biggest of all iridescent plants. A single leaf can grow up to 50 cm long and wide! It also gets its blue coloring from iridoplasts, but it also appears blue from the strong sheen of its leaves, similar to the Piper earlier in this post.
Anatomy and ultrastructure of the blue iridescent leaves of Phyllagathis rotundifolia. From Gould and Lee (1996), modified.
(7) Light micrographs of transverse sections showing iridoplasts in the adaxial epidermis (arrows). Bar = 50 µm.
(8), (9) Electron micrographs of sections through iridoplasts showing thylakoids. Bars = 0.5 µm and. 0.25 µm, respectively.
Our knowledge of these structures is limited to relatively few studies in a small number of species, and it is quite possible that additional, novel mechanisms exist. There are several plants in my collection that have never been studied to determine how they produce their iridescence and show characteristics that suggest mechanisms other than the known ones are at work.
On the other hand, there are also plants that have these structures but do not have visible iridescence. For example, quite a few plants have the helicoidal cellulose fibers like Danaea nodosa, but show no iridescence at all.
It also seems that all begonias form iridoplasts, even those that have no iridescence at all to the eye.
Begonia leaves of iridescent (B. pavonina) and non-iridescent (B. polygonoides) species observed under visible light microscope (left column) and confocal laser scanning microscope (CLSM) (right column).
Arrowheads indicate vividly-colored iridoplasts as blotches in epidermal cells. Dashes mark the edges of individual epidermal cells, which have artificially blue-stained iridoplasts in the cells. Bars in the visible light micrographs = 100 μm. Bars in CLSM images = 10 μm.
From Phrathep (2020), modified.
In the examples, you can see how much effort these plants have to put in to produce these highly ordered structures. But why actually? This question was unresolved for a long time, and is still controversial! The first thought would be that the color itself has a function. That wouldn’t even be that unusual. Structural colors are widespread in nature.
In animals (think of the splendid feathers of male peacocks or the blue wings of Morpho butterflies) they almost always serve to attract special attention, for example to attract mates. Almost every blue animal is blue not by pigment but by structural color.
There are also examples of structural colors in plants that are meant to be eye-catching.
For example, regular buttercups (Ranunculus spp.) use interference on thin layers (the same mechanism as Selaginella) to make their yellow flowers even more yellow and showy.
There are also examples of plants that use blue iridescence to stand out. The fruits of some plant species use iridescence to produce a very striking blue color. The mechanism for this, in most examples studied to date, is helicoidal cellulose fibers, as in Danaea nodosa.
However, the plants that are the subject of this article, do not have blue flowers or fruits, but leaves. When it comes to flowers and fruits, it makes sense for a plant to stand out; after all, it wants to attract pollinators for its flowers on the one hand, and dispersers for its fruits (or seeds) on the other. With leaves, however, it makes no sense to want to stand out. Rather the opposite. No plant wants leaves that are conspicuous to herbivores!
For this reason, it is assumed that the very noticeable blue coloration is an unintended side effect and the actual function of the structures that cause this effect is different. And it must be quite a beneficial function to make the very noticeable coloring worthwhile!
What these blue plants (almost!) all have in common is their habitat: they grow in the very shady understory of tropical rainforests.
Very, very little light arrives there. Tree canopy filters about 99.7% to 99.9% of sunlight before it reaches the jungle floor (Björkman and Ludlow, 1984). It has long been suspected that the nanostructures interacting with light in these understory plants have some role in capturing or focusing this sparse light.
As early as 1984, Hébant and Lee showed that the structures, and thus the blue coloration in Selaginella uncinata is expressed only when it receives weak light, and this light corresponds to the spectrum of light on the jungle floor (For the experts: 12 μmol-m-2·s-1 with R:FR of 0.35).
It was also shown that the formation of these structures is possible in both directions (non-blue specimens turn blue, and blue specimens lose their color), and is reversible in the same plant.
I have also observed this with many of my own plants: With a lot of light, the coloration disappears; with little light, and the right spectrum, it reappears. The change of color takes about two to four weeks. It is also possible to have both iridescent and non-iridescent leaves on the same plant, depending on how much light they receive. Even within one leaf there can be differences!
So it seems clear: These nanostructures help the plant in some way to cope with very little light.
How exactly these structures function and what benefits they have was only described in 2016 by Jacobs et al. on the basis of the iridoplasts of some begonias.
According to the authors, these structures promote the efficiency of photosynthesis! They do this in two ways: First, by improving light trapping at the predominantly green wavelengths available in the shade, and second, by directly increasing quantum yield by 5-10% at dim light.
Sounds absolutely crazy. And it is.
The iridoplasts with their regularly arranged membranes form a so-called photonic crystal. Photonic crystals can greatly increase or decrease the absorption of light by greatly slowing down the light of certain wavelengths (yes, it can be done, speed of light is constant only in vacuum!) and concentrating it into standing waves.
This so called slow light is effectively “captured” in the photonic crystal and passed on in concentrated form to the chloroplasts. The thylakoid membranes in the iridoplasts are precisely spaced to capture green and red light. Exactly what mainly arrives at the bottom of the forest floor.
They also do this trapping from a relatively big angle of 30°, much bigger than the 15° angle at which they reflect the blue light back. This seems to prove that the blue iridescence is, as already suspected, only a side effect.
Increased absorption in the green region of the spectrum serves to intercept residual light on the forest floor more effectively. And yes, plants still use green light quite effectively for photosynthesis! It is a myth that green light should be completely useless for photosynthesis.
Reflecting blue light is probably not a big disadvantage, compared to the stronger absorption of green and red, considering that blue wavelengths are only present in very small amounts on the forest floor anyway.
A very fascinating and also for the first time convincing explanation for the pretty appearance of our plants, but (and there is always a but) this study only examined the structures in begonias. Other plants, with different mechanisms that produce iridescence have not been studied. Maybe these structures have the same function in these plants as the iridoplasts of begonias, but maybe not.
Here are a few examples of plants that likely use an as yet unknown mechanism for iridescence and/or whose iridescence likely has a different function than that described above:
Microsorum thailandicum, a fern, and the plant in my collection with the absolute most intense blue hue, shows no angle dependence. The leaves are blue from every direction, and photographing them with a camera with flash tends to make them less blue in the pictures.
Whether the mechanism described above can be responsible for the blue color is doubtful.
Microsorum siamense is not as intense blue as its close relative above, but shows the same effect. A flash or a certain angle are not necessary for the appearance of the iridescence.
Another difference: in many iridescent plants (especially Selaginella) their shimmer partially or completely disappears when the leaves get wet. This is not so with these ferns, the leaves are still blue even when wet.
Begonia sp. 'Temuyuk'
This begonia, as of yet not scientifically described, shows the same effect as the two Microsorum above. Its iridescence is very difficult to capture because the leaves are very shiny, which covers the dark blue hue.
Just like Microsorum, the color here is not dependent on angle and is not reduced by water on the leaves.
This is interesting in that the above explanation was specific to begonias, but there appears to be at least one begonia that uses a different mechanism to appear blue. Are differently structured iridoplasts responsible here, or a completely different mechanism? And is it blue to capture more light or does its color have another function? We don’t know.
This miniature orchid from the large genus Bulbophyllum also shows a similar effect as Microsorum. The blue sheen can be seen from almost any angle, and is not diminished by water on the leaves.
Masdevallia caesia, a highland orchid, expresses a rather strong iridescence, but only in (relatively) intense light. In low light, it remains green!
Its iridescence is thus exactly opposite to what is found in shade plants, which, after all, lose their blue shimmer in strong light. The explanation that the blue hue is a side effect, and the nanostructures that produce it serve to trap weak residual light, cannot be true here either.
A possible explanation would be that the main function here is actually the reflection of blue light, as protection against too intense radiation.
Absurdly, its iridescence is more pronounced when the leaves are wet.
Stegolepis hitchcockii, a plant closely related to grasses grows, only on the highest mountain in Brazil, Pico da Neblina, in absolutely intense, bright sunlight…and yet has a very strong blue iridescence.
It uses its iridescence perhaps similar to M. caesia, as a sunscreen.
Alright, that’s enough examples of plants that may use other mechanisms for iridescence, or that use their iridescence differently than previously known.
That many plants, especially begonias use their iridoplasts to capture more light as shade plants is of course undisputed. Also, the other structures shown in various species that make these shade plants appear blue most likely have a similar or even the same function as those shown by Jacobs et al. in some begonias.
However, as we have just seen, there are apparently several other mechanisms in plants to produce iridescence. These apparently function differently at the nanoscale than the known examples. At least for Microsorum thailandicum it is known that the very intense blue color is caused by a known structure, namely helicoidal cellulose (Steiner et al. 2018).
What we have also seen is that iridescence most likely does not have the same function in all plants. In some examples, the blue reflective coloring actually appears to be the function itself, and not just an unintended side effect. Here, the blue sheen may serve as a sunscreen in habitats with intense light.
Another possible function of iridescence is to confuse herbivores. It has been shown that bumblebees have problems detecting highly iridescent objects. Iridescence could therefore have a protective effect. To the human eye, of course, the blue coloration looks very striking (that’s why I collect these plants!), but to the often completely differently structured visual systems of other animals, these plants could look very different than they do to us!
It should also be considered that it is quite possible for iridescence to perform several functions simultaneously. Even seemingly contradictory functions such as capturing dim green light in deep shade and reflecting blue light in glaring sun could be beneficial in the same plant.
At least this is what Glover and Whitney (2010) suggest. They argue that plants adapted to low light conditions could be damaged in a very short time by sunspots shining through the canopy. This assumption was made before it was known that at least some begonias have a function of capturing extra light, but it still makes perfect sense. These inconspicuous specks are 100 to 10,000 times brighter than what these shade plants are normally exposed and adapted to. So it could well be that the iridescence in the same plant both protects against too much sun and helps with little sun (or the nanostructures help, after all, with little light the blue shimmer is only a side effect).
Steiner et al. speculate that the blue sheen of Microsorum thailandicum serves this dual function, and Phrathep (2020) suggests that this may also be true for begonias.
As so often in biology, it will most likely come down to the fact that there is no single explanation, but many reasons and many mechanisms are behind the phenomenon of iridescent leaves. There are still more than enough unanswered questions and certainly many exciting details to keep researchers busy for many years to come.
If this blog has now also aroused your interest, or even enthusiasm, for this really great and diverse group of plants: Feel free to take a look at our store.
We regularly have different iridescent plants in stock, and also constantly expand our selection. With over 30 species (unfortunately not all of them in the store yet), we already have probably the biggest selection of shimmering botanical beauties in all of Europe!
And last but not least, a small gallery with pictures of other iridescent plants that I didn’t get to put in the text :)
(1) – (4) Begonia metallicolor
(5) Begonia dinhdui
(6) Begonia ‘Silver Lace’
(7) Begonia sizemoreae
(8) Vine from Costa Rica with iridescence
(9) Begonia ‘Black Knight’
All images in this post, unless otherwise noted, and post text: © Alexander Ruppert