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Was recently chatting with a colleague on Twitter. He was wondering about the color of plants under suns with different colors (e.g., red vs. blue vs. Earth’s yellowish-white sun). It’s an interesting and complex question you’ll have to consider if you might be interested in trying to plausibly describe what plants would look like on other planets. Here’s my attempt to describe some of the key factors that affect the answer.

In summary, three main factors will affect what you’d see:
  • the peak wavelengths of light absorbed by the plants

  • the peak wavelengths of light emitted by the sun, and their intensities

  • the subjective effects of background colors

  • First, what wavelengths of light will the plants absorb? (Everything they don’t absorb will be mostly reflected -- which creates the colors that we see -- and partially transmitted.) For most terrestrial vegetation, chlorophyll is the most common and most important light-absorbing photosynthetic pigment. Its peak absorption occurs in the blue wavelengths (between 400 and 450 nm) and the orange-red wavelengths (between 620 and 680 nm), which is why green light is mostly reflected -- it falls between these absorption peaks -- and this is why most leaves look green.

    But plants have a serious problem with light absorption: if they don’t absorb enough light energy, they can’t manufacture enough sugars and they starve, but if they absorb too much light energy, it damages them*. Fortunately, plants have evolved ways to dissipate the excess energy by emitting heat and by various other complicated means I won’t go into here (e.g., non-photochemical quenching). So plants must evolve photosynthetic pigments that strike a difficult balancing act between capturing enough energy for the plant to thrive but not so much that the excess energy kills the plant. On Earth, that means developing complex mechanisms to capture just enough blue and red light and reflect most of the unnecessary green. Evolution will inevitably pick pigments that achieve this balance, but for alien plants, that balance won’t necessarily be achieved by anything that resembles chlorophyll. Or it might. Convergent evolution means that organisms will “choose” similar solutions to a given problem.

    * I can imagine a situation in which you disabled the plant’s energy-dissipation mechanisms, causing the plants to spontaneously self-combust. Hmmm... interesting possible mechanism for seed dispersal! At the end of the growing season, when the seeds are ripe, the genes that encode energy-dissipating structures stop working and the plant harvests so much energy that it literally blows up, scattering its seeds to the wind. Too bad if you’re hiking through the woods at that time of year and haven’t been on the planet long enough to discover this phenomenon. Houston, we have a problem!

    Second, what wavelengths are emitted by the sun and at what intensity? Red suns will emit primarily red wavelengths, which is why they look red; similarly blue suns will emit primarily blue wavelengths, and look blue. And so on for other colors. (That’s vastly oversimplifying, of course, since suns tend to fall into certain categories of color output that are determined by their age and the fusion conditions inside the star. But the description is accurate enough for the purposes of this blog entry.) If the sun emits too much energy at its peak wavelength, then by definition (of “too much”) plants will need to find ways to reflect or transmit most of that light and capture only the remaining parts of the spectrum (i) that provide enough energy for survival and (ii) that they can safely absorb without spontaneously self-combusting.

    So, for example, an alien sun that emits roughly the same amounts of blue and red light as our sun is likely to lead to the evolution of plants similar to ours that absorb the blue and red light and reflect the green. But a high-intensity blue sun might produce plants that reflect most of that blue light to avoid overheating and that instead emphasize absorption of red light; their leaves would therefore appear blue. The opposite would occur with a high-intensity red sun: too much red, so the leaves would need to reflect the red wavelengths and would thereby appear red. The intensity of light received in a given wavelength band will also depend on how far the planet is from its sun and atmospheric chemistry: different chemistry will lead to different absorption profiles.

    Third and last, color is only an objective parameter if you’re a spectrometer. (That is, light with a wavelength of 450 nm will always be blue to a spectrogrometer.) The problem for us humans is that our perception of light is highly subjective: light receptors in our retina that specialize in wavelengths around 450 nm (nominally, blue light) will always detect that wavelength, but we might not perceive it as “blue”. The problem is that we account for background colors and the color of the light source when we judge a color. If you’ve ever been shopping for house paint, you’ve encountered this problem: a swatch that looks great in the showroom looks horrible in your living room, with different light.

    Let’s bring this back to plants. Have you ever noticed how the same leaf seems to change color between high noon and sunset on a sunny day, and how both these colors differ from those that you perceive on a heavily overcast or rainy day. (The reductio ad absurdum is, to borrow a phrase and mix a metaphor, “all cats are black after midnight". When there’s no light or only dim light, you can’t see the color at all.) We still see roughly the same part of the color spectrum (i.e., some shade of green), but the details of how we would describe that color change remarkably. The problem is sufficiently important that people who do prepress work for print publications use scrupulously color-adjusted monitors and background lighting to ensure that what they see on the screen is what they’ll get from the printing press.

    So what’s the poor science fiction author to do when it comes time to figure out plant colors on their alien world? Come up with plausible descriptions of the three abovementioned factors and use them to infer a defensible plant color.

    For example, let’s assume a primarily red sun that’s far enough from the planet in question that sunlight arriving at the planet is weak, and plants have to focus on capturing the dominant wavelength to ensure that they get enough energy. (How far is that? Personally, I don’t care. If you need to know, buy an astrophysicist a beer and ask them to do the calculations for you. If you're a writer rather than a scientist, the only answer that's important is this one: "Far enough to create the color I described. You're the scientist; you tell me!") In any event, this means the leaves will primarily absorb the red wavelengths; there might not be enough blue and green light to justify the cost of producing blue- and green-absorbing pigments, so it would be reasonable to predict that the plants would appear a pale blueish-green because they’d reflect those colors. On the other hand, if the light energy reaching the plant is low enough, the plants might also have to harvest all the blue and green light too if they want to capture enough energy to survive; as a result, their leaves would tend towards black (because very little light is reflected). Contrast this with plants that evolved close to a blazing red sun: in this case, there’s probably too much red light, and they’ll need to reflect most of that light to avoid spontaneous combustion from absorbing too much energy. As a result, the leaves will appear red.

    If you’re feeling particularly bold (or perhaps just masochistic), throw in complications such as light absorption by the atmosphere or the ability of plants to shift their dominant pigments over the course of the day to optimize light capture as the sunlight’s spectral characteristics shift*, thereby ending up with leaf colors that change over the course of the day. I haven’t even thought about the third factor (subjective color perception)... that way lies madness. But I imagine you could manipulate a vegetation image in Photoshop by color-shifting all the leaves and the background color (perhaps using a subtractive filter) to create a credible subjective image of your alien vegetation that would guide your description of the fictional vegetation.

    * Because more of certain light wavelengths will be absorbed by the atmosphere as the sun approaches the horizon.

    Diehard plant physiologists will undoubtedly still tell you that you got it all wrong, but if your assumptions are plausible, you can look them in the eye and confidently state that you chose plausible conditions for your working assumptions and plausibly extrapolated the perceived leaf color based on those assumptions.


    Some additional thoughts on this subject from people who have actual PhDs (with thanks to John Freeman, @MadScientistJo):

    Colors of Alien Plants by Vikki Meadows

    Alien plants may come in all colours but blue by Heidi Ledford
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