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Comment by montjoy | Hacker News Reader

> The researchers found that the effect varied with color and peaked at a particular wavelength of green light.

I wonder if this affects photosynthesis in any way? IIRC plants are green because chlorophyll does not absorb that color. Maybe that’s partially because green light induces too much evaporation?

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No. Plants reflect green to balance luminosity extremes.

Green is by far the most luminous color in the visible spectrum with dark purple being the least luminous. Plants that reflect purple absorb the most visible light and can produce the most sugars. Most plants must undergo a chemical transition to shift from production to dark mode and by choosing to reflect green maximum energy production is sacrificed for internal chemical stability.

Whats the mechanism here?

I found this which seemed informative: http://www.esalq.usp.br/lepse/imgs/conteudo_thumb/Why-did-ch...

C02 levels. We use C02 to turn our weed buds purple.

> most luminous color

What does luminous mean exactly in this context?

I know that our eyes are most sensitive to green. For example a green LED needs less power to seem as bright as a red LED.

But our eyes don't matter in this subject. So I am scratching my head about what you mean that green is the most luminous color?

Here is the math used by W3C for computing acceptable contrast: https://www.w3.org/TR/WCAG20/#relativeluminancedef

I suspect luminosity to mean percentage of brightness for a given frequency from a total light source assuming white light. For a more precise definition I would look to physics.

That’s for sRGB, which as for most colour spaces is defined relative to CIE XYZ, which is itself defined in terms of the perception of a “standard human observer”. In XYZ, the Y coordinate is the luminance (brightness) of the colour and that first equation is computing Y from the RGB value

I think this is the color (wavelength) that corresponds to the sun's emissivity peak (irradiance in W/m²).

The fact that our eyes are most sensitive to green is probably not an accident, but is it due to the color of leaves, or is it due to sunlight?

Maybe, but evaporation is critical for plants. They don't have a heart to pump blood and supply nutrients, they have to rely on evaporation to move nutrients from the roots up

I imagine the leaves being green would encourage this evaporation. The upper layers of cells are probably effectively translucent and might be awash in green light, perhaps increasing the rate of evaporation and improving overall transpiration.

That’s my guess as well. The green chlorophyll is inside of the cell. It absorbs sunlight for food but reflects the green light back through the water-air boundary. Perhaps the effect on evaporation or water movement provides a better benefit for the plant than absorbing the energy of that spectra. For instance, by increasing the efficiency of respiration.

At its simplest, reflecting green light would send the light through the water twice instead of once. Neat trick, plants.

Well, the chloroplast was evolved when all life forms were submerged in the sea

The sea is on the other side of a very carefully policed barrier.

Still, it's less of a challenge to pump water from your roots if you're in the sea. Did the earliest green plants even have roots?

> Did the earliest green plants even have roots?

No. Not even all land plants have roots.

No doubt. I’m just toying with the idea of how this might impact our understanding of how plants work.

That’s a good point actually. I never thought about the fact that a tree is evaporating 100L a day without any heat. They must have figured this out.

It still uses heat. Evaporation actually helps plants keep cool by using up the heat from sun that would otherwise heat the plant too much.

Plants rely on on capillary action to bring water up: https://www.usgs.gov/special-topics/water-science-school/sci....

There's probably some aspect of evaporation, but the density of water is much, much higher when it's kept in liquid form.

That's part of the process, but not all of it. The overall process of water movement through vascular tissue is called transpiration [0]. Yes, capillary action plays a part, but the underlying process is due to differences in water potential. To avoid saturation (which would halt the flow of nutrients), water is released through pores in the leaves called stomata [1], where it evaporates, creating a differential that results in additional water being pulled in from the soil.

[0] transpiration: https://en.wikipedia.org/wiki/Transpiration

[1] stoma: https://en.wikipedia.org/wiki/Stoma

Disclaimer: I'm not a biologist. I'm a third-generation home gardener; my grandfather was the youngest of his family and the to leave the family farm in Kansas to go to college. He always maintained a large garden and loved sharing his knowledge along with his vegetables.

Edit: added links and a disclaimer

Watch Veritasium’s awesome video on this

https://www.youtube.com/watch?v=BickMFHAZR0

It's an old video, more than ten years ago, but it aged well IMHO. I re-watched it and re-learned things I have forgotten.

or the opposite, it likes the green light because reflecting it keeps the plant cool? I also recall a study where trees emitted a sort of mist into the air, perhaps the greenery helps with that process too

I’ve always wondered why inefficient green is the way plants evolved. Interesting question.

[green] plants are damaged by green light, is the way i've heard it explained. So they "block it".

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