Grass

The sun gives most of the energy in the green spectrum.

Too much energy from sunlight causes DNA damage (i.e. often resulting in cancer).

Plants don't want to spend a ton of energy repairing DNA all of the time. It's more energy efficient to reflect a lot of the light.

Atleast, that's what I read here, and the explanation seems reasonable:

scienceline.ucsb.edu/getkey.php?key=500

>it reflects all other wavelenghts

Wait... that part is wrong. It reflects green light and absorbs all of the other wavelengths.

Here is a chart of what it absorbs:

i.imgur.com/Xol2c5Y.jpg

We are only referring to a specific type of chlorophyll (chlorophyll-a) when we are talking about the green color.

Meh, couldn't solve it.

The chloroplast coordinates magnesium-ion with the organic part. Magnesium is the one that gets excited by the photon. Electron jumping one level corresponds to red, and jumping two levels corresponds to blue. Green is between the levels, so won't happen.

But this doesn't really answer the question at all. Zero answer actually. Useless answer. Not a single new thing understood about the universe.

Why magnesium? Why not other ion? There's plenty of ions you could introduce to your pigment that could absorb green light.

>But this doesn't really answer the question at all. Zero answer actually. Useless answer. Not a single new thing understood about the universe.

Well, science is fun that way.

> "Sun emits most of its energy in green so it protects the plant! plants would overheat so they reflect it away!!"

Even though you didn't like that answer, I think it is a decent one. That is sort of how human pigmentation works too (i.e. skin color evolved for optimal sun absorption).

You probably had notes on how the chemical mechanism is optimized as well. If you find them, do share! I took a class a very long time ago and do not remember the exact mechanism myself.

Sure, plants have problems with heat because they dry, but their enzymes won't get denatured until it is very hot. Most of the heat come from IR wavelengths, and green wavelengths represent just about 10-20% of Sun's energy. Even if heat would be a problem, green absorbing plants would have adapted in temperate zones.

If overheating would be the *only* reason, there would be black plants in oceans, but there aren't. There are green, red, yellow, blue, purple and brown organisms doing photosynthesis, but black ones are very rare.

Organisms have a way to use similar molecules for different kinds of purposes. Porphyrins are a group of cyclic molecules with lots of free pi electrons (about 20). They'll coordinate a metal ion in their center and do some electronic shit like absorbing photons, getting excited, and exciting the ion. Fe -ligand is used heme, Co-ligand in B12 -vitamin, Ni -ligand as a cofactor in doing methane, and Mg in chlorophylls. None of the porphyrin related molecules absorb best at 500 nm though (green light).

There's also carotenoids and few other accessory molecules involved in photosynthesis that I don't know about.

Sorry, I just have notes on simple biology.

One interesting hypothesis that's been kicking around for a while:

Plants reflect green light because way back in evolutionary history, other bacteria were already filling the "harvests green wavelengths" niche.

This is a good hypothesis, but we need to find a bacteria species that can absorb green light. At least in your picture nobody absorbs green light effectively.

talking out of your ass aren't you?

livescience.com/1398-early-earth-purple-study-suggests.html

>If overheating would be the *only* reason

Well, I wasn't referring to literally overheating and drying out. I thought you were referring to the DNA damage that could be caused by absorbing too much energy,

> Chlorophyll-a and other pigments are easily destroyed by too much energy, and when the pigments break down and stop absorbing light entering the plant, that energy can cause damage to other plant tissues as well, including the plants DNA. Think of it as a sort of plant sunburn. Plants have elaborate mechanisms to repair DNA that has been damaged by too much sun energy, but these repair mechanisms are costly, and require extra nutrients. A plant that is stressed by too little nutrients or too little water can actually die from excess sun exposure. Plants have adapted to balance their need for the suns energy with their need to protect themselves from sun damage by using regions of the spectrum that are not as abundant.

Cool. Thanks for sharing some of the notes. I guess that would make sense that the porphyrin molecules don't absorb green light well since the green light is reflected?

Grass isn't just green. There's purple grass and yellow grass too.

If you leave the roots of some grass in water with food dye, then the grass will change to the food dye color. The chemical makeup of chlorophyll gives off a green hue.

that's it, I'm watering my plants with red food dye from now on.

True, retinal -like molecules can absorb green. Problem is magnitude. While chlorophylls use systems with over 20 cyclically conjugated electrons, retinal is just carbonyl -play at the end of a conjugated chain. That's why animals mostly use retinal in signaling where you need low-energy low-threshold excitement/relaxing.

But it could be, I have no idea about the relative magnitudes of their absorption spectrums. Bacteria nowadays do use chlorophylls though.

True, too much heat will be bad. But think about cactus. Those motherfuckers just chilling no problem in 50 degrees Celsius. They've adapted to resist drying, but cactus 'shell' doesn't make a good insulator. Neither does cactus have any heat dissipating mechanisms like increased surface area (which would be bad for evaporation).

Yes, haven't heard of porphyrin molecules that would absorb green light well. But that doesn't really answer the question, just like the magnesium ion didn't answer the question either. Why didn't plants develop other cyclic conjugated systems that coordinate ions and end up absorbing that green light which is what Sun gives us the most.

>True, too much heat will be bad.

You keep on saying "heat" and referring to things related to temperature, which is not what I am referring to. Temperature does not cause DNA damage.

DNA damage that the plant is trying to avoid by reflecting green light,

pnas.org/content/102/40/14127/F1.large.jpg

If OP is only asking for the physical reason, that is answer enough.

>Electron jumping one level corresponds to red
So it absorbs red light. Which means that red light will be missing from the spectrum. Which in turn will make it look green. There you have it.

Grass is green because the HOMO-LUMO gap in the coordinated Mg has the same energy as red light.

Because the chlorophyll in most plants used go be purple in the jurassic era and after a more effecting chlorophyll was found (green) they were slowly outclassed by green plants.
Sources: (trust me bro)

>it reflects all other wavelenghts
that's not how it works, pleb