Discussion Topic: Why Are Grass and Plants Green?

[andy10917]

So, we all know that grass and most plants are green. But why?? It's really interesting if you study it, and I have as part of my planted aquarium hobby (the lawn and gardens don't require much winter attention!).

Anyone with ideas?

[sc4dr]

That's one of those things that has crossed my mind, but never really followed the thought to any logical conclusion. I would like to know why.

[Green]

Green is a special color, no doubt. If it wasn't, I wouldn't have been inspired to use it as my username on this forum. I've been captivated by the color ever since I can remember. I'm into lawn care for the simple reason that grass is green and I like looking at nice green grass.

The fact that chlorophyll is green tells us it most readily absorbs non-green light. In fact, if you look it up (I just did) you can see graphs of absorbtion spectra for plants, and you see there are peaks at both red and violet wavelengths.

In contrast, the human eye's rod and even to some extent cone photoreceptors are tuned to absorb mostly green wavelengths. The protein responsible, rhodopsin, is a violet-red color.

Traditionally, color photographic film has tended to be most sensitive to green light as well. Likewise, most digital camera sensors have twice as many green photosites as compared to red or blue.

Everwhere you look, green is the color of life.

[andy10917]

Everwhere you look, green is key to life.

OK. But why Green?

[Green]

Everwhere you look, green is key to life.

OK. But why Green?

Might as well have said, "OK. But why, Green?"

Why green?

My own hunch is that the short and long of it has to do with utilizing the ends of the spectrum, the long waves, and the short waves, of near IR and near UV, respectively. Perhaps this gives the plant its energy more readily than using green light. Energy from wavelengths that might burn our skin or heat us up can be captured by one single pigment. The plant gets to absorb heat and energy at the same time. Nature likes simplicity.

But not all plants are green, and even those that are, are not always green in all cases...

Why does iron make plants more green?

[andy10917]

Nah. Occam's Razor would argue that the opposite would be the most likely case - the middle of the usable spectrum would be the most likely candidate for use by life forms - not the edges of the spectrum.

And therein lies a hint...

[GaryCinChicago]

Photosynthesis and chlorophyll.

As to why it actually makes green? Who really gives a shitt why?

[Barley]

Are you thinking of the theory that all current terrestrial vegetation evolved from aquatic plants? The latter survived because the green wavelength does not penetrate water well, so they needed to make due with other wavelengths?

I can get on board with that theory, with a few modifications.

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[andy10917]

Who really gives a shitt why?

Nobody is forcing you to participate in this discussion. Feel free to take a pass.

Are you thinking of the theory that all current terrestrial vegetation evolved from aquatic plants?

Yes.

The latter survived because the green wavelength does not penetrate water well, so they needed to make due with other wavelengths?

Green light penetrates WAY better than red light does. So why would plants want red light and pass on green light?

Hint: stick with the aquatic plant angle, but broaden your search...

[MorpheusPA]

Which set of theories do you want?

One, plants do use yellow-green light to an efficiency of about fifty percent. That's not shabby, they merely use less of it than they do blue or red. They also use light spikes in the ultraviolet, so to a point their exact color is an artifact of our vision. Photographing a plant in the infrared range shows that it glows very brightly there--plants don't use it, probably because it's pretty low-energy stuff, even though the Sun throws it around at much higher rates than blue or violet light.

But I get what you mean. If our vision encompassed radio waves to gamma rays, plants would still be green/infrared as that's their highest output of reflected sunlight.

I tend to go with the simple "evolution goes with what works, not with what's optimal, and what's optimal isn't always obvious." Although solar output peaks in the yellow-green range, evolution never tuned algae's chlorophyll to use it at extreme efficiency. Evolution never hit on the correct trick to do so, or there's another reason why not. I tend to go with the latter in this case--there's another reason, as evolution DID hit on the trick.

Blue makes sense, it's high-energy photons compared to red light. Red doesn't make a great deal of sense because of the lower energy. Reds scatter in water, blues penetrate well--so again, blue makes sense. Red still doesn't.

Plants do use blue/red ratios to determine growth or blooming, which makes sense given the differences as the sun's declination changes through the seasons. It doesn't at all limit a plant's use of green light if evolution had rendered green light growth/blooming neutral.

Thermoregulation arguments come into play. It's easier to modify your temperature by throwing away the largest amount of total energy. Green light contains far more total energy (and hence heat) than the less-radiated blue light does. Land plants and shallow water plants would find this a useful feature (so evolution might not be as dumb as plain energy comparisons would imply). For this reason, there are on-average color differences between tropical and tundra plants.

Cyanobacteria evolved first and tend to use the green/blue range of the spectrum. Your average later-evolving algae, trying to depend on green light, is going to find it slim pickings after the cyanobacteria absorb it. Red light is still ample, blue is still worth it due to the high energy of the photons, but green isn't worth bothering with. It's already absorbed, undergoing some scattering by the water, and generating the pigments beyond the basics costs the algae more than it's worth in terms of energy production.

Algae, not cyanobacteria, are the parents of all other plants. Hence, they're green--and evolution didn't much bother with generating other pigments when red/blue worked well enough with a fifty percent absorption in yellow/green.

Land plants also have an issue with resource gathering. There's only so much water, nitrogen, and so on to go around, so extreme energy gathering is self-defeating. You can't do anything with it (and raised your own temperature and by extension metabolic rate to gather it), so it's useless. The shortest resource on land is phosphorus--the energy-transfer molecule. So there's doubly no reason to bother over-gathering energy. Nitrogen isn't in abundant supply in most soils, so forget massive growth.

And absolutely last, there ARE black plants (http://lifeonthebalcony.com/awesome-bla ... ontainers/ for black plants you can grow easily at home). Black, as they note, being a misnomer in this case, but they're pretty close. There are also red-leafed, blue-leafed, and every other color you can think of.

[andy10917]

Bingo.

First of all, a little background on why I got into this. I started studying it because I am into the DIY use of LED's in the planted aquarium. I had a 400-watt SHO fluorescent system that runs 12 hours a day, and produces marginal results on high-light requiring plants at a depth of 24". I had a choice - up the wattage and costs of solving the light needs of the plants, or try to locate higher-efficiency LED's that output light at the specific ranges that plants want, and lower my costs by not generating ("waste") light at colors that plants don't want.

I found that plants have peak efficiencies of light use at 462nm ("Royal Blue") and 660nm ("Deep Red"). Which got me to thinking - why not in the middle of the spectrum, where the "simplest is best" concepts play out in nature?

Why didn't algae and early plants go to the obvious and instead developed two types of Chlorophyll (more complex)?

It turns out that it was to avoid competition, mostly likely. The early oceans were way saltier than today, and Oxygen wasn't a major element in the atmosphere. The earliest photosynthesizers were bacteria that used a purple pigment called Bacteriorhodopsin. Purple indicates that the red and blue wavelengths were available but the middle of the spectrum already had a major user. Hence, Cholorophyll(s) developed to use what was still available without big-time competition. When the oceans and atmosphere contained more Oxygen and less salt, the already-developed chlorophyll(s) and betacarotenes didn't need to adapt - the existing pigments worked fine. There has been some movement in the betacarotene pigment toward the middle wavelengths in the past billion years, though.



Believe it or not, the photosynthetic process in plants of converting CO2 to Hexose is not that efficient. Theoretically, it can be 30% efficient. In the lab, 25% has been achieved. In real-world plants, the best is the Winter Evening Primrose (in Death Valley) at 8%. Sugar Cane is 7%, and most crops achieve 1%-4% efficiency.

By the way - the original project for the aquarium now achieves double the useful ("PAR") light for 171 watts of electricity than I was getting from 400 watts of specialty SHO fluorescent lighting. The same lessons are directly applicable to the lighting used for germinating/growing annuals and perennials...

[MorpheusPA]

Believe it. I use a mix of warm and cool fluorescent on my sprouting decks, 160 watts per shelf, which is sufficient to grow sprouts and even set buds (but not many).

LED would certainly be more efficient if I could get the wavelengths right. With most off the shelf LED solutions, you'd be better off using a fluorescent. Specialty LED solutions work very well, but they tend to be expensive. LED's major issue is (as you know) the single wavelength output (well, a very tight band with a massive spike at one wavelength, anyway). That's nice if the plant happens to use that particular wavelength. Given the mix I grow, that isn't assured, and there are differences even between cultivars of the same species.

Efficiency in plants is low--but there's no reason to go much higher. You need the resources to support that, and they simply aren't there. Chlorophyll's temperature limitations mean there's no reason to be black in most cases. You'd get too hot. It would be an advantage for winter-growing plants, and those tend to be darker anyway.

Even our lawns and evergreen shrubs tend to deepen in color during fall and winter, and lighten in summer to throw away more heat.

If you've ever played with the Daisyworld simulations, you can see that in action. Arctic daisies are black. Equatorial are white. There's a transitional zone between the two in the temperate latitudes, depending on what you set solar output and greenhouse effect to be. In moderating their own colors, they also serve the purpose of moderating the Daisyworld simulation's planetary temperature--to exactly the optimal for daisies, if they can.

[John_in_SC]

My own dumb answer is... It worked more than it didn't...

I think efficiency wise - It's just not important... If trees or grass needed to produce more energy - they would change colors slightly.. and you do see plenty of purple and yellow leaf plants that do fine..... but as you travel south through the USA - you see basically all green vegitation going lighter and lighter green... (I am from South Florida... One of my surprises was just how deep green plants up north are.... They need to be... but plants far south tend to run apple green/yellowish - telling me that they are trying to absorb far less sunlight...)

I have also fooled with plants under lights plenty... One of my conclusions was that if you didn't have the "Perfect" lamps available - any old Fluorescent would do.... You just needed to leave the timer running longer... like maybe an extra hour or 2 every day.... and you can use $8.00 lamps instead of $50.00 lamps....

Now.. It might be a different discussion if I needed to give the fish/critters some dark... but that wasn't a concern...

Thanks