Black Plants

A few years ago, I was waiting for a bus and staring at the grass, when I remembered being told in a biology class that the chlorophyll and accessory pigments of plants were able to absorb nearly all of the visible light spectrum that actually reaches the Earth. I immediately realized that this contention was “full of stool”, in that the plants obviously reflected enough green light to appear green.

I worked at a textbook company at the time (which employed a disproportionate number of biologists), and the best we could come up with is that it’s because plants evolved in water, and therefore developed pigments to absorb the light that made it to them through the water. Combined with light not being the limiting factor, it seemed to make sense… but I’m still open to a better answer.

Are there black plants (by which I mean the light-gathering portions, not, for example, bee trails on flowers)? Could we someday engineer black plants that absorbed more light than their green cousins, and therefore produced more sugar?

Complex question.

What you were told in biology class is quite true. I suspect you just misinterpreted it, or perhaps the person relaying the information misinterpreted what she read. The chlorophyll and accessory pigments of plants are able to absorb nearly all of the visible light spectrum that actually reaches the Earth. The misunderstanding is that you assume this means that any individual plant can do this. Of course this isn’t true. But plant A can absorb all the wavelengths from violet to green, while plant B can absorb all wavelengths from green to red. Between them they have pigments necessary to absorb all visible light.

Plants are green simply because most of the visible light reaching the Earth is red. It’s that simple. Plant cells act like little radio receivers, except of course what they receive is visible light. They contain chlorophyll that absorbs red light, and antenna pigments that serve to absorb other wavelengths and shunt them onto chlorophyll. The cell is of course a finite size, so any given plant has to ‘decide’ what type of antenna it installs. It can’t just pack in antennas for every possible wavelength. Or rather, it could, but that would mean having antennas that pick up no signal at all most of the tie, while the most effective antennas are overloaded and burning out and signals are passing straight through without being intercepted. That’s a waste, of both the materials used to build the antennas that don’t work and the signal that is being lost.

So of course plants pack their cells with whatever pigment receives on the most common wavelength. For canopy plants this is just chlorophyll, and the leaves are green, although other pigments are usually present as well for low light days and because they are ‘cheaper’. Plants at lower levels in the forest replace more and more chlorophyll with antenna pigments for the green wavelengths, since most of the red light has already bee absorbed by the upper canopy. By the time you get to the forest floor there are large numbers of plants with red leaves, packed with pigments designed to absorb the green light filtered down. These plants can get pretty damn dark red, but they are never black. The reason is that there is no point in wasting space in the cell with large amounts of red absorbing pigments because there is no red light at this level. Without both red and green absorbing pigments the pant can’t appear black.

So no, we can never engineer black plants that absorbed more light than their green cousins, and therefore produced more sugar. We could alter the pigment mix to suit the conditions available, and so produce red wheat for example. But to make a black plant we would need to engineer a plant with a lot of red absorbing pigments and a lot of green absorbing pigments. Wherever the plant grows one of these sets is going to be largely wasted. The plant will necessarily produce less sugar because a lot of its pigments will be sitting idle, while all of the wild form’s pigments will be absorbing at peak efficiency.

Great answer… but I still have the problem with it as with the original description. If the green-absorbing pigments would be a waste, why does grass reflect so much green light (and, therefore, pass up so much energy)? Do they need as many carotenoids as chlorophyll molecules? No. But it seems like a grass with a perfect proportion of carotenoids could absorb most, if not all, of that green light.

I might buy that there simply isn’t enough space in a leaf for all of the accessories… but I seem to remember seeing a lot of “clear” space when I looked at chloroplasts under a microscope (or saw images thereof in texts).

So, if there’s space, and there’s obviously energy available that they aren’t absorbing (if they were absorbing all of the available energy, they’d be black), it must be, as one biology PhD guessed, that they simply don’t need to absorb all of that extra energy–they can’t, for example, absorb enough CO2 to use all of that absorbed energy (in fact, if I’m not mistaken plants with the chlorophyll-heavy setup often have to get rid of some of the extra energy they’re absorbing already). However, say we also engineered the plants to absorb extra CO2 (something I’m not saying is necessarily possible, but lets just assume it is), making light-gathering the limiting factor. It still seems to me that the eventual end result would be black super-wheat…

I promise not to argue with every reply… I just think Blake’s reply still skirted the issue, because, darnit, even if there’s more red light around, those green plants obviously aren’t absorbing all of the available light.

Long assed reply coming up. Be warned.

Precisely because they are a waste.

Look at it like this. Every cell (ignore chloroplasts, they complicate things) can hold a maximum of 1000 pigment molecules. In full sunlight at the equator a chlorophyll molecule can absorb 100 units of energy/minute. A carotenoid molecule can absorb 50. A cell with only chlorophyll molecules can absorb 100, 000 energy units/min. A cell with only carotenoids can absorb 50, 000. A cell with 50% of each will absorb 75, 000 units.

The plant isn’t passing up energy by reflecting green light. It’s absorbing the maximum amount of energy possible given the material available. It can’t possibly absorb all energy in all wavelengths due to space considerations, so it maximises what it can absorb.

I’d have to see the slide to know what the ‘spaces’ were. Most likely they were lipid or starch granules, not empty space. Whatever the were, can we just accept that they are necessary for the cell to function? I don’t think anyone is suggesting that plants produce superfluous empty spaces when they could pack the same area with energy producing structures.

Even if you don’t accept that, that there is an upper limit on plant cell sizes. No matter if we did replace those empty paces with pigment molecules it would still make more sense to replace them with chlorophyll than carotenoids. Why replace the spaces with carotenoids that only produce half the energy?

As I pointed out, it would never be to a plant’s advantage to be black. Yes there is always energy available that plants aren’t absorbing. At best a plant will absorb 3% of the light that falls on it. Most plants only absorb 1% or less. This is an inherent problem with the system being used and it can’t be overcome easily. What plants do is utilise as much energy as possible, and that means utilising pigments that absorb the maximum amount of energy. Producing pigments that absorb green light when 80% of the red light is being lost is not efficient. That is what would be required to produce a black plant, and it obviously wouldn’t work.

Need is a pretty meaningless term in this case. Obviously the don’t need to because they haven’t become extinct. Would it give them a huge evolutionary advantage to be able to capture even another 3%? Undeniably. We’ll get into the relative advantage of C4 vs c# plants in a minute, but it proves that even a small increase is advantageous.

That is related to one of the problems, but it doesn’t in any way favour black plants. If a plant could absorb 3% more carbon dioxide, it would still make more sense to pack in pigments designed to maximise the energy absorbed/unit area. That way the rest of the cell could be devoted to other functions, rather than having to bulk up the cell simply to maintain the same energy collection for les efficient pigments.
Absorbing CO2 isn’t the real problem. Problems are encountered when too much CO2 is absorbed and the photosynthetic enzymes start running in reverse. C4 plants have overcome this by pumping CO2 away from the site of photosynthesis. This only works at high light levels, and it allows these plants to harvest more light. But C3 plants are just as green as C3 plants. They don’t pack in red absorbing pigments.

Not really. This is really only a problem when plants are moved from the light into the dark. Then there is a problem with not having sufficient water or quenchers available. In normal circumstances the energy can be lost via luminescence with no problems. The extra energy may not be captured, but that is due to a whole range of limitations.

Well, theoretically but that’s very different to what you asked in the OP. Your OP asked whether a black plant would be more efficient. No it wouldn’t. If we could engineer a plant to utilise all available light, then we would presumably need to engineer it with a C4 pathway so it didn’t suffer from CO2 overload. Then we could engineer it to not lose excess water through the massive stomata that it needs to absorb sufficient CO2 to run these reactions. Then we would need to grow it in hydroponic solution so it could get enough water to run this photosynthetic reaction. Then we would need to re-engineer the entire plant cell, so that it is so totally filled with chlorophyll and associated green pigments that it can capture all the visible light. Then…

If we could do all these things so that the plant became 100% more efficient than it is now, and was able to absorb all the visible wavelengths then yes, it would be advantageous to have a black plant. But that’s just rephrasing the question as the answer: If a plant were able to absorb and utilise all wavelengths, would it be black. Well yes, of course.

The trouble is that none of this is feasible. The biggest problem is that, even overcoming the water balance problems etc, a plant cell has finite space. If we increase the efficiency of the existing chloroplasts and photosystems at absorbing red light, then it will still be more efficient to pack in more green chloroplasts, because they are simply more efficient. Only when we have packed in so many green chloroplasts that the return from adding more is less than the return from the red will it make sense to start adding red. The trouble is that I suspect that by the time this is achieved there will be no more room in the cell for anything else at all.

Basically black plants will never have an advantage at normal light levels unless you can fundamentally re-engineer the thylakoids themselves so they take up only 10% of the current space. That’s real nano-technology stuff. Theoretically possible the same way that time travel is, but in no way relevant to your question as posed in the OP.

No that’s right. They aren’t absorbing even small fraction of the most common light frequencies. As a result any increase in efficiency will dictate upping this absorption to a reasonable level first. Another words just adding more green pigments. Only after these are maxed out will it become feasible to add red, and that’s the only way to get black plants.

Look at it like this. Your car is only using about 20% of the energy in the gas you buy. It only uses 2% of the energy in wood (with a gasifier etc). gas tanks and wood tanks are the same size. If you wanted to make your car travel further, would you add more gas tanks, or would you add more wood tanks?

The thing is that your car’s range isn’t at maximum because it doesn’t have enough fuel. Adding more tanks for a less efficient fuel isn’t a solution. Similarly plants aren’t photosynthesising at maximum because they aren’t absorbing all the red light yet. Adding more pigments for a less efficient wavelength isn’t a solution.

Thank you. The lightbulb finally went on with the “look at it like this” paragraph.

What I wasn’t seeing is that those green plants aren’t absorbing 100% of the red light, either… they just look green because so much more green is reflected than red. Only once all of the red (and blue, actually–if I’m not totally forgetting things I really should know, chlor a tends to absorb red better than blue, and chlor b blue better than red, or vice versa) was being absorbed would it be advantageous to start packing in antennae to gather the other wavelengths. And, as you said, there simply wouldn’t be space (heck, if you just look at the space taken up by a single molecule of chlorophyll, it’s big enough compared to a photon that that single molecule probably doesn’t absorb every red photon that hits it… so even if the thylakoid were somehow completely coated with chlorophyll, it wouldn’t absorb all of the red, and therefore would have no reason to absorb the green).

Thank you! This has bugged me for 3 years (the “they get by without green” argument just never satisfied me, because it seemed like something more efficient should have come along and out-competed those plants that were getting by :slight_smile: )

Glad to be of service.

Terrific answers!!

To answer the other part of the question, though, there are some black plants. Black mondo grass is the first that comes to mind, and it’s definately black.

Indeed. Wow! :slight_smile: How can you add to that?