Why aren't plant leaves black?

Optimal use of photosynthesis would allow for plant leaves to be black and absorb all wavelengths of sunlight. But at best plant leaves are a very dark green.

So what mechanism prevents plants from having black leaves and absorbing green light from sunlight?

[WAG]It would be too much light and heat and/or they can’t do anything useful with ultra/infra green wavelenghts. [/WAG]

Chlorophyll is a molecule that has special properties suitable for absorbing light and harnessing its energy for use in the cell. Unfortunately for plants, it happens to absorb light best at red and blue wavelength, even though sunlight is more intense at green wavelengths. Since chlorphyll reflects green wavelengths, it makes plants appear green.

Plants use supplemental red pigments called carotenoids to collect the green wavelengths that chlorophyll can’t absorb, and pass the energy on to chlorophyll.

Even so, because they have so much chlorophyll, plants reflect more green than any other wavelength, which is why they appear green.

In the fall, when chlorophyll breaks down, the carotenoids are revealed.

Something I never got a satisfactory explanation for in science lessons (despite repeatedly asking it) - how do the few plants with non-green leaves (copper beech trees, for instance) manage it?

They have chlorophyll, just like other plants. They just have an inordinate amount of carotenoids (I’m not sure why) which masks the green color (just as chlorophyll masks the red carotenoids in most plants).

Grrrrrrr… :slight_smile:

Light is not usually the limiting factor on plant growth. Nitrogen, CO[sub]2[/sub], water, phosphorous etc. are all needed in addition to photons. There’s no evolutionary advantage to capturing a higher percentage of photons, if the plant doesn’t have anything useful it can do with the extra energy.

A black plant in full sun would have a higher temperature than a green plant. All other things being equal, this would lead to increased water loss from the foliage.

Green plants do alter the size and arrangement of their photoreceptive arrays (thylakoids) depending on lighting conditions. This helps them match the output of photosynthesis to the rest of their metabolism.

I believe that copper beech itself is a cultivar, so if you want a “why” it’s because humans bred them that way.

Pigments are very expensive to make. They require a lot of nitrogen. Second to water, nitrogen is the limiting factor for plant growth. Also, because pigments are organic compounds, a plant would have to allocate carbon–and thus energy–to their production.

Black absorbs heat more readily than other pigments. Plants are thirsty creatures already; you don’t want them to sweat more than they have to. Also, all the energy black pigments would absorb would be damaging to the delicate molecular machinery of the photosynthetic pathway. A plant would have to make more caretonoids to protect itself, which would again require the expenditure of valuable energy and nitrogen.

The cost-benefit ratio of black pigments is probably too high for them to have widespread usage in plants.

Just as an odd side note, since several people have commented the heat issue. Presumably, this would be because no plant pigment is able to convert the energy of light into useful chemical compounds (whatever they might be) at 100% efficiency - some of it is ‘wasted’ as heat.

Any idea what the efficiency ratios are for chlorophyll and other plant pigments of that sort? Is there a theoretical upper limit on the efficiency achievable? :slight_smile:

I never knew that. And obviously nor did any of my science teachers :slight_smile:

Yes, it seems to be called Fagus sylvatica var. *purpurea, atropurpurea, cuprea, * or other names.

My understanding was that way back in the single-celled alge and bacteria stage, a lot of organisms used purple pigments (and still do)- that is, pigments that absorbed most of the middle spectrum of usable light (green) leaving red and blue left over. And that aother group of alge evolved chlorophyl to absorb the red and blue light (reflecting green), and that as an accident of evolution most higher plants are descended from the green-colored alge.

Addendum: chlorophyl using organisms also produce oxygen as a byproduct (purple photosynthesizers do not), so “accident of evolution” would have to include the fact that green photosynthesizers pumped out a waste product poisonous to many of their competators.