[Bert Latamore]

Bert Latamore asked the Naked Scientists:
   
Why are leaves green? Theoretically they should be black to maximize energy absorption for photosynthesis.

Bert Latamore

What do you think?

[Chemistry4me]

Good question, I think the answer is because the chlorophyll pigment is green. Also maybe the leaf might get too hot and burn up if it was black. But don't just take my word for it...

[RD]

http://www.thenakedscientists.com/forum/index.php?topic=2993.0

[lyner]

Along with the rest of us, plants have a problem with thermal energy balance. Overheating can be a serious problem for some of them, so reflecting the light of wavelengths they don't need makes sense.

[Madidus_Scientia]

Plants don't use the heat to photosynthesise, but rather the photons themselves interact with molecules in chlorophyll to cause an electron to jump to a higher energy level, which then allows a chemical reaction to occur. So there's no use in the leaves being black to absorb more heat, as it's only these interactions that matter in photosynthesis. And I would suppose that the green wavelength of light is the least likely to interact in this way, so it wouldn't be absorbed by the plant as much as the other wavelengths.

[Don_1]

There are very few plants with black leaves or flowers. Most of those which are described as black are in fact a very deep, near black, purple.

Black Nightshade has a near black flower.

As for the black leaf, there is the Ipomoea batatas 'Blackie', the Schefflera elegantissima, the Sambucus nigra 'Eva' (Black Elder) and the Ophiopogon planiscapus 'Nigrescens' (Black Lilyturf) which has a very deep coloured leaf which turns black in full strong sunlight.


Black Lilyturf (Ophiopogon planiscapus 'Nigrescens')
Picture from Shrubland Park Nurseries http://www.shrublandparknurseries.co.uk/shop/index.php

Here is an abstract from a study of the Black Lilyturf, which is not, as the name suggests, a grass.

Black-pigmented leaves occur only rarely in nature, possibly because their efficiency of light capture for photosynthesis is low. Using near-isogenic morphs of black- and green-leafed Ophiopogon planiscapus Nakai ‘Nigrescens’, we tested the possibility that black pigmentation restricts the transmission of PAR within the leaf. We measured chromaticity coordinates of black and green leaf phenotypes, quantified their pigments and optical properties, and followed the transmission profiles of red, blue and green light through lamina tissues. Chroma and lightness values for the black leaves were comparable to those of a black paint standard, and were lower than those for the green phenotype, or for green and anthocyanic leaves of three other species. The adaxial surface of black leaves absorbed 95% incident quanta, and reflected 4% across the entire 400–700 nm waveband. There were no obvious structural differences between black and green leaves. Black coloration correlated with luxuriant concentrations of both chlorophylls and anthocyanins in superficial mesophyll. Profiles of transmission of red and blue light were similar in green and black leaves. In contrast, green light was restricted to uppermost palisade mesophyll layers in black leaves, but was transmitted to more central mesophyll in green leaves.

Source: CSIRO Publishing http://www.publish.csiro.au/paper/FP06220.htm