Lifeboat Foundation

Black salad grows faster, looks funny. Most plants are green, and most people can appreciate the healthy green glow of a vigorous plant. Would you say that plants like green light? Not so — green plants are green because they reflect green light, while absorbing red and blue. Consequently the green light goes into the eyes of the viewers, who thus see plants as green. By reflecting green light, plants are rejecting it. They use light that they absorb as solar energy, powering a process that sucks carbon dioxide out of the air and converts it, ultimately, into plant contents using a process known a photosynthesis. Perhaps weirdly, it now appears that photosynthesis, the biological process that terraformed the Earth in the distant past, creating and now maintaining at 21% the oxygen in the atmosphere so necessary for human life, uses the esoteric physics phenomenon known as quantum entanglement to do its work. A green color may be a sign of health, but it is also a sign of inefficiency. If the could only absorb and use green light better, it would be using solar energy more effectively and could grow faster, for either its own or human purposes. If a plant absorbed and used all light falling upon it, it would be black, not green, because black is what we see when all colors are absorbed and not reflected into our eyes.

If black plants would be better, then what are the prospects for mountains, plains, and rolling hills of deep black instead of striking green? Plants absorb light using special pigments which begin the solar energy harvesting process. Well-known and prevalent pigments are chlorophyll, of which there are several varieties named chlorophyll a, b, c, c1, c2, and others. Different chlorophylls (chloro– from a Greek word for green, –phyll from the Greek for leaf) are of varying shade depending on the type. However there are also other pigments useful for photosynthesis. Various xanthophylls (xantho– from the Greek for yellow) are also used. Carotenes, which make carrots orange, are another. There are hundreds of them. Opsins (ops– from the Greek for sight, as in ‘optical,’ and –in which indicates a biochemical substance) are not only used for sight in the human eye (namely rhodopsin, rhodo– for rose-colored, also named “visual purple”), but for photosynthesis (using bacteriorhodopsin, a misnomer since the organisms using it are archaea, not bacteria). Bacteriorhodopsin preferentially absorbs green light and reflects red and blue, thus appearing reddish-blue (that is, “purple”). Bacteriochlorophylls are related to chlorophylls and can be greenish or purplish. Phycobilins are used in photosynthesis in certain microorganisms and come in a variety of colors, from red to blue. Phycocyanins are another category of pigment used in photosynthesis (cyan meaning blue-green). Phycoerythrins too, which are red (-eryth from Greek and meaning red, as in erythrocytes — red blood cells).

Even invisible light is important. The water-dwelling microorganism Acaryochloris marina contains chlorophyll d, which is particularly good at absorbing infrared light for photosynthesis. On the other hand, some plants reflect infrared light. But they could reflect more, enough to actually change the world’s climate significantly. It seems that leaf hairs can help reflect infrared while allowing visible light through for photosynthesis. By breeding plants with leaves that are quite hairy, more infrared could be reflected back into space, with a cooling effect on the climate. Extra hairy soy varieties have already been bred such that if they and other crops similarly bred were grown extensively enough in temperate regions, the average temperature of those regions would decrease by about 2 degrees F. It is a tough call which would be better, crops with pigments that use infrared light for photosynthesis, or crops that reflect as much of it as possible away from the Earth.

Be that as it may, a plethora of pigments exist to support photosynthesis and there seems no intrinsic reason why a plant could not eventually evolve naturally or be genetically engineered to mix and match the pigments so as to absorb and use nearly all light. Such plants would potentially have an evolutionary advantage over other plants that waste resources by taking the trouble to grow leaves and then not use all the light that falls upon them. Thus future plants that solve this problem would tend to take over, both in nature and in agriculture. Though the real action would be at the biomolecular level, visually such plants would be near-black, not green. Would black salad taste better than green? Only a future taste test will resolve this important question!

Next time: Alternamorphs: plants with options.

References

“Perhaps weirdly, it now appears that photosynthesis…uses the esoteric physics phenomenon known as quantum entanglement to do its work.” M. Sarovar, A. Ishizaki, G. R. Fleming and K. B. Whaley, Quantum entanglement in photosynthetic light-harvesting complexes, Nature Physics, vol. 6, pp. 462 – 467, 2010, executive summary at http://newscenter.lbl.gov/feature-stories/2010/05/10/unt.….anglement/.

“Acaryochloris marina uses chlorophyll d, which absorbs infrared light for photosynthesis”: R. Mohr, B. Vosz, M. Schliep, T. Kurz, I. Maldener, D. G. Adams, A. D. Larkum, M. Chen, and W. R. Hess, A new chlorophyll d-containing cyanobacterium: evidence for niche adaptation in the genus Acaryochloris, The ISME Journal (27 May 2010), http://dx.doi.org/10.1038/ismej.2010.67.

“Extra hairy soy varieties have already been bred such that…the average temperature of those regions would decrease by about 2 degrees F.” C. E. Doughty, A. McMillan and M. Goulden, Climate Management Through Agricultural Albedo Manipulation, Eos Transactions of the American Geophysical Union (2007), vol. 88, no. 52, Fall Meeting Supplement, Abstract GC52A-10, http://www.agu.org/meetings/fm07/fm07-sessions/fm07_GC52A.html. See also: Super-hairy plants could battle global warming, New Scientist, issue 2637, Jan. 9, 2008, http://www.newscientist.com/article/mg19726370.700-super.….rming.html.