Did you know? Have you ever wondered why most plants are green? It’s because, when we look at plants, we ‘see’ the green-yellow light that the chlorophyll is reflecting, not the red-blue light that it is absorbing.
During photosynthesis plants absorb visible light, mainly through the pigment chlorophyll, and use its energy to convert carbon dioxide and water into sugars required for metabolism and growth. The visible light spectrum (a.k.a. the rainbow) is made of colours with varying wavelengths—from Red (650 nanometers) through Orange, Yellow, Green, Blue and Indigo to Violet (400 nanometres). Chlorophyll mainly absorbs red and blue light and reflects yellow and green light.
Since chlorophyll doesn’t absorb yellow or green light, a plant ‘fed’ only one of these wavelengths would die as quickly as one kept in the dark.
A plant receiving only one wavelength of red or blue light could struggle along for a while, but it wouldn’t grow normally, and it wouldn’t be green.
Did you know? The word "green" is derived from an Old English word "growan", which comes from the Germanic "grun" for "grow". "Green" means, literally, the "colour of living plants."
Sunlight provides a fine balance of red and blue light, to which plants are genetically attuned. The intensity and length of exposure to these wavelengths has a strong influence on plant growth, flowering, and seed production. Experiments have shown that plants receiving excess red light become tall and spindly. Excess blue light has the opposite effect, producing plants that are short and stocky.
Photosynthesis is calibrated to the type of light reaching a plant’s environment, which is determined by the intensity of sunlight, the filtering effects of the atmosphere and, for aquatic plants and algae, the filtering effects of water. Water is very efficient at filtering red light, so beyond three feet deep algae actually photosynthesize by absorbing green, violet and blue light through accessory pigments called phycobilins which "pass" energy along to chlorophyll. Guess what colour those algae are? Red!
Did you know? Some land plants also have red or burgundy leaves. These plants still photosynthesize using chlorophyll, but have abundant accessory pigments (flavins and carotenoids) which give them their colourful nature.
NASA scientists speculate that on planets with different kinds of solar radiation and atmospheric chemistry, photosynthesis could be powered by diverse wavelengths of light, producing bizarrely coloured vegetation. On planets circling low energy stars (red dwarfs) photosynthesis might require "plants" to absorb all available wavelengths of light, making them appear black. Slimy green aliens would sure stand out in a black forest!
- Cosgrove, D.J. and Green, P.B. 1981. Rapid Suppression of Growth by Blue Light. Plant Physiology 68:1447-1453.
- Nozuel, K., Covinton, M.F., Duck, P.D., Lorrain, S., Frankhauser, C., Harmer, S.L., and Maloof, J..N. 2007. Rhythmic growth explained by coincidence between internal and external cues. Nature 448, 358-361.
- Kiang, N.Y. 2007. The Color of Life, on Earth and on Extrasolar Planets http://www.giss.nasa.gov/ research/briefs/kiang_01/