Why are plants green?

This is a frequently posed question, that has no unique or simple answer. Prof. Lars Olof Björn has written a section on this in his book Photobiology: the science of life and light which is much more detailed than this short post. The problem with this question is that its meaning can be different to different persons. I will start by separating different aspects of this question into separate, and better-defined, questions that are easier to answer:1) Why do plants “harvest” energy from solar radiation with a pigment that has maximal absorbance in the red band of the spectrum?

If we assume that it is not possible from the physicochemical  point of view for a pigment to have a very broad and flat absorption region, then the location of the red absorption peak of chlorophyll can be explained based on that this region of the solar spectrum is where photon irradiance is maximal. To understand this one must remember that for the rate of a photochemical reaction, what is important is the number of photons (quanta) and not the total amount of energy.

2) Why do plants reflect and transmit very readily radiation in the far-red and short infra-red bands of the spectrum?

A possible explanation is that photons of such long wavelengths have too little energy to drive the photochemical reactions of photosynthesis. This is true, only if we restrict the analysis to photosynthetic reactions using water as a source of electrons, and thus releasing oxygen. On the other hand there are some phototrophic organisms that can use as a source of energy for metabolism radiation of longer wavelengths than terrestrial plants (see 4 below).

3) Why do plant leaves reflect and transmit more green light than blue or red light?

One thing to consider is that even though plant leaves absorb less green-, than red- or blue light, they do absorb quite a lot in the green waveband. Reflecting 5 to 10 % of the solar green light is enough for us to see plants as green, but plants do use green light for photosynthesis. So, this is really a difficult question to answer, especially knowing that some plants have whitish leaves (because of hairs or waxes) that demonstrate that in some habitats reducing the amount of solar light absorbed can improve fitness.

4) Do we know counter examples of organisms that use pigments with maximal absorbance in other regions to “harvest” energy from solar radiation?

Yes, some algae have chlorophylls different to those in higher plants, with absorption maxima shifted towards longer wavelengths. A possible explanation for this is the shift in the light spectrum with depth in water bodies. Some purple bacteria contain bacteriochlorophylls, with absorption maxima even at wavelengths longer than 1000 nm, however photoreactions in these organisms do not lead to the release of oxygen from water, they use sulphur compounds as sources of electrons.

Rhodopsins are pigments usually having a peak of absorption in the green region of the spectrum. In most organisms where it is present rhodopsin behaves as a sensory pigment (frequently located in the eyes), used to obtain information, but in a few bacteria, its role is somehow equivalent to that of chlorophyll, although driving different photochemical reactions. These species of bacteria use the proton pump bacteriorhodopsin to “harvest” energy. The spectral tunning of rhodopsins is also a well described phenomenon.

In conclusion, an universal answer is almost impossible because: 1) we cannot assume that maximizing the absorption of solar radiation is always beneficial, 2) we do not know what were the historical and physicochemical constraints present during evolution, 3) we know that light harvesting by chlorophylls and auxiliary pigments depends on energetic gradients to funnel excitation to reaction centres, so pigments cannot be considered in isolation, their interactions need to be taken into account, considering each light driven metabolic process as a whole.