Additional to the Chlorophyll a and b there are accessory pigments - carotenoids, xanthophylls, and flavonoids, and many others that absorb other wavelengths to convert the energy of those wavelengths. There are also pigments which reflect harmful wavelengths such as IR and UV.

The Barr Report Newsletter March 2005 discusses Photosynthesis.

"...Blue light stimulates malate synthesis in guard cells. Under red light, guard cells synthesize glucose and sucrose mainly; this metabolic pathway is deactivated by blue light, and metabolism switches to starch degradation and malate biosynthesis..."

Evidently, red light stimulates CO2 fixation while the addition of blue stimulates organic acid, amino acid, and protein building.


"...These other pigments allow a much greater range over the spectrum than Chlorophyll alone. SAMs can and do change the concentrations and type of these pigments to suit their
environment. Algae essentially do the same thing at the single or colonial cell level. Thus changing the spectral output does not gain the aquarist an advantage over the algae.. Furthermore, green microphytic algae have the same pigments as the SAMs..."

This is more or less consistent with what I observe. I see no difference in bushiness or legginess as a result of spectral variation. This makes sense because the spectral emission of the sun is wide and changes during the day being red(ish) in the morning blueish at midday, however due to the local environment of a plant at any given location, the spectral quality that plant receives can vary wildly from that of the sun due to shade or reflection. It would therefore be self defeating for a plant to only be able to use one or two narrow bands of light.

Cheers,