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red & green light
We have slowly progressed: from the guy who believed plants are green because they absorb green light, to more useful contributions like Roger Miller's quotes from Whatley & Whatley, and the graph of chlorophyll's absorption spectrum (with the caveat that the action spectrum may be different).
The general conclusion is that for _vegetative_ growth, you are best lighting with the red & blue parts of the spectrum (ie, you get most growth per watt of electricity used), because the plants do not use the green light as efficiently, even though they can use it. However, for other plant functions, such as inducing flowering, it can be another matter. I read somewhere that commercial flower & fruit growers use changes in lighting as well as manipulation of the photoperiod in order to induce flowering. If I remember rightly, the near infra-red has specific functions for some plants despite its uselessness for photosynthetic purposes. By analogy, the functions of different spectral bands may also be important for aquatic plants, even if flowering is of less interest. There must be research literature on this, but I don't have access to it. Any else in a position to do some library hunting?
PS: On looking back, I have just seen that David Brooks mentioned this idea back in APD #861 - but no followup info yet as far as I've seen.
PPS: The chlorophyll absorption spectrum JPG file looks like the diagram in Lehninger, Nelson and Cox: Principles of Biochem. No attribution - tut tut.