Re: What does 6500K actually provide for plants?
Some generalized comments from a plant physiologist.
Socratic Monologue hit the nail on the head with color temperature. Color temperature as used in lighting is based on appearance, and as such winds up being rather loosely related to spectral composition of the light. This looseness happens partly because it's based on how it appears to our eye, which does not have a flat spectral response. And of course it has nothing to do with intensity of the light as a whole, or intensity at some particular wavelength.
For plants, we generally care about intensity (photons per unit area per unit time across a specific waveband) and for the most part do not care about spectral composition. Many find that last bit surprising given how we are taught photochemistry in school: that chlorophyll is the only pigment that matters and it absorbs light in the red and blue region of the visible spectrum, thus plants are green. But in practice, while chlorophyll is the primary photosynethic pigment, the various chlorophylls (plural in both numbers and forms) are packaged into antenna arrays associated with the previously mentioned photosystems (though these are not as described in the previous post of DPfarr) and associated with those are a variety of other pigments bound at the periphery. These additional pigments serve a few different roles, one of which ends up extending the range of useful wavelengths capable of driving photosynthesis.
The "action spectrum" mentioned above demonstrates this behavior quite clearly. These are measurements that relate light absorption at a specific wavelength to some measure of photosynthetic activity (eg carbon assimilation or oxygen evolution) when the leaf is illuminated by narrow band light at that wavelength. They are unrelated to intensity of the light and it is not predictable from just pigment composition alone. I suspect when DPfarr referred to these, they were actually thinking of a light response or AQ curve. That relates light intensity to photosynthetic activity with a specifically limited regard to spectrum (see below).
It was early measurements of action spectrum that led to the formal definition of PAR (photosyntheticly active radiation) and PPFD (photosynthetic photon flux density), and the idea that regardless of wavelength all photons with a wavelength between 400 and 700 nm have equal ability to drive photosynthesis. That means you can even drive photosynthesis with green light, despite a lack of absorption specifically by chlorophyll at those wavelengths.
To the mention of fluorescence, this is actually a very import energy dissipation pathway in plants and it is safe to say that if a plant is under illumination it is fluorescing. If our eye had a flat spectral response and we could see into the near infrared, it is possible plants would appear to glow pink to us! We are unfortunately strongly biased in sensitivity toward green light and our vision cuts out right where the bulk of the fluorescence emission that makes it out of the leaf starts.
It is also worth noting that plants have other "light receptors" that function unrelated to photosynthesis, but that can have real impacts on growth and development. Phytochrome for example, serves a role in detecting neighboring plants (by conversion between P680 and P700 in the presence of red versus far-red light) and as a result partly drives biomass partitioning between roots and shoots.