Lighting Research - Preliminary Studies: White versus Red/Blue

I've been doing quite a bit of reading about the lighting requirements of plants.  I'll provide some details on my findings, plus references, in another post.  For now, some notes.  My goals are to build LED lighting assemblies that will provide adequate and appropriate light for my plants.  As there will be a variety of vegetables, plus fruiting plants and hopefully some root-plants, the lighting must be able to accommodate them all.

I investigated HIDs and fluorescent fixtures, and ruled both out as too costly in terms of setup and upkeep.  The former are power-hogs.  The latter's bulbs reportedly lose value after 6 months and require replacement.  LEDs to the rescue, then!  But, it's not so simple.

Firstly, manufactured LED assemblies are horribly expensive.  Second, the majority of them are red-blue fixtures.  Some also incorporate green.  I did a great deal of search about the best growth spectrum, the typical spectra for chlorophyll, and eventually found myself on a very detailed site regarding photobiology.  Here's what I have so far...

Chlorophyll-1 and -2 do indeed absorb rather specific portions of PAR light.  One takes the majority of its energy from around 450nm, the other from the 640nm area (very roughly, don't count on those numbers AT ALL).  However, their precise absorptance spectra depend on the solvent in which they are dissolved, if considered in vitro.  When one considers the effects of PAR spectra in vivo, it has been observed that a very wide range of the 400-700nm wavelengths are indeed absorbed.  These are not necessarily absorbed by the chlorophyll itself, but could be absorbed by accessory pigments and other chemical processes within the plant.  Furthermore, the presence of other proteins and chemicals either attached to the chlorophyll, or within close proximity to it, will affect (sometimes to a great degree) the spectra that the chlorophyll can and will absorb.  We cannot rely on in vitro data alone to build our lighting, as it does not accurately characterize the actual operating conditions of chlorophyll.

Some plants also make specific use of the far-red region of the spectrum.  This is evidenced by phytochromes, namely Pr and Pfr (for the red and far-red spectra).  The two convert to one another in the presence of the right light, and if I remember correctly Pfr converts without light input to Pf.  The relative quantities of these two phytochromes determine the expression of certain traits, such as the straightening of plant parts.  But the key take-away is this: we cannot alone rely on the spectra relevant to chlorophyll to determine the appropriate lighting.

Given that this will not be a monocrop, and given that the sun's spectrum in the PAR region is generally flat (as apposed to the spiked spectra for chlorophyll-1 and -2), it becomes evident that focusing on only the blue and red spectra will not be for the best of the plants.  In other words, white light with a fairly even spectral distribution will be the optimal light, with what limited information I have about the goings-on inside the plants themselves.  While it will certainly be sub-optimal from the point of view of chlorophyll - that is, I am technically "wasting" power on light that isn't immediately absorbed by the chlorophyll - it will also be the kind of light the plant has evolved to expect.  Given the number and variety of inner-plant processes that we either do not know, or do not understand, it is possible the spectra-restricted (red/blue) lighting is itself deficient, and therefore wasteful.

Many questions remain:

• How much light, in terms of PPF per day, will the plants require?
• What will happen if too much light is provided?
• What will be the best LED light source for the spectra chosen?  Will I need to augment that spectrum with additional, spectra-specific LEDs?
• That is, most of the white LEDs I've surveyed to-date (all from Cree), tend to show dips in the spectrum after blue, but before yellow/red.  This probably means emission of the green spectra is greatly reduced.  I would have to compensate with additional green LEDs, but only if that really matters in the end.  It's possible it does not.
• What should be the minimum and maximum distances for the light sources from the crops?
• On the DWC, there will be no choice
• Will the even, consistent lighting prevent fruiting?  If so, will it be necessary to implement additional lighting (in specific spectra) that can be triggered on an as-needed basis?
• If spectra-specific lighting is required for fruiting, will it cause non-fruiting plants to bolt?

The math also looks like it will be fun.  Lots of 10-to-the-very-large-number exponents.