Photomorphogenesis is actually any growth that is controlled by light. I assume that you are referring mainly to the effects of stretching and etiolation due to insufficient light, and how to distinguish these from overall plant growth. The method used in most controlled studies is to measure overall plant mass as well as plant height, since a tall spndly plant weighs less than a shorter healthy plant. At a more detailed level, production of different plant materials such as root matter, cellulose and glucose are measured. I'm sure you understand the kinds of controlled studies that would be done.

The simplistic blue vs red interpretation of the spectrum that is often seen on the web completely fails to describe the effects of different light wavelengths on plants. See the link for some hint of all the different processes that are mediated by different types of light receptor. I don't think it is possible to define "the ideal spectrum for plants", but a starting point would be typical daylight in the plant's natural environment.

At a gross level, the most important types of light for plant growth regulation are blue, red, and far red. In particular the relative levels of red and far red (or near infrared) light are most important for controlling etiolation, but by far the most important factor is the intensity of the light. Monochromatic red light of sufficient intensity will grow plants of normal stature. Experiments into levels of plant growth under specific narrow wavelengths bands of light have shown that plants grow well with predominantly red light around 650nm and 10%-20% blue light around 420nm. These particular wavelengths have been chosen because they are the most efficient for plant metabolism, for most plants anyway. Other monochromatic wavelengths, at sufficient intensity, have also been shown to be sufficient for producing healthy plant growth.

There are various opinions about whether the use of strict narrow bands of wavelengths to stimulate peak photosynthesis are more effective than a broad spread of wavelengths to mimic a natural environment. A few experiments comparing gro-lux lamps, that concentrate most of the light emission in narrow red and blue bands, with standard cool white tubes, which have a spread of wavelengths in the red and blue and also some in the green, have shown no worthwhile difference. Note that a standard cool white fluorescent tube has very strong peaks at a few wavelengths and quite weak emission at other wavelengths. Some growers swear that nothing is as good as the plant lamps, others choose a full spectrum bulb such as the GE Chroma series. My preference is for sufficient light intensity to produce the necessary plant growth without etiolation with some effort to provide light over the whole spectrum. I achieve this by a mix of warm white and daylight bulbs, which has been the standard advice for decades.

It is interesting to note that blue light is far less efficient, in terms of glucose production and subsequent overall plant growth, than red light. One photon of blue light produces, after transfer through numerous intermediate molecules and subsequent energy losses, exactly the same chemical results as one photon of red light. It requires twice as much energy to produce the blue photon as the red one. This is despite the often-seen contention that blue light is for growth and red light is for flowers. I have successfully and rapidly grown seedlings under just warm white lamps, which produce only a tiny amount of blue light. The blue light, however, is responsible for mediating a number of processes that are important to plant metabolism and experiments have shown that using solely red light is not the most efficient for the plant. A mix of warm white and cool white lighting provides more red than blue, but still provides sufficient blue light and other wavelengths for good growth.

Here is a link that might be useful: Photomorphogenesis

Thank you very much for your reply (and time!)! I realy appriciate this. Altough your reply answers most of my question, there are still some left:
1) you wrote: "Monochromatic red light of sufficient ntensity will grow plants of normal stature. Experiments into levels of plant growth under specific narrow wavelengths bands of light have shown that plants grow well with predominantly red light around 650nm and 10%-20% blue light around 420nm." What do you mean exactly with 10-20%...number of photons? or energy? Since "blue photons" are much more energetic there is of course a big difference.
2) Is it right to conclude from you e-mail that if my lightbulb is an exact copy of the sun in both intensity and composition (hypothetical). Or in other words IS the sun, you would prefer to sacrifice some of the green (even slightly the blue) light and increase the intensity of the red light? Or would you still stick with just the "sun"?
3)Can you explain to me what influences the growth of flowers/fruit? B,R or FR light?
4)Can you give me any references on this topic (non internet based)?

1. That would be photons., meaning more like 15%-30% blue light in power terms. Studies seem limited in both time, plant species, and spectrum range, so I would take these numbers as a guide rather than the bible.

2. Research has shown that, to some extent at least, lower intensity light of a suitable spectrum can be more effective in producing plant growth. However, cost is a factor and since I don't know exactly which spectrum will work best for which plants at which stage of growth, I aim for the best light intensity at the lowest price. I grow seedlings under lights, not mature plants, so my opportunities for detailed experimentation on the best bulbs are limited. So far I have settled for a mix of simple warm/cool bulbs that is bright enough to avoid etiolation. Growers of mature plants (and aquarium owners) tend to develop attachments to specific bulbs that have worked best for them, but often their experiences seem contradictory.

3. Flowering in many plants appears to be induced not so much by a specific spectrum, but by the interplay of red and far-red light mediated through the phytochrome receptor. This occurs naturally during the dawn/daylight/dusk/night cycle. Growers under artificial lights use lights with plenty of red and a day/night cycle to encourage flowering and fruiting.

4. Hope these are available to you in print. Most recent publications tend to provide detailed molecular information about individual receptors rather than an overview of the subject. You might try hunting up "phytochrome" in a general text book and go from there. Also, as you probably know, just following the references chain and reading abstracts can give you a good overview.

Goins, G.D., Brown, C.S., Sanwo, M.M., Yorio, N.C. 1997. Photomorphogenesis, photosynthesis, and seed yield of wheat plants grown under red light-emitting diodes (LEDs) with and without supplemental blue lighting. J. Exp. Botany, 48: 1407-1413.

Potter, J.R., and J.W. Jones. 1977. Leaf area partitioning as an important factor in growth. Plant Physiol. 59:1014.

Fankhauser, C., and Chory, J. (1997). Light control of plant development. Annu. Rev. Cell Dev. Biol. 13, 203Â229.