Hi Az. Since plants are not able to choose their home, they have become extremely good at adapting to their immediate environment in order to increase their chances of survival. One of the best ways they do this is by using their Phytochrome proteins to sense their environment. This category of photobiology is called photomorphogensis. This is completely different than photosynthesis, but it is a very important part of the plantís lifecycle. With photosynthesis, we are focused on using the right type and right amount of light to grow the plant. With photomorphogenesis, we are focused on using specific light signals (photo) to impact the plants development (morphology). The Phytochrome gets a lot of attention, but we should also be aware of the Cryptochromes and Phototropins, as they both play an important part in plant development.
Back to your original question, I donít pay too much attention to the Red to Far Red (R:FR) ratio for most lighting applications, since most people are focused on Photosynthesis (i.e. bigger plants). If you are trying to induce a Phytochrome-related response (i.e. induced flowering, delayed flowering, taller plants, etc.), then you need to pay attention to the R:FR ratio. Per my earlier post, green and far red (especially far red) are transmitted through the plant canopy at a higher rate than blue and red. Over time, plants have figured out that a high level of far red light probably means they are being shaded by taller plants. Since they need light for photosynthesis, the plants realize they need to shift their energies into growing taller to get above the other plants. If they donít, they probably wonít survive very long. Once above the canopy, the R:FR ratio changes, and the Phytochromes tell the plant to stop growing tall/spindly, and to put their energies back to growing more leaves/branches to catch more light for photosynthesis. This is called the Shade Avoidance Response, and it is a really interesting facet of photobiology. As mentioned, you can also use 660nm and 730nm to induce or suppress flowering in short day plants (SDP) and long day plants (LDP). I have worked on large commercial photomorphogenic lighting applications, and it is really amazing stuff.
Hope that helps