Besides using light to drive photosynthesis, specific wavelengths are required for things like avoiding stem elongation (we want short fat plants, not tall skinny plants), proper leaf shape/size/orientation, stomatal opening (the pores underneath leaves that allow the plant to breathe), and flowering (in some plants). Some responses like flowering vary widely from plant to plant while others like stem elongation and stomatal opening are almost universal. Most of those responses can be had with minimal amounts of blue light (again, it depends on the plant)

Besides all that, I believe you have the right idea: A yellow LED of a high enough efficiency could be more efficient at driving photosynthesis than a deep red. Unfortunately, there is no such thing as a yellow LED - only red or blue LEDs with phosphors to convert a portion of the light. As such they will never be as efficient as the base LED.

If we look at our bean plant data from before (if someone has this data for cannabis I'd love you forever) we can see that 700 nm light causes the plant to consume about 34 molecules of CO2 for every 1000 photons and 650 nm light causes the plant to consume about 103 molecules of CO2 for every 1000 photons. From this, one might guess that given 1000 photons of 700 nm light and 1000 photons of 650 nm light then the plant would consume 137 molecules of CO2, but actually it consumes a bit more. This effect was discovered in 1957 by Robert Emerson. This effect exists because all higher plants have two photosystems and a full photosynthetic cycle (not sure if I'm saying that correctly) can not proceed unless both systems are stimulated. photosystem I, containing more chlorophyll a, uses the 700 nm light more efficiently while photosystem II, containing more chlorophyll b, uses the 650 nm light more efficiently.

Anyway, I'm using about 20% 460 nm blue LED and the rest 660 nm red.
thepaan Reviewed by thepaan on . Importance of 460nm red LEDs? Here is something very confusing to me. Many LED proponents loudly proclaim the need for 460-470nm red LEDs over the cheaper 630nm LEDs. I understand that this better coincides with the accepted chlorophyll B absorption peak. So far so good. LEDs veg very well given appropriate wattage & color. So far so good. LEDs do not compare (generally) to HPS flowering as far as yield. (Please - let's not go off in that direction here. Thank you!) Rating: 5