Im on a 20/4 schedule right now, thanks for pointing that out, another thing I hadn't considered!Dark periods. 18/6 will likely allow for faster root development than 24/0.
Ethylene (female flowering hormone) and the hormone which controls roots which is eluding me right now dissipate in or are less present in parts of the plant that are exposed to light.
Mycorrhizae, if you're organic.