This is something that I suspect of being responsible for when HPS growers report higher resin/quality when adding UVA/UVB fluoros fixtures. HPS just lacks alot of blue, and the blue plant photoreceptors are likewise stimulated by UV, and the UVB chromophor overlaps into that as well. So HPS may not be enough to give a full response, whereas white LED, which puts out more blue and also has usually a higher PPF, should be better to do so. I remember a study by Fluence where they compaired LED to HPS in terms of also quality, and LED gave a better result. I agree alot with the raised opinion that many photobiological studies leave alot to be desired, often the result is based on the set parameters of the study. For example, when they pick wrong genetics, don't include Farred in their "full-spec" light used for control. When the science already tells us FR is the only bandwidth that can reduce heatstress from the core of PSI, which will be increased when a more blueshifted spectrum is going to be used. It then also doesn't surprise me that each and every study done with Cannabis outdoors or greenhouse -ie. under the sun- shows a positive correlation with Cannabinoids under elevated UV exposure. Personally I believe it's mostly down to genetics but the environment must give sufficient impulses to "trigger" these up to their max potential, and this may not always been the case with past indoor artificial lighting recipes.
