Far red is somewhat tricky since it has several effects on the plant:
Emerson: far red + photo red; seems like total photosynthesis when both wavelengths are present is higher than what it would be by measuring total photons given. A kind of synergy effect. But I haven't seen the science of it for cannabis, especially in flowering. But there seems to be some references that this happens mainly when total light levels and photosynthesis are low, at higher light levels like cannabis flowering it's almost nothing.
But all the other effects are still in play:
Flower inducing: seems like it favors the flower response, speeding up flower a bit. But if it's not well balanced it can speed it up so much that the early harvest means some yield reduction.
End of day treatment: pr/pfr switching is what the plant uses for knowing when it's time to shut down and sleep. In normal conditions far red is very high at dusk, higher than 660nm, giving the plant a signal to go to sleep quicker by about 2 hours compared to no far red. Some use this to flower quicker, some use it to give the plant a couple of extra hours of daylight for more yield.
Shade avoidance: high far red, compared to photored, also triggers shade avoidance syndrome: stretch and yield lost. This happens as an adaptive response to far red being high in shady conditions. Best to be avoided, one of the reasons you want things to balanced.
Offset to uv: seems like far red and uv to some extent of set each other's morphological reactions; blue and uv shrinks leaves, far red increase size; far red stretches and uv inhibits stretch. Not sure if it is on anymore level than this.
The key seems to be to balance all things out and remember that a little is a lot.
Hey mate, good to see ya! You're right about the Emerson Effect being more pronounced at lower PPFD levels, but if memory serves me correct, those tests were conducted on lower PPFD plants and not cannabis, so it may stand to reason that the effect is greater at higher PPFD levels depending on plant species. I'm speculating, but I think I've read somewhere else that it still has an effect at higher PPFD levels. I'll have to go and research now . . .
Shade avoidance is something that can be harnessed and is not "best avoided" when you are trying to promote larger leaf area (as opposed to greater internodal length), as large leaves capture more photons. Actually, even longer internodal length can help with yields if it means better light penetration into the lower canopy. I have seen scientific tests where this was cited as being a probable cause for higher yields in cannabis species when compared to LED lighting with no far red. In fact, I think it was a Bugbee study – again, I'll have to come back to this.
Yes, UV can offset far red shade avoidance to an extent as it opposes cell expansion (far red shade avoidance promotes cell expansion). But I need to mention that when I first noticed the faster flowering times under the Nichia LEDs, I was comparing them to HPS and neither of the two light sources (LED, HPS) had any significant amounts of UV or near UV. (SPAM alert) Our new boards do have UVA and near-UV – as well as over 9% far red – and all reports are that flowering times are reducing by up to 10%.
I've actually been telling growers to run higher amounts of far red through ALL hours of flowering – including SOD and EOD – as red:far red ratios of typical LEDs is up to 10:1 or more, whereas sunlight is typically 1.3:1 and you don't normally see much stretch outdoors (although there is much more UV around).
Just my 2c
