u/SuperAngryGuy

https://www.mdpi.com/2223-7747/14/10/1469

tl;dr- Yield goes up fairly linearly as you add lower light. I have theory below.

Paper highlights

The raw electrical efficiency (REE) values for FDW production were 1.49 g·kWh−1 (top only), 1.61 g·kWh−1 (subcanopy), and 1.48 g·kWh−1 (inter-canopy). These results indicate that subcanopy was the most power-efficient strategy, providing an advantage of about 8.2% over TL and ICL for producing dry inflorescences.* (see section 2.5, page 10)
There could be a slight decrease in potency with increase yield. This is often called the dilution effect. THC yield per area goes up by adding more light.
Lower flower terpenes went up by ~12%.
n=24 which is better than most cannabis studies

Other related paper on lower lighting:

https://www.reddit.com/r/BudScience/comments/1hjtai0/improving_cannabis_bud_quality_and_yield_with/

my anecdote and what not to do

I am very, very skeptical of any gimmick lighting claim with the exception of just blasting the lower leaves in addition to the top. Papers like this are backing the claim that subcanopy and inter-canopy lighting will work for improving your yield per area or volume, and maybe per energy input. As a skeptic, I would want to see more testing to support the claimed improvement in yield per unit of energy from subcanopy lighting.

As anecdote of what I found won't work well, I used to (~2009-2013) design "bud blasters" where I would try to up close and directly illuminate the buds themselves without light blasting the leaves as hard to try to improve yield locally. They were six 3 watt high power LEDs mounted on a small linear heat sink and a 3 layer heavy duty aluminum foil reflector was used to strongly couple the light into the buds. I tried a variety of wavelengths like 660 nm red and including novelty ones like warm white combined with green. They basically didn't work nor should they because they were only producing a relatively small amount of sugar through additional photosynthesis. (those cheaper LEDs I used were around 25% efficient at most compared to lowers 80s today for SOTA).

I was running them maybe 5-8 watts true inches away from buds and there was not much if any difference. I've also tried high power diode lasers with beam spreaders up close to try to really localized on individual buds to see if direct bud blasting works to improve yields. There was no apparent difference in my personal experiments and the equipment was a hassle to work with.

Blasting the lower leaves with light in addition to the top light is the only technique that I know of that will obviously improve yields per area or volume.

If people are interested I could post some pics of what didn't work.

About this paper getting published

This paper was published with MDPI, which is a publisher that does not have a great reputation. Their standards are generally lower than publishers like Springer Nature, and the fast turnaround can lead to rushed and reduced peer review quality. But most MDPI journals are indexed in Scopus, so they are treated as legitimate academic sources.

Whenever I see something published in MDPI, I always ask why since they have had past strong allegations of being predatory. Many early career scientists publish with MDPI which is understandable. Serial publishing in MDPI is generally not how you get academic tenure in most fields, though.

This paper was produced by a private biotech company in Spain, Phytoplant Research S.L.U., and it still got published in an academic journal. That’s pretty normal for applied science and engineering. It is less common in basic plant biology, because the top-tier journals tend to favor work from universities rather than applied commercial studies.

Being a private company might explain why they used MDPI as a publisher.

They were funded through a government grant as part of a larger research project, which does add credibility. These grants often include money to cover the APC (Article Publication Charge) charged by journals, and MDPI’s fees as a publisher are a few thousand dollars. $2000-3000 is what it typically costs the author to get a paper published, higher with the higher-tier publishers.

The paper itself looks solid and the authors seem well qualified. The study was n=24 plants, which is larger than you see in most cannabis studies.

The optical bottom side of a leaf and gas exchange

I frequently see the internets (sic) say that only the top side of a leaf can use light and only the top side has photoreceptors. That’s not true at all. In a plant like cannabis, the main difference is that the top side has higher chlorophyll density and the bottom side has most of the stomata, which handle gas exchange of mainly CO2 and water vapor.

The higher chlorophyll density means that in a healthy dark green high nitrogen cannabis leaf, roughly 90-92 percent of red and blue light are absorbed by the upper surface, and upper 80s to 90 percent in some cases of green light is also absorbed, so just a little less than red and blue (remember that our eyes are about 5 times more sensitive to green than red/blue and we are biased to see green). That is based on my own spectrometer readings on cannabis. The underside of the leaf looks lighter green, so maybe 65-70 percent of green light is absorbed there, with red and blue still close to 88-90 percent at absorption dips. And once your lighting is inside the canopy, it barely matters which side is facing the light because the leaves around it will absorb the photons anyway.

Green light only becomes a potential factor with lighting from below the canopy where you might actually lose more of that reflected light. But this is mitigated if the subcanopy light facing upwards is also on a more reflective surface.

The stomata are pores mostly on the underside of the leaf, and they mainly let in carbon dioxide for photosynthesis and let out water vapor from transpiration and as a byproduct of photosynthesis. It's right around 98% water vapor from transpiration and maybe 2% from photosynthesis. Something close.

The stomata guard cells are blue light sensitive, and higher amounts of blue light will force them to open more than normal. Green light tends to push them toward closing. I am not aware of any studies showing that this blue versus green effect creates meaningful real world differences in plant performance or if their is a difference in gas exchange. I have seen it come up in discussions, though, so I wanted people to understand what the argument actually is. Ask for data and not theory in any discussion like that one way of the other.

Pressure flow hypothesis

You can light the lower leaves to increase photosynthesis in the lower canopy, and those produced sugars can be transported to the buds through the phloem. This is the pressure flow hypothesis:

https://en.wikipedia.org/wiki/Pressure_flow_hypothesis

If you want to easily test the pressure flow hypothesis in action, with a smaller plant maybe topped once, grow it under a light as normal, but leave space on the sides and set up PAR38 bulbs to blast the lower leaves. I have multiple Bridgelux Vero 10 at 5 watts on a 40 mm heat sink that I set up with smaller plants for under lights. Get a light meter with a remote sensor head down there. You are going to need to water more often.

But stuff that works when you’re just tinkering gets much more complex at commercial scale. SCRoG with under-canopy lighting adds labor, adds hardware to maintain, and increases heat load, which drives up AC costs.

With taller plants you have to determine what height the inner light bar is producing the best, and if you should be use multiple heights of light bars.

Safety

If you are going to pack lights into or below the canopy, they either need to be low voltage safe, or fixtures rated for wet locations. It needs to be IP rated for IP65 or above, and should say "Suitable for Wet Locations" on the safety label. Many Mean Well LED and other high quality drivers are going to be rated for this.

All intracanopy lighting should be on a GFCI/RCD circuit. I also tape cord caps so no energized copper can be exposed, using Scotch Super 33+ for long-term durability (from a safety stand point, this is no different than hard wiring a fixture).

In the past, I used custom 24 volt DC lighting powered by a fully isolated Mean Well supply, which is the safest architecture you can use below canopy.

What you absolutely should not do is place junk lights like the Mars Hydro TS600 below the canopy or other lights that do not use an external LED driver. Fixtures like these place line voltage directly on the LED circuit board with no isolation from ground, protected only by a thin conformal coating. That design is dangerous in dry conditions and outright lethal in a wet canopy. It's a stupid design. I discuss testing these lights in my lighting guides. (only the Mars Hydro TS250 is also designed like this)

Also keep in mind that quantum boards are mechanically fragile. When mounted in or below the canopy, it is easy to crack or knock LEDs off the board. If an LED or series segment opens, current can redistribute unevenly across the remaining parallel strings, pushing some LEDs above their rated current potentially causing thermal runaway and cascading failures.

It's best to use lights that have some sort of rigid cover over the LEDs to protect them.

I just got off the phone with tech support on the massive data usage with DriveSafe

Subcanopy and Inter-Canopy Supplemental Light Enhances and Standardizes Yields in Medicinal Cannabis (Cannabis sativa L.)