Net photosynthesis is the amount of glucose generated by a leaf during the process of photosynthesis minus the glucose used by the leaf during the process of photosynthesis.
The Chandra paper was a big deal for a variety of reasons and one of the impacts of the paper is the data presented in the paper/displayed in the chart indicate that there's a rapid decrease in the value of increasing light levels on a cannabis grow. Put another way, the rapid falloff in net P as light levels increase was the law of diminishing returns showing that there was rapidly decreasing benefit in growing cannabis at high light levels.
Reading through the details of the Chandra paper revealed that the measurements of photosynthetic output were taken by putting leaves in a device about the size of a shoe box and measuring the output. That struck me as odd - was it valid to extrapolate the out of of glucose from a few leaves to the yield of a cannabis plant?
My response was "I'm not harvesting net photosynthesis" and I looked around for research that dealt with yield rather than net P from a few leaves in a box.
What was playing in the back of my mind was that emphasis that Bugbee was putting on growing cannabis under high light levels. Bugbee never put forward yield data but everything he talked about was growing at four digit levels (albeit that was in CO2). And I came across the description somewhere (to this day, I can't cite the source) that the light saturation point for cannabis is "800 to 10000µmols, depending on the strain.".
It wasn't until I found "Frontiers in Plant Science - Yield, Potency, and Photosynthesis in Increasing Light Levels" (attached) that, pardon the pun, the light went on. The authors, one of whom is Zheng who is a former Bugbee student IIRC, tackles the issue head on. The key phrase is "plasticity". Search the PDF for that word and that part of the text (one of the many highlights) states that net P of a few leaves cannot be taken as a proxy because cannabis has tremendous "plasticity" when it comes to yield and increasing light levels.
A step aside - essentially all light recommendations "on the internet" appear to rest on the results of the light levels in the Chandra paper. No doubt many of them have never heard of the Chandra paper but the light levels that are recommended are, my word, "modest". The lights at Migro appear to be specifically designed to work at those levels. If you look at the Migro lights, the drivers are lower powered than many of the competing products and if you read the Migro blog, you'll see that Shane stays well away from 1kµmol.
Another source that deserves mention is the data presented in the charts at growlightmeter.com. When I emailed the programmer a few years ago, asking for sources for their recommended light levels, I was told to check for footnotes on the bottom of the page where the data were presented. At the time, there were none and I haven't bothered to check back since for the reason that the research data that's been published is overwhelmingly different than what growlightmeter.com presents on their site.
Back on the research track - the other attached papers support the findings in the Frontiers paper. Their approaches are different but, overall, they buttress the assertion that cannabis yield tends to increase a light levels increase, as long as light is the limiting factor.
The most recent addition to the canon is the work done by Mitch Westmorelan, who is (was?) a PhD candidate under Bugbee. A year ago, he released a pair of videos in which he discusses some of the research that he conducted for his thesis. He makes it quite clear that his research indicates that cannabis yield increases as light levels increase.
This video is one of the two longer ones but he's done shorter, interview-length videos with Shane (Migro) on different topics. The topics are similar but Westmoreland adds little tidbits here and there.
Somewhat of a long response, eh?
The shorter version:
The table below is from the cited paper. If you plot the curve, you'll see that the law of diminishing returns is quite evident but the curve rolls off at a much slower rate than the curve of net P in the Chandra paper.
View attachment 5439959
Clearly, I'm an enthusiastic supporter of growing using high light and I do follow that practice with my grows. I run my grows as close as I can get to 1kµmol on average, or higher, and have no complaints about the results.
Growers have commented that high light tends to reduce the quality of the crop but the research doesn't support that. My belief is that Westmoreland addresses the issue, based on his research in 2021 and subsequent years, which he discusses in the videos I've cited.
The issue with reduced quality that some growers (including me) experience when cannabis is grown under high light is not due to high light levels but is a function of not keeping the temperature of the flower tops to <=78°. The graphic below is from his recent video and it substantiates the results that he reported in his 2021 video.
View attachment 5439960
Grow lighting for cannabis is a very deep topic but the Westmoreland videos, at only 50± minutes in length, are an excellent summary of the body of research that I've been able to dig up.
