Defoliation criteria - 200 mmol/m2/s

Just listened to a Growcast episode from 7/18.

The guest mentioned that there is a study showing that an effective criteria for determining whether to defoliate a particular leaf is 200mmol/m2/s. This is because below this level it takes more energy to maintain that leaf than the leaf provides back to the plant.

Just wondering if anyone has seen this study and where I could score a read - sounds like a very interesting idea. Thanks in advance.

 

[Bruce Campbell]

The guest on the Growcast episode was David Kessler, who is the Chief Science Officer at Agrify. He was referring to "work done by Allison Justice".

One source for this seems to be CANNABIS CULTIVATION AND SCIENCE PODCAST Episode 43 (starting at 38:30), where she discusses defoliation practices while working at OutCo. She uses the term "research" to refer to her reading up on reasons why defoliation is performed, and then field work measurements with Michael Jenkins of Rubisco Consulting who came out with a LI-COR meter. They found that "leaves receiving less than 200 micro-moles were not photosynthesizing efficiently".

She states that Jenkins was going to "publish a paper on this", which may be this one:

Photosynthetic Performance and Potency of Cannabis sativa L. Grown under LED and HPS Illumination
January 2021 Agricultural Sciences 12(03):293-304
DOI:10.4236/as.2021.123019
Authors: Michael W. Jenkins, Curtis B. Livesay

However, she then seems to make a leap from "< 200 PPFD = not efficient" to "this means they were a net sink" (i.e., consuming more than producing). She doesn't really discuss or mention how much they consume normally or whether that is also reduced at lower light levels, etc. She also goes on to say that OutCo was primarily defoliating for humidity control, and they never bothered to investigate effect of defoliation on final yield.

Based on that, 200 PPFD doesn't seem to be a hard / well-established / set-in-stone limit for when that crossover point actually occurs. There have been a number of other studies measuring photosynthesis efficiency vs. light intensity. Here's another one:

Chandra, Suman; Hemant Lata; Ikhlas A. Khan and Mahmoud A. Elsohly (2008) "Photosynthetic response of Cannabis sativa L. to variations in photosynthetic photon flux densities, temperature and CO2 conditions" Physiol. Mol. Biol. Plants 14(4)

But, I haven't run across any yet that then compare it to net leaf consumption for the purposes of defoliation. It seems to be more of a "it's good to defoliate for humidity control, so if you have a PAR meter, go ahead and set yourself a PPFD value to follow."

 

[WildBill]

The guest on the Growcast episode was David Kessler, who is the Chief Science Officer at Agrify. He was referring to "work done by Allison Justice".

One source for this seems to be CANNABIS CULTIVATION AND SCIENCE PODCAST Episode 43 (starting at 38:30), where she discusses defoliation practices while working at OutCo. She uses the term "research" to refer to her reading up on reasons why defoliation is performed, and then field work measurements with Michael Jenkins of Rubisco Consulting who came out with a LI-COR meter. They found that "leaves receiving less than 200 micro-moles were not photosynthesizing efficiently".

She states that Jenkins was going to "publish a paper on this", which may be this one:


However, she then seems to make a leap from "< 200 PPFD = not efficient" to "this means they were a net sink" (i.e., consuming more than producing). She doesn't really discuss or mention how much they consume normally or whether that is also reduced at lower light levels, etc. She also goes on to say that OutCo was primarily defoliating for humidity control, and they never bothered to investigate effect of defoliation on final yield.

Based on that, 200 PPFD doesn't seem to be a hard / well-established / set-in-stone limit for when that crossover point actually occurs. There have been a number of other studies measuring photosynthesis efficiency vs. light intensity. Here's another one:


But, I haven't run across any yet that then compare it to net leaf consumption for the purposes of defoliation. It seems to be more of a "it's good to defoliate for humidity control, so if you have a PAR meter, go ahead and set yourself a PPFD value to follow."

That's probably a pretty logical way to proceed if you want to go further than just defoliating for airflow.

 

[Bruce Campbell]

Below are two relevant plots from the Chandra article.

I believe these are per leaf measurements from leaves on the uppermost portion of the plants. When net photosynthesis goes negative, that means the leaf is respiring instead of photosynthesizing, which is probably the crossover point Kessler and Justice were referring to.

Justice's main point was that the leaves continue to transpire at 0 PPFD, which meant they were still contributing to humidity issues at OutCo.

 

[WildBill]

Below are two relevant plots from the Chandra article.

I believe these are per leaf measurements from leaves on the uppermost portion of the plants. When net photosynthesis goes negative, that means the leaf is respiring instead of photosynthesizing, which is probably the crossover point Kessler and Justice were referring to.

Justice's main point was that the leaves continue to transpire at 0 PPFD, which meant they were still contributing to humidity issues at OutCo.

View attachment 1372458

View attachment 1372459

That does make sense of the growth jump you get when you just open the girl up from simple LST from pulling the stems outwards.

Take two clones and use this 'standard' for defol or LST for one girl.

 

[CannabisMingus]

Take two clones and use this 'standard' for defol or LST for one girl.

Someone should do that (not me - don't have the room/bandwidth). Better yet, do 10 -20 for each group (experiment and control). I'm sure someone will pick up the mantel.

 

[WildBill]

Someone should do that (not me - don't have the room/bandwidth). Better yet, do 10 -20 for each group (experiment and control). I'm sure someone will pick up the mantel.

Same here! Gotta be some 70's porno girls though.
There are some strains that this has little to no impact on, but those aren't the norm.

 

[CannabisMingus]

There are some strains that this has little to no impact on, but those aren't the norm.

 

[Delps8]

I've posted two plots from the publication entitled "Photosynthetic response of Cannabis sativa L. to variations in photosynthetic photon flux densities, temperature and CO2 conditions" written by Suman Chandra, Hemant Lata, Ikhlas A. Khan and Mahmoud A. Elsohly as published by Springer in the journal "Physiol. Mol. Biol. Plants", volume 14(4), dated October, 2008.

These plots differ slightly from what Bruce Campbell posted above, which provide detail lacking in the two that I have posted. What all of the plots indicate to me is that the light compensation point is well below a PPFD of 100 and that gives credence to this page which puts the LCP at 63µmols and the LSP at 780µmols.

It does concern me that 780µmols is stated as being the LSP for cannabis. I've found no research to confirm that and my experience, though limited, indicates an LSP of just over 900µmols for both Gelato autos and Gorilla Glue autos. If the LSP is incorrect, that makes me question the LCP. In defense of the state value of 63 µmols, we do have the Chandra plots that strongly buttress that values as being correct.

I have watched the interview with Allison Justice and was quite taken by how quickly she danced away from her statement about 200µmols. IIRC, the interviewer became quite animated in anticipation of a discussion of LCP but she left him high and dry. There seems to be very little discussion of the LCP.

Re. using the LCP as a criterion for defoliation - that's why I started looking for the value of LCP but, after taking readings with my Apogee, that curiosity quickly ended. I don't have my own data but my data does match what Shane at Migro has reported, to whit the PPFD on the underside of a leaf is about 5% of the PPFD value on the leaf surface. He discusses this in his YouTube video on "penetration". To my way of thinking, there is, essentially, no "penetration" and that's the focus of Shane's video. The only way to get light to the inflorescence below even one leaf is to ensure that there are no leaves above.

Further, using a criterion such as LSP disregards one of the main functions of leaves, that being to act as stores of mobile nutrients. The lower leaves play a significant role in that respect.

Dr. Justice did state/allude to a sound approach for defoliation - remove dead or dying vegetation and prune for air flow so as to reduce chances of mold.

 

[kfigerm]

nice work putting all this together.
This summer I learned a bit about this, I think:

Further, using a criterion such as LSP disregards one of the main functions of leaves, that being to act as stores of mobile nutrients. The lower leaves play a significant role in that respect.

Had/have an ultimate that was fed perfectly and very controlled, when I dropped N too early, but just a little, so I could observe the move of N at this stage very nicely. And what I saw was, that the yellowing happened at the biggest (not the oldest or first ones) lower fanleaves, that grow on the stem directly. Those big ones got drained first.
And it literally went from leaf to leaf. You get a clue how you feed movable nutritients from them and have some time to react till it spreads to other leaves. As long as you feed well, you won't need other leaves as storage and can clean out the canopy for more light. Significant role..yes indeed, but as we are not in nature and can control the feed, the role may be" just" a "warrant canary". By doublequoting "just" I want to stress the importance of watching these leaves together with the new growth to keep the plant healthy. Having them there to watch them is good value!
Leaving them and defoiliating the smaller rest (ref Dr Justice , only if I think needed) in the shadows is what I will do the next runs - now I have some numbers for the light penetration to add to that, thanks!