Do cannabis plants really 'cannibalize' their own leaves?

Mañ'O'Green · Nov 27, 2022

Olderfart said:
There is another complexity to this issue. I have read (but don't recall a link) a research paper that confirmed that any fans seeing less than ~200 PFD are sinks rather than a sources of photosynthetic products, which I take to mean that heavily shaded leaves (at least the ones I have checked with my par meter) might as well be chopped because they are not net producers, and may be using resources better spent elsewhere.

As a consequence, I do not feel bad about thinning the lower canopy to make sure that there is enough air circulation, and I do not worry about whether the leaves are dead/depleted yet. I will provide the plant with what it needs with the salt nutes. Other than ensuring air circulation, I don't remove leaves.

The other aspect to the 200 PFD finding is that any leaf that does received 200 pfd is a producer, and removing it will reduce the photosynthetic production of the plant. @Mañ'O'Green has posted commentary before showing that even his lower buds not receiving significant light do very well in his grows, and that suggests, if not confirms, that production by all leaves is available to a degree to all buds, not just the adjacent ones.

Happy growing mate, and thanks for bringing this interesting topic up.

Yes my lower plant areas do quite well. I attribute most of that to the efficiency of the Orca Grow Film that lines my walls. I have great light penetration. I still had good lower growth in mylar but this Orca really upped my game.

I don't take leaves until late flower for air flow.

Delps8 · Nov 27, 2022

Humanrob said:
If you don't mind my asking, how do you know this is true?

It's in at least one of the Bugbee videos and, IIRC, it's in a research paper that discusses different defoliation strategies.

Delps8 · Nov 27, 2022

Olderfart said:
There is another complexity to this issue. I have read (but don't recall a link) a research paper that confirmed that any fans seeing less than ~200 PFD are sinks rather than a sources of photosynthetic products, which I take to mean that heavily shaded leaves (at least the ones I have checked with my par meter) might as well be chopped because they are not net producers, and may be using resources better spent elsewhere.

That's the "light compensation point", the complement to the "light saturation point", which is the level below which the leaf in a plant consumes more glucose than it can generate. The LSP for cannabis is 63µmols and the only place I've found it published is here. That's just a page on the internet (one of billions) so I took that with a grain of a salt but I noticed that values is about the value of the intercept on the X axis of Figure 1 A in the attached Chandra paper so I think that adds some validity to the 63µmol figure.

Olderfart · Nov 28, 2022

Delps8 said:
That's the "light compensation point", the complement to the "light saturation point", which is the level below which the leaf in a plant consumes more glucose than it can generate. The LSP for cannabis is 63µmols and the only place I've found it published is here. That's just a page on the internet (one of billions) so I took that with a grain of a salt but I noticed that values is about the value of the intercept on the X axis of Figure 1 A in the attached Chandra paper so I think that adds some validity to the 63µmol figure.

Thanks for the links. I will see if I can dig up the paper that suggests 200 micromoles, and I'll post it here if I find it.
Since at the low end of ppfd, Chandra et. al. only tested at 0 and 500 micromoles, I don't think that his graph is that helpful. Extrapolation that far from 500 seems a bit excessive to me. The paper has no information on plant response anywhere near the suggested compensation point, other than at zero micromoles, which does not help. To clarify this issue, several data points showing trends nearby the compensation point on both sides of it are needed.

Anyway, I'll do some digging and report back. :pighug:

Olderfart · Nov 28, 2022

Olderfart said:
Thanks for the links. I will see if I can dig up the paper that suggests 200 micromoles, and I'll post it here if I find it.
Since at the low end of ppfd, Chandra et. al. only tested at 0 and 500 micromoles, I don't think that his graph is that helpful. Extrapolation that far from 500 seems a bit excessive to me. The paper has no information on plant response anywhere near the suggested compensation point, other than at zero micromoles, which does not help. To clarify this issue, several data points showing trends nearby the compensation point on both sides of it are needed.

Anyway, I'll do some digging and report back.

No luck, I can't find the paper that I ran into earlier. I'll still feel free to trim the lower canopy though. :biggrin:

Redeye Master · Nov 30, 2022

Am I stupid in saying that any plant or tree would do the same?
If the leaf/branch is only using energy and not creating energy I always assumed that said plant/tree kind of makes it redundant?
Also…
The more leaves your plant/tree have creates more food? (With Other elements like water obviously).

its weird because I have been growing fruit and veg for best part of 14-15 years (I’m 34) and only in the past year or 2 have I started looking into the actual science of growing so tbh in my head I think “you done pretty well without the science so y now” lol

in my own conclusion…. It’s your grow (regardless of what your growing) do what you thinks best lol

Delps8 · Dec 1, 2022

Olderfart said:
Thanks for the links. I will see if I can dig up the paper that suggests 200 micromoles, and I'll post it here if I find it.
Since at the low end of ppfd, Chandra et. al. only tested at 0 and 500 micromoles, I don't think that his graph is that helpful. Extrapolation that far from 500 seems a bit excessive to me. The paper has no information on plant response anywhere near the suggested compensation point, other than at zero micromoles, which does not help. To clarify this issue, several data points showing trends nearby the compensation point on both sides of it are needed.

Anyway, I'll do some digging and report back.

That would be great if you could dig that up. The only cite I've seen was on that vendor's page and that's only one data point.

The Chandra chart - I agree about the "iffiness" (new word).

The Chandra paper - the LCP didn't get any attention because their focus was on the other end of the scale.

BTW, I took the fact that slope of the curve for net P started to roll off ≈ 500µmols to indicate that the yield curve would also roll off. That's not the case. Check out the yield "curve" (it's a scatter plot) in "Cannabis Yield, Potency, and Leaf Photosynthesis Respond Differently to Increasing Light Levels in an Indoor Environment".

Their yield was pretty low but the curve is 86.8+0.24x. If you fiddle with that in Excel, one of the interesting things that falls out is that they had 4.5%± increase in yield for every 50µmols of PAR.

Delps8 · Dec 1, 2022

Olderfart said:
No luck, I can't find the paper that I ran into earlier. I'll still feel free to trim the lower canopy though.

Same here.

I don't do anything with the under-developed buds so I remove small branches that won't mature into buds. The description from one grower was to remove anything that was thinner than a pencil. That seems to work out pretty well for me.

No question that growth could be used to make edibles and other fun things but even a little 2' x 4' tent can generate a "whole lotta weed" so the extra effort to turn the undercarriage growth into THC isn't worth it for me. Heck, I just tried some herb from my January harvest and I was toasted after just a couple of hits.

stressed · Jan 25, 2023

Humanrob said:
For years I and others have referenced that when a cannabis plant is lacking nutrients in the soil, they will take nutrients from their existing leaves. I realized I have no idea where I got that information, and that while it seems on the surface to 'make sense', I have no idea if it's true. From a botanical standpoint, is this accurate? Or just Bro Science? I tried to do google searches on this and I can't find any information to back that up.

An alternative paradigm would be that the the fan leaves that change color and eventually die when a plant is lacking available soil based nutrients, are simply being abandoned by the plant. This would imply that the plant is able to prioritize where limited resources are distributed, and it focuses on it's reproductive imperative.

This is relevant in terms of defoliation techniques, because either (A) removing leaves removes a resource for the plant in terms of accessible stored nutrients, or (B) we are enabling the plant to direct its limited resources away from the leaves, which take a lot of energy to support, and 'helping' it focus its energy on the buds. Of course, it's entirely possible that neither of those scenarios represent how plants actually work, I really have no idea.

i believe the answer is A. despite the glowing reports from a handful of mostly newish growers, i'll believe it when it's proven, and simple tests do not meet the parameters of a scientific study. time will tell. the good thing is, i grow enough for me, which is the only thing that matters.

i do schwazz my pubes. it works!

happy experimenting! it's what makes us human.

kfigerm · Jan 25, 2023

Hey people, I just came around this topic on another thread also...so here my 5 cents.
schwazzing/defoiling will be beneficial in the one or other way, So why not do it.
For that we need tools to see how our plants do and we like to have some fast reacting indicators, before we have a mess with our plants.
So the lower fans are good reservoirs and yes, they are used when possible(available)/needed.
The loss of one leaf below is less worse than have a mess up. So let them be your tools to estimate whats going on.
So for me, i do defoiling and all the nasty stuff if needed, but try to keep the lower big ones as early indicators..if they are good, all is good. If they get sucked, action!

Do cannabis plants really 'cannibalize' their own leaves?

Mañ'O'Green

Kia ora. Retired to watch the grass grow.

Delps8

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DIY whenever possible, and sometimes when it isn't

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