Is 1500 μmol really the optimal light density for MJ ?

OK so I am mostly hoping some science buff who could clarify this for me. Based on the (I would assume popular) study I am about to quote, 1500 μmol would be the optimal light density for growing MJ.

Here is my question :

My interpretation of their methodology is that they figured first the optimal light density and then added CO2 and concluded elevated CO2 raised photosynthesis at this optimal light density.

Now I am wondering if it is safe to conclude that putting aside genetic difference, assuming you have the exact same cultivar used in the study, with higher CO2 levels (than the 750 used) and higher PPFD than 1500 μmol, is it not POSSIBLE that the rate of photosynthesis might be higher as well ?

I might be wrong and I am questioning myself based on the conclusion of the study that 1500 μmol is the optimal PPFD. However I am under the impression that the methodology figured the optimal photodensity before adding CO2 but that in adding it, more light might means even more photosynthesis.

Also, I am aware it is actually the PPFD and the temperature (at 30 C) that is considered optimal so I guess my question would be more precisely formulated as :

Did they proved that it is impossible for this specific cultivar to get more photosynthesis at higher PPFD than 1500 μmol, given CO2 levels higher than the maximum tested (750) and the optimal temperature for this hypothetical condition was found ?

Thank you to anyone trying to answer this. Now the study :

Photosynthetic response of Cannabis sativa L. to variations in photosynthetic photon flux densities, temperature and CO2 conditions


http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3550641/pdf/12298_2008_Article_27.pdf

ABSTRACT :

Effect of different photosynthetic photon flux densities (0, 500, 1000, 1500 and 2000 μmol m−2s−1), temperatures (20, 25, 30, 35 and 40 °C) and CO2 concentrations (250, 350, 450, 550, 650 and 750 μmol mol−1) on gas and water vapour exchange characteristics of Cannabis sativa L. were studied to determine the suitable and efficient environmental conditions for its indoor mass cultivation for pharmaceutical uses. The rate of photosynthesis (PN) and water use efficiency (WUE) of Cannabis sativa increased with photosynthetic photon flux densities (PPFD) at the lower temperatures (20–25 °C). At 30 °C, PN and WUE increased only up to 1500 μmol m−2s−1 PPFD and decreased at higher light levels. The maximum rate of photosynthesis (PN max) was observed at 30 °C and under 1500 μmol m−2s−1 PPFD. The rate of transpiration (E) responded positively to increased PPFD and temperature up to the highest levels tested (2000 μmol m−2s−1 and 40 °C). Similar to E, leaf stomatal conductance (gs) also increased with PPFD irrespective of temperature. However, gs increased with temperature up to 30 °C only. Temperature above 30 °C had an adverse effect on gs in this species. Overall, high temperature and high PPFD showed an adverse effect on PN and WUE. A continuous decrease in intercellular CO2 concentration (Ci) and therefore, in the ratio of intercellular CO2 to ambient CO2 concentration (Ci/Ca) was observed with the increase in temperature and PPFD. However, the decrease was less pronounced at light intensities above 1500 μmol m−2s−1. In view of these results, temperature and light optima for photosynthesis was concluded to be at 25–30 °C and ∼1500 μmol m−2s−1 respectively. Furthermore, plants were also exposed to different concentrations of CO2 (250, 350, 450, 550, 650 and 750 μmol mol−1) under optimum PPFD and temperature conditions to assess their photosynthetic response. Rate of photosynthesis, WUE and Ci decreased by 50 %, 53 % and 10 % respectively, and Ci/Ca, E and gs increased by 25 %, 7 % and 3 % respectively when measurements were made at 250 μmol mol-1 as compared to ambient CO2 (350 μmol mol−1) level. Elevated CO2 concentration (750 μmol mol−1) suppressed E and gs ∼ 29% and 42% respectively, and stimulated PN, WUE and Ci by 50 %, 111 % and 115 % respectively as compared to ambient CO2 concentration. The study reveals that this species can be efficiently cultivated in the range of 25 to 30 °C and ∼1500 μmol m−2s−1 PPFD. Furthermore, higher PN, WUE and nearly constant Ci/Ca ratio under elevated CO2 concentrations in C. sativa, reflects its potential for better survival, growth and productivity in drier and CO2 rich environment.

I can't give you a specific direction, but I've read discussions of this research, on this forum I believe. IIRC, 1500 is way high.

Thank you for your reply. I stumbled upon this :
Source : https://www.icmag.com/ic/showthread.php?t=306152

Beta Test Team said:

Oh yea, about PPF (the same thing as PPFD), both Gavita and ePapillion are way off the mark when they suggest 1,000 as the goal. You want between 700-800 at all times, with around 700 for veg and 800 for flowering (to account for daylength in terms of DLI), with the outside range of 600-900.

Not only is 1,000 PPF too much when its provided all day long, with the way Gavita and ePapillion calculate number of fixtures they overshoot 1,000 by normally at least 100, so in reality, some areas of the canopy get >1,100 PPF, which is way too much radiation all day long.

The claim of 1,000 PPF as ideal is flawed, based upon a flawed study on Cannabis. Who's authors now use 700 PPF to grow Cannabis, not the 1,500 PPF they suggested(!) as ideal from their flawed study.

Click to expand...

I was hoping Beta Test Team would pass by this thread and maybe shed some light as to which ways this study was flawed and maybe some sources.

Still, I would be curious to know IF the study was reliable WOULD my interpretation of it and my conclusions be right. Sadly I do not posses the academic background to be confident enough in my own conclusions.

In any case, I appreciate that you pointed me to the concensus that 1 500 μmol is too much.

Also, while searching around I found this too :
Source : https://www.icmag.com/ic/showthread.php?p=4684503&langid=1

delta9nxs said:

Vegetative light schedules

24/0-20/4-18/6, and now 8/4/8/4.

For some time I have been using a vegetative light schedule of 8 hours on, 4 hours off, 8 hours on, and 4 hours off. Or 16 hours of light per day.

My plants are reaching the same size in the same time frame with this schedule as they as they did with the 20/4 schedule I was using immediately prior to the change to 8/4/8/4.

Everyone has seen the numerous threads over the years about which vegetative light schedule is best. They always degenerate into arguments that are never resolved. One person swearing by 24/0 and another saying 18/6 is the best.

The arguments for or against any regime are flawed without considering the amount of light actually received by the plant. And, as we shall see, all the arguments are futile and rendered obsolete simply by using a tool to measure your light.

If you are using powerful lighting at the right distance you need less time to achieve the size plant you desire than someone using weak lighting.

What is the right distance? Dr El Sohly at the University of Mississippi has determined that the maximum rate of photosynthesis for cannabis occurs at 1500 umols of photon bombardment, which is an instantaneous measurement of flow. The only way to determine this is with a quantum sensor. This distance to produce 1500 umols will be different for every type of lighting or bulb.

I can do the 8/4/8/4 routine because I use a 1k hps hortilux during the vegetative period and run my light at 14” to the closest part of the plant, which is the distance producing a 1500 umol flow with my light.

Someone using fluorescents or low wattage mh or hps bulbs in the vegetative stage will have to give the plant more time to reach the same goal. If the lights you use are very weak you may not be able to reach the same growth I get even if you go 8 more hours and give the plant 24/0.

All of this is because of the daily light integral. The total amount of accumulated light a plant gets in a 24 hour or diurnal cycle. The maximum daily light integral for cannabis is not currently known but we know that it is a “high light” plant. We know that the most sunlit places on earth rarely get more than about 60 moles/day and that plants in most greenhouses get no more than about 30 moles/day.

In order to determine the correct vegetative light exposure for your plant with your light you need a quantum meter.

Using one I was able to determine that, at 14”, with my light, I was able to deliver approximately 32 moles in each 8 hour period. Or 64 moles/day. A huge amount of light that , if given in one continuous period, would probably be more than the plant could process.

But I believe that the rest or dark periods allow the plant to process enough photosynthate in between light periods to be able to process that light.


There is really nothing to argue about. Just to
measure.

d9

Click to expand...

That would implies that Delta9nxs used 1 500 μmol with no ill effect.

If the study was reliable, that would means that between 1 500 μmol and 2 000 would a point of were photoinhibition starts occurring (at least that would be what I would assume to have caused the diminishing yield at 2 000 as opposed to simply no gain, if anyone feels like correcting me you are welcome to). Not the point of light saturation, even less the point of diminishing returns.

If I was to assume that the study was flawed and that indeed that value was to be lower than the 1 500 μmol, than an observed reduction in the grow rate would be even more prononced and obvious at 1 500 μmol. Yet it seems it was not the case to d9.

Could it be that the authors of the study and the reason it is now discredited was that they claimed that 1 500 μmol was optimal which would actually be the point of diminishing return. And that would be the reason Beta Test Team suggest 700 μmol would be the ideal light density ?

I know I ask lots of questions and all this might seems like unjustified conjectures but consider this :

You have budget issue and want to get the most bang for your buck and that means you want to go up to the point of diminishing return and no further (in my previously stated hypothesis that would be the 700 μmol.) at which point it gets more profitable to add more light to other plants.

On the other hand if you are limited in your plant count and what you are looking for is getting the most out of every plants and don't care about your electric bill than you want to go further than the point of diminishing return up to the saturation point.

I am mentioning all this because it actually crossed my mind the first time I've read this study, if they figured what gave the highest rate of photosynthesis it could not be called optimal because that would means they past the point of diminishing return.

Anyone sharing any insight to that subject will be greatly appreciated.

R043,

I believe you're on the right track with both PPFD and DLI. I'm thinking that high PPFD (along with CO2) that doesn't exceed 60 DLI should be about optimal. In the screen capture below, I've calculated the DLI from eight Cycloptics Greenbeams in a 4 x 8 tent with Orca on the inside and you can see that it is slightly high during vegetative growth at 17 hours per day. you could reduce the "daylight" or reduce PPFD somewhat. Perhaps the higher PPFD (up to 1,500 PPFD) and reduced exposure time is better based on how things work in the natural world? That's how plants evolved in any case.



The numbers for the 8 Greenbeams is an extrapolation from some modeling that Cycloptics provided to a ICMag member scrappy doo (see below). The DLI calc came from Beta.

Getting close to 3 per at 650-800

Thank you timmur I appreciate your input and the information you share.

Thank you also Buckowens at least I know great results can be achieve at those values.

Still, I am left wondering if :

650 - 800 PPFD is

- the point of diminishing return ?

- the point of saturation ?

- the point at which photoinhibition starts occuring ?

and what are the value of the two missing points.

Not mentioning all my philosophical interrogation regarding the flawed study.

I think I will head up in the lighting section and ask for those value.

In the spirit of sharing, I also intend to ask should the optimal DLI be fixed at around 60 mol. If you play with shorter circadian cycle (for instance flowering under 7 hours lights on and 12 hours off) could you raise the PPFD above the accepted optimal value. So if you would like an answer to that question as well, see you there.

Dr. Mahmoud A. Elsohly is one of the worst examples of somebody scientifically studying cannabis, so much misinformation over his career it is astounding he still holds his position.

R043,
Quick math says that at 1,500 PPFD you'd have to limit "daylight" to about 11 hours. Sounds just about right for an equatorial sativa in flower! I know that doesn't really answer your question, but I don't think the issue has been adequately studied. The other parts of the equation are CO2 levels and leaf temperature. I'm not sure if max DLI goes up with CO2 levels, but PPFD can scale with increasing CO2 and temp.

Follow this link for some additional info on the interaction between light, CO2, and, temperature.

And here.

MrBelvedere said:

Dr. Mahmoud A. Elsohly is one of the worst examples of somebody scientifically studying cannabis, so much misinformation over his career it is astounding he still holds his position.

Click to expand...

i call him dr assholy! he has opened a consulting firm across the street from the U of Miss. and gotten a patent on rectal suppositories as the ideal way to administer cannabis for medical use.

and because he has misused the public trust while being paid with public funds to be the sole legal researcher in the U.S. for many years.

the paper is flawed in several ways. while 1500 umols does produce the highest rate of photosynthesis in cannabis and 2000 umols is the point of photoinhibition the paper does not address the time lag for assimilate to be used or routed by the plant.

over long photoperiods the use of such intense lighting will reach a point of diminishing returns.

as the earth turns under the sun the plant is exposed to various levels of light. giving parts of the plant time to "catch up" while others are being bombarded.

with fixed emitters the bombardment is relentless. you can do it all day long at 1500 umols and not hurt the plant but you won't necessarily get more overall growth by doing it.

i have found that by backing the lights off to about 1000 umols and/or flipping them back and forth during the photoperiod i get better overall growth.

you get a better footprint and more even distribution.

taking 3600 (#of seconds per hour) x 1000 umols of flow x 12 (average photoperiod) you can deliver 43.2 moles per period. a very large amount of light for a plant.

and you are probably delivering it at a rate the plant can use continuously. by that i mean processing the assimilate in a steady manner without eventually overloading the apparatus. overloading and photoinhibition are two different things.

the good dr. did his research with single leaves in a chamber and not with intact growing plants.

there is also the issue of co2 in that paper. he determined that 750 ppm produced the highest rate of photosynthesis found in that study but he only tested up to 750 ppm.

while i believe 1200 and up to be a waste of resources i have found that around 900-1000 ppm produces fast growth.

i set my set point at 750 but with a 50 ppm deadband and the sensor in the middle of the grow it will surge upwards of 900 ppm before the device shuts off.

i get big, heavy, healthy plants at that setting.

i agree with his temperature finding of 86f with co2. but i use it as a median point letting it run anywhere between 84f and 88f.

in my previous location i was running vertical bare bulbs at approx. 16-18" from the plants. 1500 umols. large trees. i installed an extra bulb in each position and backed them off to about 1000 umols. i flipped the two bulbs off the same ballast on a schedule of 3 hours up, 3 down, 3 up, and 3 down.

from harvest records i saw an approximate yield increase of 12-15%.

delta9nxs said:

i call him dr assholy! he has opened a consulting firm across the street from the U of Miss. and gotten a patent on rectal suppositories as the ideal way to administer cannabis for medical use.

and because he has misused the public trust while being paid with public funds to be the sole legal researcher in the U.S. for many years.

the paper is flawed in several ways. while 1500 umols does produce the highest rate of photosynthesis in cannabis and 2000 umols is the point of photoinhibition the paper does not address the time lag for assimilate to be used or routed by the plant.

over long photoperiods the use of such intense lighting will reach a point of diminishing returns.

as the earth turns under the sun the plant is exposed to various levels of light. giving parts of the plant time to "catch up" while others are being bombarded.

with fixed emitters the bombardment is relentless. you can do it all day long at 1500 umols and not hurt the plant but you won't necessarily get more overall growth by doing it.

i have found that by backing the lights off to about 1000 umols and/or flipping them back and forth during the photoperiod i get better overall growth.

you get a better footprint and more even distribution.

taking 3600 (#of seconds per hour) x 1000 umols of flow x 12 (average photoperiod) you can deliver 43.2 moles per period. a very large amount of light for a plant.

and you are probably delivering it at a rate the plant can use continuously. by that i mean processing the assimilate in a steady manner without eventually overloading the apparatus. overloading and photoinhibition are two different things.

the good dr. did his research with single leaves in a chamber and not with intact growing plants.

there is also the issue of co2 in that paper. he determined that 750 ppm produced the highest rate of photosynthesis found in that study but he only tested up to 750 ppm.

while i believe 1200 and up to be a waste of resources i have found that around 900-1000 ppm produces fast growth.

i set my set point at 750 but with a 50 ppm deadband and the sensor in the middle of the grow it will surge upwards of 900 ppm before the device shuts off.

i get big, heavy, healthy plants at that setting.

i agree with his temperature finding of 86f with co2. but i use it as a median point letting it run anywhere between 84f and 88f.

in my previous location i was running vertical bare bulbs at approx. 16-18" from the plants. 1500 umols. large trees. i installed an extra bulb in each position and backed them off to about 1000 umols. i flipped the two bulbs off the same ballast on a schedule of 3 hours up, 3 down, 3 up, and 3 down.

from harvest records i saw an approximate yield increase of 12-15%.

Click to expand...

Great information thanks man.

It depends on the genetics and the kind of light which make a huge difference.

Of course a sativa can handle more light than an indica for example
and plants can handle more light if it comes from more angles
or exposed to strong light from the beginning they can handle it better
or if it is from a broader spectrum
or if the plants have more rootspace
or if the VPG (not VPD) is lower
or if it is more diffused
or of course if it is colder or the RH lower, etc, etc.

IME under ideal circumstances the plants can handle an extreme amount of light, way above the 2000 μmol from the flawed study.
Probably that isn't very optimal, which is dependant on all those and more factors and would differ per situation.

The point of diminishing return is very low, in one study they had a yield of 1.6 gr/w with 270 watt HPS and 0.9 gr/w with 600 watt.
The 330 watt more only yielded 0.3 gr/w.

You may want to play with a longer circadian ritme instead of a shorter one -which won't work anyway- and more diffused light from wider angles based on the remarkable results we get at 53N.
So remarkable Sam still can't believe it, so that has to mean something.

Thank you all for those interesting replies, I have been monitoring closely my other post in the lighting section but forgot about this one until I stumbled back on the wealth of interesting information that was added since.

@ MrBelvedere
I appreciate you confirming the dubious credibility of Dr Elsohly

@ timmur
It's good to know that there is still room to experiment and that those aspect were probably not study enough. I entirely concur that there might not be a linear correlation between to max PPFD given the right temp and CO2 concentration and the max DLI. And I of course agree it is such an interesting topic.

Also thank you for the links, I was already a big fan of Silas Sativarius and everything I've read so far on his blog has been extremely interesting.

I did read already the first link but I must have missed the second one so I really appreciate that you posted it. Its quite relevant to the discussion and confirms what my intuition tells me. I guess most growers would put some lights and then will find the appropriate optimal CO2 levels and then be satisfied with that. That will probably prevent most of them to test adding more lights and raising CO2 (and temp) and lowering RH to the point of noticing an increase in photosythesis and I guess that is why it is generally agreed that certain values are optimal when they could be pushed over. Not mentioning that for a lots of grower it might be a lot more simpler to just add more light and grow space instead of shooting for the maximal gain per space/plants.

So once again timmur I find myself thanking you for the pertinence of your information.

@ delta9nxs
Thank you for this extremely interesting and detailed post. I appreciate you passing by and letting us know you do not use 1500 PPFD anymore as some of my reasoning were based on your post I quoted.

I can't stress enough either how much sens it makes to me to let the plant have some rest period from high intensity in order to process the assimilate. I make the distinction between Photoinhibition and the overload phenomenon you mentioned thanks to the way you explained it and I think it is an extremely valuable information and something to consider when trying to push the limits of production. I am left wondering tho why you chose to reduce the intensity to 1500 PPFD when you could have flip/flopped the lights but let them at that intensity. I am convinced you had good reasons to do so that are most likely related to the DLI you give your plants yet you would still had some benefit from alternating the light source. Yet again, by furthering the lamp to 1000 PPFD you did get more coverage overall and that might have been the reason why you did it.

I have to say it reminded me of the way Heath Robinson was growing his trees, instead of alternating vertically stacked bulb, he had placed them around his tree and would swap between 2 light sources of the 3 every 6 hours. He was interested in maxing GpW.

I also noticed the 750 ppm max of the study and also thought that going further might produce better results. Have you read the second link from timmur ? It is quite interesting. Maybe it will leads you to experiment with higher PPFD again and higher CO2 levels.

In any case, your inputs were extremely valuable so really appreciate you sharing them.

@ Bubbleblower
I was wondering about the point of diminishing return so I appreciate this. I have to say tho, even if the measure is imprecise, the study you mention seems to go against the generally agreed upon 50W to 60W per squ feet for HPS, and that it was somewhere above that value that you find the point of diminishing return.

I also appreciate that you shared your experience with above 2000 μmol, which implies it would be worth to experiment with those high levels given other conditions are maximized.

I do wonder what VPG is. I couldn't find information on google besides a plant virus thing that seems off topic. I am familiar with VPD but I would appreciate some clarification on VPG.

What remarkable results are you referring to at 53N ? I am a bit lower than that but still curious since you mentioned them. Any link will be welcome.

Finally I am not sure why you are convinced shorter light time won't work. I have yet to experiment with it but I've read intensively on different forums and it seems that everyone is getting about the same results, namely that shorter days = faster flowering time and lower yield and that longer days = longer flowering time and higher yield.

Still there are numerous reasons why one would go at it one way or the other. I won't spam my own thread with this but I go in more details about them in this thread (second post) should you be curious to hear them :

https://www.icmag.com/ic/showthread.php?t=312411

Lack of edit button forces me to repost.

THIS : I am left wondering tho why you chose to reduce the intensity to 1500 PPFD

SHOULD HAVE BEEN : I am left wondering tho why you chose to reduce the intensity to 1000 PPFD

I opine I mean I am not a real scientist but I was under the impression that lumens means very little to optimal photosynthesis.
Lumens is the amount of light us humans see.
I mean it can be hard to cypher what with all the terminology in talking light.
But what matters most is (PAR) that stands for photo-synthetically active radiation.
That is the light spectrum that photosynthesis actually takes place.
Actually a cannabis plant will only utilize light energy from 400 thru 700 nanometers.
There are two primary compounds that drive photosynthesis.
Chlorophyll A and B.
These compounds absorb primarily blue and reds light.
Most other spectra are reflected.
Peak absorption points for chlorophyll A occurs @439 NM and667 NM.
Chlorophyll B occurs @469 NM and 642 NM.
Like I said I am self taught and I forget a lot but these things are what we should be basing our bulb choices on.
Not all bulbs will have a PAR chart but they should.
Ratz

Ratzilla said:

I mean I am not a real scientist but I was under the impression that lumens means very little to optimal photosynthesis.
Lumens is the amount of light us humans see.
I mean it can be hard to cypher what with all the terminology in talking light.
But what matters most is (PAR) that stands for photo-synthetically active radiation.

Click to expand...

That's why nobody mentioned lumens, but ppfd and umol, which are used to quantify PAR.

Sativied said:

That's why nobody mentioned lumens, but ppfd and umol, which are used to quantify PAR.

Click to expand...

Sorry this is the first time that I've seen PAR discussed using those measurement.
Thought umol was a measurement of lux.
You talk about cutting edge.
Right church wrong pew.
Ratz

R043 said:

Lack of edit button forces me to repost.

THIS : I am left wondering tho why you chose to reduce the intensity to 1500 PPFD

SHOULD HAVE BEEN : I am left wondering tho why you chose to reduce the intensity to 1000 PPFD

Click to expand...

howdy! for a larger footprint. this whole subject of allocation and distribution fascinates me and i have always wanted to understand it better.

in my last place where i was able to try this up/down flipping i was getting 2.5-3 lb plants regularly.

i'm building a room now with more bulbs around the plant and intend to flip them only once during the period instead of every 3 hours but flip them opposite as in one up and one down instead of both up and both down. also with 4 positions around the plant instead of 3 as i was doing previously.

so 8 total bulbs around each plant. should be interesting. a checkerboard pattern with 20 plants on 6' centers with four light sources around each plant. all vertical bare bulb.

It is a very promessing set up.

I am fascinated as well with the subject of optimal light density and coverage.

You know, all those games (Videogames, boardgames, cards deckbuilding games, etc.) where you are trying to build the ultimate set up. I am convince growing MJ appeals to the same part of the brain those game are.

In any case, I am convinced you will get great result. It won't mean much to you, yet I also would have alternate adjacent light source one Up one Down and so on. And for numerous reasons I am also convinced checkerboard pattern with Vertical bulbs as lateral lighting is the most efficient (along with colliseum vert grow but how wants to deal with that many clones).

As soon as possible I am planning on exactly that type of set up, only going perpetual (with less plant count) and one thing (out of many) I want to experiment with, is optimal additionnal lighting (probably will go Plasma).

The interesting thing with the additionnal lighting on a perpetual set up is that the plant you choose to supplement is actually a specific stage (portion) of the flowering cycle. I doubt that adding extra lighting for a week or two at any point during flowering will give the same increase in yield. Then it means there is a point during the cycle where it'll give more bang for the bucks. My money is on the second and third weeks after flipping for optimal results. But without testing, there is no way to tell.

If I remember your other thread correctly you used quite a high light density in veg right ? I think if you grow threes (like you do with your set up) it is the way to go to max veg time. So how long will you veg before flipping ?

Anyway thanks for passing by, I've read some of your post before and always found them valuable.

Oh, and if I count correctly you are dealing with 40 lights ? That is quite the set up !!