[spurr]

High,

I haven't posted much lately, but I thought this topic was important enough to write a thread. I have written about this topic quite a lot in the past, but I have never made a thread just for this topic.

My goal: to try and kill the myth that 1,500 ppm CO2 is ideal. I want to get it known in the cannabis world, that it's important to not use > ~1,100-1,200 ppm CO2.

In short, the claim that 1,500 ppm CO2 is ideal for cannabis is total hogwash. I challenged anyone a while ago to find a single scientific study showing 1,500 ppm is ideal for C3 flowering plants, or even just to find the reasons why it's claimed 1,500 ppm is ideal in the cannabis world. I assume the myth (yet another!) came from the liked of Ed Rosenthall or George Cervantes or Mel Frank, etc.

If there is interest in the 'whys' I can explain why it's important to not use > 1,200 ppm, ideally we would use ~1,000-1,100 ppm. The effects from "super-optimal" CO2 concentrations range from reduced rate of photosynthesis, to reduced yield, reduced root growth, reduced stomatal openness, increased photorespiration, etc. In other words, nothing good.

The reason why we should ideally use ~1,000 ppm, is for most C3 species (and C4 I think), CO2 "saturation" occurs at ~1,000-1,100 ppm. That means more than ~1,100 ppm (up to 1,200 ppm) isn't going to help the plants, it's only going to waste CO2 and hinder plants if CO2 is about > 1,200 ppm.

The reason why we should ideally use < 1,200 ppm CO2 is the effect high (super-optimal) CO2 has on "Rubisco activase", the substance that turns inactive "Rubisco" into active Rubsico. At CO2 > ~1,200 (and temp > ~89'F) Rubsico activase is inhibited, which in turn inhibits conversion of inactive Rubisco into active Rubisco. And active Rubisco is needed for high rate of photosynthesis, which in turn leads to high growth rates and high yield, etc.

So, to sum up: It's important to keep CO2 below ~1,200, and to be safe and most efficient, keep CO2 at ~1,000 ppm. Night time CO2 should be < ~500 ppm, unless one is trying to reduce dark respiration and stretch, then upwards of 750 ppm can be used for short periods (otherwise leaf chlorosis can set in).

I can fully cite all those claims, if anyone wants to read the academic lit. For now, here are a few good studies looking at C3 wheat and rice plants:

• Note: 0.0001% CO2 = 1 ppm CO2 = 1 micromole mol^-1 CO2 (mol^-1 is written as "mol-1" below)

CO2 crop growth enhancement and toxicity in wheat and rice
Bugbee B, Spanarkel B, Johnson S, Monje O, Koerner G.
Adv Space Res. 1994 Nov;14(11):257-67.

Abstract

The effects of elevated CO2 on plant growth are reviewed and the implications for crop yields in regenerative systems are discussed. There is considerable theoretical and experimental evidence indicating that the beneficial effects of CO2 are saturated at about 0.12% CO2 in air. However, CO2 can easily rise above 1% of the total gas in a closed system, and we have thus studied continuous exposure to CO2 levels as high as 2%. Elevating CO2 from 340 to 1200 micromoles mol-1 can increase the seed yield of wheat and rice by 30 to 40%; unfortunately, further CO2 elevation to 2500 micromoles mol-1 (0.25%) has consistently reduced yield by 25% compared to plants grown at 1200 micromoles mol-1; fortunately, there was only an additional 10% decrease in yield as the CO2 level was further elevated to 2% (20,000 micromoles mol-1). Yield increases in both rice and wheat were primarily the result of increased number of heads per m2, with minor effects on seed number per head and seed size. Yield increases were greatest in the highest photosynthetic photon flux. We used photosynthetic gas exchange to analyze CO2 effects on radiation interception, canopy quantum yield, and canopy carbon use efficiency. We were surprised to find that radiation interception during early growth was not improved by elevated CO2. As expected, CO2 increased quantum yield, but there was also a small increase in carbon use efficiency. Super-optimal CO2 levels did not reduce vegetative growth, but decreased seed set and thus yield. The reduced seed set is not visually apparent until final yield is measured. The physiological mechanism underlying CO2 toxicity is not yet known, but elevated CO2 levels (0.1 to 1% CO2) increase ethylene synthesis in some plants and ethylene is a potent inhibitor of seed set in wheat.

Super-optimal CO2 reduces wheat yield in growth chamber and greenhouse environments
Grotenhuis T, Reuveni J, Bugbee B.
Adv Space Res. 1997;20(10):1901-4.

Abstract

Seven growth chamber trials (six replicate trials using 0.035, 0.12, and 0.25% CO2 in air and one trial using 0.12, 0.80, and 2.0% CO2 in air) and three replicate greenhouse trials (0.035, 0.10, 0.18, 0.26, 0.50, and 1.0% CO2 in air) compare the effects of super-optimal CO2 on the seed yield, harvest index, and vegetative growth rate of wheat (Triticum aestivum L. cvs. USU-Apogee and Veery-10). Plants in the growth chamber trials were grown hydroponically under fluorescent lamps, while the greenhouse trials were grown under sunlight and high pressure sodium lamps and in soilless media. Plants in the greenhouse trials responded similarly to those in the growth chamber trials; maximum yields occurred near 0.10 and 0.12% CO2 and decreased significantly thereafter. This research indicates that the toxic effects of elevated CO2 are not specific to only one environment and has important implications for the design of bio-regenerative life support systems in space, and for the future of terrestrial agriculture.

Very high CO2 reduces photosynthesis, dark respiration and yield in wheat
Reuveni J, Bugbee B.
Ann Bot. 1997 Oct;80(4):539-46.

Abstract

Although terrestrial CO2 concentrations, [CO2] are not expected to reach 1000 micromoles mol-1 for many decades, CO2 levels in closed systems such as growth chambers and glasshouses, can easily exceed this concentration. CO2 levels in life support systems in space can exceed 10000 micromoles mol-1 (1%). Here we studied the effect of six CO2 concentrations, from ambient up to 10000 micromoles mol-1, on seed yield, growth and gas exchange of two wheat cultivars (USU-Apogee and Veery-l0). Elevating [CO2] from 350 to 1000 micromoles mol-1 increased seed yield (by 33%), vegetative biomass (by 25%) and number of heads m-2 (by 34%) of wheat plants. Elevation of [CO2] from 1000 to 10000 micromoles mol-1 decreased seed yield (by 37%), harvest index (by 14%), mass per seed (by 9%) and number of seeds per head (by 29%). This very high [CO2] had a negligible, non-significant effect on vegetative biomass, number of heads m-2 and seed mass per head. A sharp decrease in seed yield, harvest index and seeds per head occurred by elevating [CO2] from 1000 to 2600 micromoles mol-1. Further elevation of [CO2] from 2600 to 10000 micromoles mol-1 caused a further but smaller decrease. The effect of CO2 on both wheat cultivars was similar for all growth parameters. Similarly there were no differences in the response to high [CO2] between wheat grown hydroponically in growth chambers under fluorescent lights and those grown in soilless media in a glasshouse under sunlight and high pressure sodium lamps. There was no correlation between high [CO2] and ethylene production by flag leaves or by wheat heads. Therefore, the reduction in seed set in wheat plants is not mediated by ethylene. The photosynthetic rate of whole wheat plants was 8% lower and dark respiration of the wheat heads 25% lower when exposed to 2600 micromoles mol-1 CO2 compared to ambient [CO2]. It is concluded that the reduction in the seed set can be mainly explained by the reduction in the dark respiration in wheat heads, when most of the respiration is functional and is needed for seed development.

Super-optimal CO2 reduces seed yield but not vegetative growth in wheat
Grotenhuis TP, Bugbee B.
Crop Sci. 1997 Jul-Aug;37:1215-22.

Abstract

Although terrestrial atmospheric CO2 levels will not reach 1000 micromoles mol-1 (0.1%) for decades, CO2 levels in growth chambers and greenhouses routinely exceed that concentration. CO2 levels in life support systems in space can exceed 10000 micromoles mol-1(1%). Numerous studies have examined CO2 effects up to 1000 micromoles mol-1, but biochemical measurements indicate that the beneficial effects of CO2 can continue beyond this concentration. We studied the effects of near-optimal (approximately 1200 micromoles mol-1) and super-optimal CO2 levels (2400 micromoles mol-1) on yield of two cultivars of hydroponically grown wheat (Triticum aestivum L.) in 12 trials in growth chambers. Increasing CO2 from sub-optimal to near-optimal (350-1200 micromoles mol-1) increased vegetative growth by 25% and seed yield by 15% in both cultivars. Yield increases were primarily the result of an increased number of heads per square meter. Further elevation of CO2 to 2500 micromoles mol-1 reduced seed yield by 22% (P < 0.001) in cv. Veery-10 and by 15% (P < 0.001) in cv. USU-Apogee. Super-optimal CO2 did not decrease the number of heads per square meter, but reduced seeds per head by 10% and mass per seed by 11%. The toxic effect of CO2 was similar over a range of light levels from half to full sunlight. Subsequent trials revealed that super-optimal CO2 during the interval between 2 wk before and after anthesis mimicked the effect of constant super-optimal CO2. Furthermore, near-optimal CO2 during the same interval mimicked the effect of constant near-optimal CO2. Nutrient concentration of leaves and heads was not affected by CO2. These results suggest that super-optimal CO2 inhibits some process that occurs near the time of seed set resulting in decreased seed set, seed mass, and yield.

[spurr]

The Cliff Notes:

In all the studies above, and others I have read on C3 plants, ~1,200 ppm CO2 is the limit to benefits from CO2. And ~1,000-1,200 ppm CO2 is the saturation point for most C3 species. For reference: 1,200 ppm CO2 = 0.12% CO2 = 1,200 micromole mol^-1 CO2.

[spurr]

@ MODS: May I request this thread be made a stickey?

[1969bubba]

Originally Posted by spurr

High,

I haven't posted much lately, but I thought this topic was important enough to write a thread. I have written about this topic quite a lot in the past, but I have never made a thread just for this topic.

My goal: to try and kill the myth that 1,500 ppm CO2 is ideal. I want to get it known in the cannabis world, that it's important to not use > ~1,100-1,200 ppm CO2.

In short, the claim that 1,500 ppm CO2 is ideal for cannabis is total hogwash. I challenged anyone a while ago to find a single scientific study showing 1,500 ppm is ideal for C3 flowering plants, or even just to find the reasons why it's claimed 1,500 ppm is ideal in the cannabis world. I assume the myth (yet another!) came from the liked of Ed Rosenthall or George Cervantes or Mel Frank, etc.

If there is interest in the 'whys' I can explain why it's important to not use > 1,200 ppm, ideally we would use ~1,000-1,100 ppm. The effects from "super-optimal" CO2 concentrations range from reduced rate of photosynthesis, to reduced yield, reduced root growth, reduced stomatal openness, increased photorespiration, etc. In other words, nothing good.

The reason why we should ideally use ~1,000 ppm, is for most C3 species (and C4 I think), CO2 "saturation" occurs at ~1,000-1,100 ppm. That means more than ~1,100 ppm (up to 1,200 ppm) isn't going to help the plants, it's only going to waste CO2 and hinder plants if CO2 is about > 1,200 ppm.

The reason why we should ideally use < 1,200 ppm CO2 is the effect high (super-optimal) CO2 has on "Rubisco activase", the substance that turns inactive "Rubisco" into active Rubsico. At CO2 > ~1,200 (and temp > ~92'F) Rubsico activase is inhibited, which in turn inhibits conversion of inactive Rubisco into active Rubisco. And active Rubisco is needed for high rate of photosynthesis, which in turn leads to high growth rates and high yield, etc.

So, to sum up: It's important to keep CO2 below ~1,200, and to be safe and most efficient, keep CO2 at ~1,000 ppm. Night time CO2 should be < ~500 ppm, unless one is trying to reduce dark respiration and stretch, then upwards of 750 ppm can be used for short periods (otherwise leaf chlorosis can set in).

I can fully cite all those claims, if anyone wants to read the academic lit. For now, here are a few good studies looking at C3 wheat and rice plants:

• Note: 0.0001% CO2 = 1 ppm CO2 = 1 micromole mol^-1 CO2 (mol^-1 is written as "mol-1" below)

CO2 crop growth enhancement and toxicity in wheat and rice
Bugbee B, Spanarkel B, Johnson S, Monje O, Koerner G.
Adv Space Res. 1994 Nov;14(11):257-67.

Super-optimal CO2 reduces wheat yield in growth chamber and greenhouse environments
Grotenhuis T, Reuveni J, Bugbee B.
Adv Space Res. 1997;20(10):1901-4.

Very high CO2 reduces photosynthesis, dark respiration and yield in wheat
Reuveni J, Bugbee B.
Ann Bot. 1997 Oct;80(4):539-46.

Super-optimal CO2 reduces seed yield but not vegetative growth in wheat
Grotenhuis TP, Bugbee B.
Crop Sci. 1997 Jul-Aug;37:1215-22.

Hey Spur
I don`t post much but everytime I check out this site I end up reading your post. I just want to thank you for all your hard work and all the help you have given to many,many growers myself included TY
1969BUBBA

[dizzlekush]

Originally Posted by 1969bubba

Hey Spur
I don`t post much but everytime I check out this site I end up reading your post. I just want to thank you for all your hard work and all the help you have given to many,many growers myself included TY
1969BUBBA

Ditto on that one. thanks Spurr for coming out of hiding and giving us another gem. your scientific approach to everything is very refreshing in this paradigm and myth based "community"

my #1 mentor for sure, and thats with zero private lessons.

[supermanlives]

another great post . thanks for the info

[spurr]

Ethylene ...

A concern with high Co2, especially over ~1,000 ppm, is ethylene buildup. Even low ppm concentration of ethylene gas can affect flower (inflorescence) size, plant growth rate, seed set (for breeders), senescence, etc. If grow rooms or greenhouses are not vented at least once, ex., at the end of the day, and Co2 is used at 1,000 to 1,500 ppm (esp the latter), ethylene can buildup to levels that will lower yield.

Here are a few good resources:

"Ethylene synthesis and sensitivity in crop plants"
Stephen P. Klassen and Bruce Bugbee
HortScience Vol. 39(7) pp. 1546-1552 (2004)



"Ethylene In The Greenhouse"
The authors explain how to detect ethylene, how to take action against it and how to stop problems before they happen
By W. Roland Leatherwood and Neil S. Mattson
April 2010
https://www.greenhousegrower.com/magazine/?storyid=3153




"Ethylene, Plant Senescence and Abscission"
Stanley P. Burg
Plant Physiol. 1968 September; 43(9 Pt B): 1503–1511.
https://www.ncbi.nlm.nih.gov/pmc/arti...00512-0034.pdf

[spurr]

EDIT:

I just fixed a typo, I first wrote that Rubisco activase is hindered when canopy temps exceed just over 92'F, but that's wrong. The correct temp is just above 89'F. So, make sure not to let canopy temp exceed 89'F

[schwilly]

Thanks for the info.

I might have missed it, but can you explain the connection between co2 over 1000 ppm and ethylene build-up?

[spurr]

@ 1969bubba, dizzlekush and supermanlives,

Thanks for the kind words, I'm glad you found this info useful.