The how to and why fors of CO2 supplementation for growers

[inreplyavalon]

Does the Sanyo work when it is snowing outside?

 

[Lazyman]

Yeah there are inverterless models that work in cold outdoor temps, but do a search for inverterless here and you'll find more ideas.

 

[Ickis]

Lazyman I am back for your opinions and thoughts.

I purchased and set up a sentinel cppm-1. Sweet unit from what i can tell. I have not set up the tank yet but i am reading the CO2 content in the room. I have the door open and the room(almost sealed) has a slight negative pressure. The exhaust in the room next door(veg) is causing it.
My point is that my cppm1 is reading between 600-1480ppm on its display. With the generator not yet powered on are even connected????
Anyone got a clue as to why this may be happening?

Thanks much

I hesitate to chime in because I created a bad debate on another site but your findings are supportive of what I said. The outdoors Co2 is something like 300-400 and in the average home it is higher because of occupants. Humans and pets produce Co2. The average home is around 600-1000 Co2. A home with smokers is 1500-2500. A non-smoking restaurant is also around 1000. A smoking restaurant can be 1500-4500. If you home is next to a highway it will have higher Co2 than average.

Before anyone asks I am not confusing carbon dioxide with carbon monoxide. The only people who need Co2 are people that have a sealed room. If you don't need a sealed room because of security there isn't a need to have one in an effort to use Co2. Pot plants max out on the amount they can use at 1500 ppm. Non smoking in a house can be near 1000. So nonsmokers in an apartment would probably exceed 1500. A house or apartment with smokers is definitely over 1500.

Unseal the room. Draw air from living quarters, use a filter for exhaust and you are set. Your readings before Co2 are 600-1480. I bet you smoke in the house or have a good ratio of people to cubic feet. If you have 1480 air and 1500 is maximum why use Co2.

I think your meter is proving what I researched before.

 

[inreplyavalon]

Thanks for the thoughtful response Ickis. I have found that with the door open and me in the room the Co2 is hovering around 1100. With the door closed and me not in the room, but still with some negative pressure it is hanging around 500.
We have no smokers in the house, occasional joint but mostly get our meds through ingestion.
I don't doubt your theory and think it is a good idea to pull air from your house, unless its winter and you are heating that air...or summer and you are cooling that air.

 

[Enevold]

Your breath(exhalation) is around 300 000 ppm of co2. So you see why hanging out in your room helps...

 

[secondtry]

Hey Lazyman,

Nice write up! However, I would like make some very important points which will effect 99% of growers who use Co2...that they use too much Co2:

(The goal of 1,500 ppm is arbartry IMO and I have yet to find a single source of since as to 'why' the like of E.Rosenthal and G.Cervanties suggest such a ridicules level of Co2. I think they suggest such an obscene amount of Co2 becuase they are pretty much full of BS)
​

I want to keep this post short, I have little free time right now. But suffice it to say Co2 levels for cannabis to reach peak Pn (rate of photosynthesis) and max carbon assimilation is 700-1,000 ppm Co2. About 1,000 ppm is the level of Co2 saturation in many higher plants. Also, an increase in Co2 won't help much if stomatal conductance is low (ie. closed stomata). Thus if one adds Co2 one should also control the VPD (Vapor Pressure Deficit) of leaf to around 0.8-1.2 kPa (see below). Further, high irradiance (PPFD over 1,000) is needed for peak carbon assimilation. That means to really get full benefit out of Co2 we need to control four things:

1. correct irradiance = high, over 1,000 PPFD
2. correct VPD = lowish [sic], 0.8-1.2 kPa
3. correct Co2 concentration = 700-1,000 ppm
4. correct temp = under high irradiance and Co2 is 30'C

The reason we don't want to use Co2 over about 1,000 ppm is that figure offers about max Pn in cannabis, and over about 1,200 ppm will reduce active "rubisco" (RuBP) and active RuBP is needed for photosynthesis and high Pn. Not to mention Co2 saturation is around 1,000 ppm. The problem with high Co2 is inactive RuBP is converted to active RuBP via. "RuBP activase" and RuBP activase is reduced by two things: 1) high heat (>89'F); and 2) high Co2 (~ > 1,200 ppm). Thus if one lowers amount of active RuBP that grower will also lower Pn. So if one adds 1,500 ppm of Co2 they are actually LOWERING Pn, not helping the plant! (see below)


Here is the effect of Co2 upon Pn of higher plants like cannabis:

Here is the effect of temperature and UV-b irradiance upon Co2 assimilation of cannabis:

--------------------------------------------------------------

About RuBP activase:

A decrease in active RuBP limits Pn and Pnmax because activated RuBP is a limiting factor of photosynthesis. The main chemical that activates RuBP is RuBP "activase". If RuBP activase decreases there is a resulting limit to activated RuBP because RuBP activase is what turns inactive RuBP into active RuBP. Total RuBP is not thought to cause "midday depression", only the level of activated RuBP is thought to be the culprit. It then follows that we want to limit the decrease of RuBP activase and lucky for us that is a trivial task. RuBP activase is decreased under two conditions: 1) high heat and 2) high Co2. I have found studies showing over 1,000-1,200 ppm can reduce RuBP activase and that matches Co2 saturation levels for cannabis (which maxes out at about 1,000 ppm of Co2). Thus to prevent a decrease in activated RuBP we should keep temperature under 89'F and keep Co2 levels below 1,200 ppm (I use 1,000 ppm).




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On the topic room RH and Vapor Pressure Deficit (which effects Pn, transpiration, carbon assimilation, etc):


We should probably be using VPD for measurement, not RH. When growing plants like cannabis we want a VPD of 0.8-1.0 kPa, i.e., 8-10 mb(ar). Below I will quote heavily from a paper I attached to this post entitled: "Understanding Humidity Control in Greenhouse". When keeping VPD between 0.8-1.0 kPa we should be able to lower the intensity/amount of "Pn peaks" throughout the day and allow for greater Pnnet (the net rate of photosynthesis over a day), also mitigating midday depression. The problem is that keeping VPD from 0.8-1.0 kPa means a high RH, around 70-80% in some instances.

VPD effects the opening and closing of leaf stomata, adjusting VPD to 0.8-1.0 kPa increases the openness of stomata and thus the amount of transpiration, Co2 uptake and rate of photosynthesis, etc.

I have yet to do a grow with a VPD as low as 1.0 kPa but this next grow I am shooting for 1.0-1.2 kPa which means I will need to use humidifiers (impeller style are best) to raise RH and also I will need to exhaust of air on regular basis, maybe once an hour or once every two hours after which point the Co2 tank will turn on to raise the Co2 to 1,000 ppm. My main worry of course is high humidity in late flowering but I will try to keep RH below 75% with strong air movement and reoccurring air changes. At the bottom of this post I offer links to products which allow for fully automated control of RH and temperature, and hence VPD in a room...on the cheap.

The most accurate way to calculate VPD is to use air temperature, RH and leaf temperature. According to E. T. Linacre (1967) leaf temperature is higher than air temperature when air temperature is lower than the so-called "equality temperature" of 30'C (86'F), and in a paper by Nobuoka Takshi et al,. (circa 1997) the authors explain how an increased air movement lowers the temperature of the leaf and increases transpiration. According to Chandra, et al., (2008) ideal air temperature for Pnmax of cannabis is 30'C when irridiance is 1,500 PPFD.


Here are two different scenarios where I took VPD into consideration, most growers should fit into either of the daylength category; these are just very simple examples, YMMV (calculated with this page: link)

(the following data was calculated from referenced studies on cannabis (i.e., ideal PPFD, Co2 and air temp) and higher green plants (i.e., VPD), I didn't randomly choose these data except for RH and leaf temp-tho both the latter are based upon science)

[Daylength]
Here is what seems to be reasonable figures (although it should be a range) for achieving ideal VPD while also achieving ideal PPFD, Co2, air temp...all of those equate to Pnmax and Pnnetmax of cannabis:

• VPD = 1.03 kPa
• PPFD = 1,300-1,500
• Co2 = 800-1,000 ppm
• Air relative humidity = 70%
• Air temperature = 30'C (86'F)
• Leaf temperature = 29'C (84'F)

[Daylength]
Here is my best judgment on ideal VPD (although it should be a range) in less than ideal PPFD which means lower air temperature (and decreased Pn in cannabis):

• VPD = 1.19 kPa
• PPFD = < 1,200
• Co2 = 300-400 ppm (no addition of Co2)
• Air relative humidity = 75%
• Air temperature = 25'C (77'F)
• Leaf temperature = 27'C (80'F)

"Understanding Humidity Control in Greenhouse"

What is Vapor Pressure Deficit?

Relative humidity is still the most commonly used measurement for greenhouse control, even though it is not a perfect indication of what the plants ‘feel’. Plants respond to the difference between humidity levels at the the leaf stomata and the humidity levels of the surrounding air. At the same relative humidity levels, but at different temperatures, the transpiration demand for water from the leaves may be double (See Figure 3.) Therefore, another kind of measurement, called the Vapour Pressure Deficit is often used to measure plant/air moisture relationships. Some environmental control companies now offer VPD measurements as a part of their humidity management programs.

Role of Humidity

The main plant mechanism for coping with humidity is the adjustment of the leaf stomata. Stomata open and close in response to vapour pressure deficit, opening wider as humidity increases. When humidity levels drop to about 8 grams/m^3 (12 mb VPD) the stomata apertures on most plants close to about 50% to help guard against wilting. This also reduces the exchange of C02, thereby affecting photosynthesis. Tables 1 & 2 (See Page 8) outline vapour pressure deficits (the difference between saturated air and air at various relative humidities). Although different crops vary in their response to humidity levels, a VPD range of 8 - 10 mb has been suggested as an optimum range. VPD can be used for both dehumidifying and humidifying, but it is particularly useful for humidifying.

Photosynthesis

Humidity levels indirectly affect the rate of photosynthesis because C02 is absorbed through the stomatal openings. At higher daytime humidity levels, the stomata are fully opened allowing more C02 to be absorbed for photosynthesis. Photosynthetic levels can vary by about 5% between VPD’s of 2-10 mb.

Products Needed:


1. Accurate hygrometer:

I like synthetic hair models, below are a few good models/brands. This comes calibrated but I like to double check and adjust if needed as so: fully soak a towel then wring it out so it's fully wet but not dripping water, then wrap it around the hygrometer fro 60 minutes, remove the hygrometer and adjust the hydrometer to 99% RH (or use the salt method).

Hygrometer/Thermometer ($60) accurate to +/- 5% and +/- 2' F
http://www.partshelf.com/taylor5565.html

Hygrometer($41) accurate to +/- 5%
http://www.a3bs.com/weather/precision-hair-hygrometer-u14293,p_83_860_862_0_1065_image_full.html

Hygrometer($75) accurate to +/- 4%
http://www.robertwhite.com/cgi-loca...0&categ=rw_thermo_hygrometers&nav_bar=weather
​

2. Infrared Leaf Thermometer ($35):

http://www.horticulturesource.com/product_info.php?products_id=4657



3. Humidifier impeller style: (reduce VPD)

I use 2 in a room that is 10x12 to raise RH over 80%, can get these at Walmart, etc:
http://www.kaz.com/kaz/store/product/77e8cc17f66a13b695100097a33ddebc/



4. Humidistat and thermostat (auto-control of humidifier and heater):

• I have used this for mycology and it worked well, I spoke with ZooMed and they claim a +/- 3% accuracy in RH score. I also use a ceramic space heater rated under 1,000 watts with this controler so when the RH drops below 70% the controller will turn on the humidifiers and turn off the humidifier once RH is at 70% and if the temperature drops below 30'C the controller will turn on the space heater and turn it off once the temperature is at 30'C.
  http://www.reptiledirect.com/zoomed-hygrotherm.aspx $75.00

• I would highly suggest this unit over the Zooomed, this unit can handle higher wattage heaters too: Sectinel EVC-1 Environmental Controller
  http://www.hydroponics.net/i/199987 $170

P.S. I have lots of references in cast anyone is interested in reading more.


I hope someone finds this as interesting as I do

 

[secondtry]

P.S.

I am guilty of not reading this whole tread (sorry!), so if this has already been covered please forgive my repeating of known info.

 

[sparkjumper]

Ickis I do this in my vegroom,and yes co2 inside is defiantely higher inside than outside,but to achieve real enrichment you cant have the swings that are going to occur within the 12 hour period.Its just like growing normally without enrichment if its not on a controller

 

[Surrender]

Incredibly interesting stuff secondtry! Gonna take a few cycles for me to absorb all that though.

Have dual-stage regulators caught on yet? Apparently they are less prone to freezing and more accurate towards the end of the tank. Popular with the reef aquarium guys:

Two stage regulators will continue to hold their working pressure steady until the CO2 cylinder is empty unlike single stage regulators which will increase their working pressure when the cylinder nears being empty.

 

[oregon024]

Just bought co2 tanks 2 weeks ago.Good thread really helped!

 

[secondtry]

Hey Lazyman,

I wanted to address some info in your first post, some info I don't think is correct (or at least useful in terms of what variables were not accounted for in the studies, ie. VPD, etc). I could be mistaken so I would like your response if you see something I posted that is not correct, etc. Thanks!

The Effect of Temperature on Plant Response to Higher Levels of CO2

Photosynthesis consists of chemical reactions. Chemical reactions procede at a higher rate at higher temperatures. The rule of thumb is that there is a doubling of the reaction rate for every 10°F rise in temperature. Plants grow faster at a higher temperature providing they have adequate levels of CO2, water, sunlight and plant nutrients. The C4 plants have a great response rate for a higher temperature than does the C3 plants.

That is true, but plants have a heat limit and for cannabis under high irradiance and Co2 it's 30'C ambient temp.

A higher temperature without adequate level of the necessary ingredients for growth might produce no response or even damage. Sylvan Wittwer, quoted above, states that under most circumstances the availability of CO2 is the factor which limits growth. Thus with a higher level of CO2 in the air plants can grow faster with a higher temperature.

I would not agree with that, it's the synergy of Co2, light (irradiance), temp, VPD and soil-water deficit (ie. the amount of plant "available water"; not all water in media is useful for plants) that limits growth.

Plants transpire water vapor to keep an even temperature. There are tiny holes on the underside of plant leaves, called somata, which are the openings through which the plant absorbs CO2. With higher level of CO2 concentration in the air the somata do not have to be open as wide. The narrower opening means that less water is transpired and thus less water is required by the plants.

This is why we should control VPD: we can have wide open stomata which increases Co2 assimilation and Pn while reducing water loss as transpiration.

In other words, higher levels of CO2 increase the efficiency of water use by plants. This was confirmed in experiments reported by K.E. Idso and S.B. Idso.

Would you mind offering links? Or full references so I can find those works? I would not agree with that statement, there are many factors which effect water use (ie. "WUE"; water use efficiency) of plants, most notability is VPD.

They found that enhanced CO2 increased growth by 31 percent in plants with adequate moisture but it increase growth by 62 percent for plants in moisture-stressed condition.

What type of moisture-stress? Soil-water deficit and available water (ie. 1-10 kPa water tension) stress?

In effect, enhanced CO2 by reducing water loss created the same effect as providing more water. Thus the effect in moisture-stressed plants was the effects of enhanced CO2 plus the effect of increased water.

I don't understated that statement, could you please clarify a little? We don't want to stress cannabis at all, thus we should control VPD (and in turn water loss and WUE) which allows for greater Co2 assimilation and less stress and greater Pn.

The effect of increased CO2 in narrowing the stomata of plants has the additional benefit that a lesser amount of pollutants in the air will make it through the narrower openings. Thus enhanced CO2 has the effect of protecting plants against damage from air pollutants such as ozone or sulfur dioxide.

Increasing Co2 doesn't close or narrow stomata AFAIK, soil-water deficit, amount of available water, and VPD are the biggest controllers of stomata. Also, open stomata doesn't necessarily mean increased pollutants to enter leaf because the leaf has protection mechanisms as "aqueous pores" and "cuticle layer", both help protect leaf from pollutants.

The effect of enhanced CO2 is even greater for plants grown under low light conditions. The enhance growth is greater than 100 percent for a 100 percent increase in CO2. This compares to less than 50 percent for plants grown in normal light conditions.

That is contrary to all info I have read, high irradiance increases Co2 assimilation. Please see the following pic about effect of PPFD (and UV-b) upon Co2 assimilation of cannabis:

The evidence that clinches the argument is that some greenhouse owner artificially elevate the CO2 level to triple what the level in the atmosphere is.

Yes I agree that is great, up to about 1,000 ppm...if one also achieves the other goals I laid out (ie. high irradiance, temp, VPD, etc).


All the best

 

[secondtry]

So having the CO2 high during the last couple of weeks is not detrimental in anyway, (ie inhibits trichome maturity) it's basically just a waste??

Is your veg room sealed too or are you exchanging the air through some sort of ventilation separate from your flower room? I ask because my entire grow area is going to be one big sealed room divided into sub rooms and the whole area has to share the same air. So if I'm running 1200-1500 CO2 ppm in flower that air (because it has the AC) has to be circulated through the veg room to cool it. So I'll have 1500 in my veg/clone room.

Unless there is a way around this. Ideally, I'd love to have 300-500 in my veg and 1500 in flower. I think that's asking a lot though. I won't really know until I get this shit hooked up. I might be over thinking it??

Keep Co2 high until harvest, plants photosynthesize their whole life time (even the flowers offer photosynthesis just like leafs!). There is no need to lower or cut Co2 in the last few weeks, that's the time when most bulk is gained. Also, like I wrote above, don't exceed 1,000 ppm (but use that level for veg and flowing); the old 1,500 ppm is BS (I would love to know where the myth started!).

 

[secondtry]

Hey again Lazyman,

The only place on plants where they uptake O2 is through the roots, so I don't think high atmospheric levels of O2 are ever really beneficial, in fact high levels of O2 are toxic to plants, since it is a waste product to them really. I think an airtight grow would be fine on the O2 front, since the plants "exhaust" plenty of it during the dark cycle. Does that make sense?

That's not accurate, leafs also take up O2 by "photorespiration" (RuBP using O2, not Co2) and that's a sure sign the grow room has problems. IIRC photoresption can be caused by photoinhibiton (too much light), too much heat, more ambient O2 vs Co2, etc. The main biological action is when the Co2 in the leaf is low (ex. < 50 ppm) the leaf start photorespiration with O2.

We want to avoid photorespiration at all costs so adding Co2 is wise.

HTH

 

[Lazyman]

Keep Co2 high until harvest, plants photosynthesize their whole life time (even the flowers offer photosynthesis just like leafs!). There is no need to lower or cut Co2 in the last few weeks, that's the time when most bulk is gained. Also, like I wrote above, don't exceed 1,000 ppm (but use that level for veg and flowing); the old 1,500 ppm is BS (I would love to know where the myth started!).

Do you have documentation on why not to exceed 1000 ppm CO2? I have read a number of studies on cannabis showing optimal uptake at 1500, so please share with us.

Thanks

 

[secondtry]

On the topic of the least expensive Co2 controller:

I found that the least expensive (non-fuzzy) Co2 controller is the HydroInnovations Co2 monitor for ~$285: http://www.horticulturesource.com/product_info.php?products_id=7772

The problem is that unit is preset for 1,300-1,500 ppm, which as I have shown is WAY too high. I spoke with the HydroInnvoations people and they are emailing me a document which shows how to hack the monitor so one can set their own ppm goals, which for cannabis should be 700-1,000 ppm. However, if one hacks the unit the warranty is void, so it's best to allow the monitor to acclimate, make sure it's working and then hack it. That way if it's not working before you hack it you can return it, once you hack it your shit outta luck if it breaks...and the HydroInnovation people said they will offer ZERO tech support to people who hack their units.

Considering the CAP PPM-3 is from ~$380-480 I think the HydroInnvoation option is good one for ~ $280.

P.S. They HydroInnovatoin people agree 1,300-1,500 ppm is too much and when quarried the reason they set the level there is that is what people ask for..even tho the people are wrong (and HydroInnvoaitns agrees with me but they are marketing to uninformed consumer wishes)


HTH, I am ordering one in two weeks and I will make a thread with picks of my hacking exploits!

 

[secondtry]

Hey LM,

Do you have documentation on why not to exceed 1000 ppm CO2? I have read a number of studies on cannabis showing optimal uptake at 1500, so please share with us.

Thanks

See my previous posts and comments, it's all there. I will post references in a little bit. But the main reason is over 1,200 ppm lowers rubisco activase (which lowers active rubisco which lower Pn) and 1,000 ppm is the Co2 saturation level of cannabis (drug type); it is also the saturation level for most other higher terrestrial C3 green plants.

Could you post the references to the studies you have read? I have tried to find the reasons why someone would suggest 1,500 ppm for cannabis and I have yet to find one.

Thanks! And thanks for being open minded, I am not trying to be a dick, I am just trying to correct misinformation (not from you, but from the like of E.Roshenthall, G.Cervantes, et al.)

All the best (ps. I am the same guy who wrote all the info about AM fungi in your sig )

 

[secondtry]

@ all,

If one buys the HydroInnvoations Co2 controller and hacks it, and buys the "Sentinel EVC-1 Environmental Controller" ($170; link) to control VPD and room temp they would spend ~ $450 for the same technology (minus the fuzzy-Co2) as the "Sentinel CHHC-1" which costs ~$650. A savings of ~$200 for about the same technology !!!

HTH

 

[*mistress*]

The problem is that keeping VPD from 0.8-1.0 kPa means a high RH, around 70-80% in some instances.

My main worry of course is high humidity in late flowering but I will try to keep RH below 75% with strong air movement and reoccurring air changes.

nice tech secondtry...

this may be helpful, to those gardeners that are unfamiliar w/ vpd:
RH importance
https://www.icmag.com/ic/showthread.php?t=112589

did a tech there on rh, or vpd...

basically, the higher the rh, the greater the actual water pressure (as vapor) is on the leaves. this will affect the amount of water taken up & transpired by the plant...

high vapor pressure deficit = low rh

low vapor pressure deficit = high rh

krusty was 1 of the 1st to actually recommend higher rh for gardening... he consistently went for 70%+...
however, this will decrease the amount of transpiration & amount of water drawn up by plants...

the strong air movement is very crucial if running higher rh... this carries away the water that plants are constantly giving off during day (lights on).

if a garden is fed w/ 5 gallons of water, that same 5 gal, or @ least 4 1/2 gallons, should be dehumidifed w/in 24-36hrs... w/ a high vpd (low rh)... as plants generally cycle thru 95% of the solution they are given w/ing 24-48hrs, the remaining 1-5% being the fertilizers supplied.

this internal water pressure is controlled by actual leaf temps, root temps & ambient temps...
stomatal opening & closing is not linear... also, plants close their stomata during mid-day, or the hottest point of the day... then re-open them... increased levels of c02 may delay the opening &/or re-opening of the stomata...

water travels thru the plant & up thru those veins on the underside of the leaf, into the pores on the leaf surface, & out into the environment... if that water is not carried away (dehumidifed) it will cause the plant to slow its water uptake, & potentially cause issues.

why? the external pressure is too great to overcome to push out the water...
this will decrease transpiration & lead to less water (& nutes) being taken up...

this is how a gardener can basically determine the health of their garden... by how efficiently the plants are transpiring... if feed 5 gal of water, that same water in bucket in 24hrs... faster w/ higher temps. if that water not in bucket,then either a) ac/fans went off; b) too cold in room...

run fermentation c02... & yes, breathing does help as well...

c02 is slightly different topic than vpd, but they both variables in total controlled environment.

there is a balance that seems to be required, between temps & rh (vpd) & amount of water given to plant. they really prefer a slight dry period between watering... this makes them draw up all possible water in the basin/trough/bucket... they also prefer somewhat drier air (lower rh)... but, if can mng slightly higher rh (60%+), good to keep fans, etc running 24/7 - thru the canopy, under the canopy, over the canopy... & dehumidification a must.

hope this helps. enjoy your garden!

 

[Lazyman]

Hey Secondtry, yep I'm looking, most of it was years ago so its proving to be tricky. I did see that decreased rca levels are more a result of high temps than high CO2, but without graphed results it's tough to tell how it slopes. Will keep you posted.

oh and thanks much for the myco tech stuff, saved me a bunch of money!

 

[secondtry]

Hello Mistress,

(thanks for the nice and good post, I hope we can let bygones be bygones )

this internal water pressure is controlled by actual leaf temps, root temps & ambient temps...
stomatal opening & closing is not linear... also, plants close their stomata during mid-day, or the hottest point of the day... then re-open them... increased levels of c02 may delay the opening &/or re-opening of the stomata...

I have already figures out how to prevent "midday depression" and "multi-peaked Pn" when growing cannabis indoors and it's all about VPD, etc (ie. what you are referring to, sometimes called "noon-break phenomenon").

Outside there is little one can do about midday depression and multi-peaked Pn, but indoors and in greenhouse we are in full control to insure the highest Pn all day long.


----------------------------------------------

This helps explain what we are talking about to others (I wrote this for the LED vs HID thread)


A very quick into:
Midday depression is when rate of photosynthesis (Pn) drops as does carbon assimilation, etc. The main goal of a grower should be to reduce the intensity and/or occurrence of midday depression and the intensity and/or number of Pn-peaks. The goal is to have plants achieve high Pn all day long at a fairly consistent rate. The flowing paper is very good and covers the topics really well, the paper offers more info of directly applicable nature than I have found so far.

The following paper is only in hardcover so I will go get the book tomorrow and photocopy the paper, then I will upload it. It is definitely worth the effort, one can read the whole paper on Google-book, well every page but the second page. That paper just answered a lot of the questions and I a few assumption I had.


Da-Quan Xu and Yun-Kang Shen

"Midday Depression of Photosynthesis"
Handbook of photosynthesis. v. 1996 - 1997. pp 451-460.​

http://books.google.com/books?id=MU...hotosynthesis&q=noon#v=snippet&q=noon&f=false

I. Introduction

Midday depression of photosynthesis occurs in many plants and significantly affects crop yields. Since it was discovered at the beginning of the century [1], many studies have been carried out, and several hypotheses, such as feedback inhibition of photosynthesis resulting from assimilate accumulation, stomatal closure, enzyme deactivation, and reversible decline in photochemical activity, have been proposed to explain the phenomenon [1-4]. In recent years, midday depression has been scrutinized with modern techniques. However, its casual mechanism is still not established [4]. Based on available data, the ecological, physiological, and biochemical factors related to midday depression are analyzed and the possible mechanisms and adaptive importance are discussed in this chapter.

II. THE PHENOMENON

A. Pattern of Diurnal Variation for Photosynthesis

Under natural conditions there are two typical patterns of photosynthetic diurnal course [5]. One is one-peaked, i.e., net photosynthetic rate increases gradually with the increase in sunlight intensity in the morning, reaches its maximum around noon, then decreases gradually with the decrease in sunlight intensity in the afternoon. Another is two-peaked, i.e., there are two peak values of net photosynthetic rate, one in late morning and another in late afternoon with a depression around noon, the so-called midday depression of photosynthesis, as shown in Figure 1 (curves 1 and 2).

---------------------------------------------------------

Here is who I will prevent midday depression and multi-peaked Pn:



(Here is the much more current paper vs. the one from my previous post above ^^^. I scanned and turned into a PDF and uploaded to my IC account: "External and Internal Factors Responsible for Midday Depression of Photosynthesis")

Midday Depression and multi-peaks of diurnal (daily) net rate of photosynthesis (Pnnet):​

Many outdoor and greenhouse plants experience what is known as "midday depression": when Pn (rate of photosynthesis) drops around midday. If the Pn does not increase later in the day the daily Pn flux is a one-peak graph. If after midday depression the Pn increases (ex. due to decreased VPD (Vapor Pressure Deficit) in the evening) the daily flux of Pn is a multi-peak graph, usually having two peaks. When growing plants inside we are still concerned about midday depression and multi-peak Pn, however, we have great control over the environment, and thus over the occurrence of midday depression and multi-peak daily Pn. If we strive to prevent midday depression the daily Pn flux will be a one-peak and flat rate after that (once "steady-state photosynthesis" is reached).


The reason we want to prevent midday depression is the higher the Pn the greater potential growth, healthy, quality and yield of cannabis. It seems surprising easy to prevent midday depression.


Below I will list what is thought to cause midday depression of Pn:



Main Physiological Cause:

• Decrease in leaf stomatal conductance, usually by closure of stomata.

Main Biochemical Cause:

• Decrease in activated rubisco (RuBP)

• Decrease in PS II QE (Photosystem II Quantum Efficiency), e.g., when a leaf uses photons within PAR range less efficiently to drive photosynthesis, often caused by light saturation and resulting photoinhibition.

Main Environmental Causes:

• VPD = when VPD is too high the stomata close decreasing stomatal conductance.

• Soil-Water Status = when the media is dryish [sic] (i.e. low plant "available water") the indirect result is decreased stomatal conductance thought to be from an increase in ABA (Abscisic Acid).

• Light Saturation = too much PPFD is not good; do not exceed 1,500 PPFD for cannabis to be safe (this should not be a concern for growers who use less than 1,000watt HID)

• Activated Rubsico (RuBP) = a decrease in active RuBP limits Pn and Pnmax because activated RuBP is a limiting factor of photosynthesis. The main chemical that activates RuBP is RuBP "activase". If RuBP activase decreases there is a resulting limit to activated RuBP because RuBP activase is what turns inactive RuBP into active RuBP. Total RuBP is not thought to cause midday depression, only the level of activated RuBP is thought to be the culprit. It then follows that we want to limit the decrease of RuBP activase and lucky for us that is a trivial task. RuBP activase is decreased under two conditions: 1) high heat, over 89'F is bad; and 2) high Co2, I have found studies showing over 1,200 ppm can reduce RuBP activase and that matches Co2 saturation levels for cannabis (which maxes out at about 1,000 ppm of Co2). Thus to prevent a decrease in activated RuBP we should keep temperature under 89'F and keep Co2 levels below 1,200 ppm (I use 1,000 ppm).

Environmental Factors That Are Not Causes:

• Daily Light Integral = the total irradiance per day is not a cause of midday depression, however, PPFD is a cause in the form of photoinhibition due to light saturation.

• Circadian Rhythm = does not seem to be a factor of midday depression

• Co2 Level = does not cause midday depression directly, but can be a cause of reduction in activated RuBP.

How to Prevent Midday Depression:

• Lowish [sic] VPD = 0.8-1.3 kPa

• Soil-Water Status = keep media moist, do not let it go lower than about 45% moisture content (by wet weight gravimetric basis; see link in my sig for more info on how to water)

• Light saturation = do not exceed 1,500-1,600 PPFD for cannabis.

• Active RuBP = keep temps at or below 30'C if using Co2 and keep Co2 at or below 1,000 ppm.

• Misting Plants = when leafs are misted it helps increase stomatal conductance and Pn. I wrote earlier that I use an hour of dark at midday in the thought it will reduce the midday depression; well I now know the hour of dark is not of much help at all to reduce midday depression. However, the hour of dark is a great time to spray plants to increase stomatal conductance (and help prevent midday depression of PS II QE). If we use an hour of dark per daylength, or maybe two half-hour instances of darkness at which time we would mist the plants it could be helpful.

This graph shows three different daily (diurnal) Pnnet (net rate of photosynthesis), the first curve is a one-peak diurnal Pnnet, the second curve is a two-peak diurnal Pnnet, and the third curve is a one-peak diurnal Pnnet with drastic midday depression. I attached the PDF to this post, I scanned it yesterday from a huge book entitled "Handbook of Photosynthesis; 2nd Edition". The paper is not the best scan job and the first two pages should be ignored, start reading on page three. The text can be a bit hard to read but I had no problems, my 'CliffNotes' are above.

HTH