# Photoinhibition threshold



## 911reagan (May 12, 2013)

Hello I am wondering if anyone could help me. I want to know what is the threshold for photoinhibition to occur in mj. i will accept measurements in lux, foot candles as well as lumens per square foot.  i read somewhere it is around 18000 lumens per square foot or foot candles, but i want to be absolutely sure. and also i want to know what are the optimal amount lux, foot candles or lumens/sqft at the top of the canopy of the plants to give maximum growth without hindering anything.


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## DrFever (May 12, 2013)

One way in which plants are categorized is by the way they gather and handle carbon dioxide. Cannabis is a C3 plant. It uses the CO2 it gathers during the light period, when it is photosynthesizing. Plants designated C4 also gather CO2 during the dark period for use during the light period. Many C3 plants, including cannabis, do not need a rest period. They continue to photosynthesize as long as they are receiving light.

The plant's photosynthetic rate determines its growth rate because the sugars are used by the plant to build tissue and for energy. Cannabis under continuous light will grow 33% faster than the same plants on an 18-6 light regime.
Now getting to your question  marijuana being a C3 plant and needing a good light source  you can look at the out put of the sun and go by them  for instance  you can grow MJ at 30 watts - 100 watts per Square foot  
The rule of thumb i go by is  min 40 watts per to 80 watts per  strain dependent for light bleaching but most importantly is room conditions 
Another measure called vapour pressure deficit (VPD) is also used to indicate humidity and is felt to be more directly related to a plants wellbeing. VPD combines the effects of both humidity AND temperature into one value and so gives a good indication of plant wellbeing without the need for the grower to do any mental arithmetic. VPD values run in the opposite way to RH values so when RH is high VPD is low.

If humidity is too low (i.e. high VPD), the stomata on the leaves tend to close in order to limit transpiration and prevent wilting. This closing of the stomata will also limit the rate of CO2 uptake and hence limit photosynthesis and consequently plant growth. Low humidity also reduces turgidity (water pressure within the plant cells) and this in turn also restricts growth. Blossom end rot in tomatoes and capsicum can also be attributed to low humidity (high VPD).

Conversely, if humidity is too high (i.e. low VPD) the stomata will fully open but even so the plants will be unable to evaporate enough water to carry minerals into the plant and so again, growth will be impeded and mineral deficiencies (particularly calcium) may occur. In addition, the plants may exhibit soft growth, fungal problems and mineral deficiency symptoms.

It is frequently stated that VPD more closely matches what the plant "feels" in relation to temperature and humidity and therefore forms a better basis for environment control. Unfortunately, VPD is extremely difficult to determine accurately as it is necessary to know the leaf tissue temperature. Attempts to measure leaf temperature reliably on an ongoing basis have often ended in disaster. One of the problems is that the plants leaves are in differing amounts of sun with some leaves in full sun, some in partial sun and others in full shade. This makes the concept of "leaf tissue temperature" particularly complex.
So you need to  dial in your room play with it  till you see the proper growth rates and plant health 
 so keep in mind  40 - 80 watts per Sq foot  proper  room temps  72 - 86 , and humidity   in veg you want it a little higher      55 - 75    and in flower    40 - 65


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## Hushpuppy (May 12, 2013)

If you go to this page:http://www.marijuanapassion.com/forum/showthread.php?t=43715
And go down until you see all of the charts, there is a long chart that gives the MH and HPS lights in wattages, then shows down the columns the level of lux measured at different heights. This chart should tell you(generally) what you need to know.


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## Hushpuppy (May 12, 2013)

Here is the specific chart that I was referring to for seeing the amount of Foot-candles(or LUX): http://www.marijuanapassion.com/forum/attachment.php?attachmentid=119317&d=1245698238  Hope this helps you


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## 911reagan (May 14, 2013)

well actually lux and fc are not the same thing, lux is lumens per square meter where fc are lumens per square foot. thanx for the input I was wanting an exact lumen rating at which photoinhibition occurs and my rh is general no lower than 55% and no higher than 70% depending on if its in veg or flower.


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## The Hemp Goddess (May 14, 2013)

I am curious why the exact number is so important?  Things like CO2 enhancement can alter how much light the plant can use.  Are you actually thinking of trying 18,000 lumens per sq ft?


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## Hushpuppy (May 14, 2013)

911reagan said:
			
		

> well actually lux and fc are not the same thing, lux is lumens per square meter where fc are lumens per square foot. thanx for the input I was wanting an exact lumen rating at which photoinhibition occurs and my rh is general no lower than 55% and no higher than 70% depending on if its in veg or flower.


:doh: That's right I forgot that lux is per sqm not sqft. Its hard to keep up with the different measures. That is why I try to have about 6000-7000 lumens per sqft in my grow tents. But measuring lux or Fc is important as I have found that the cool tubes are not as light efficient as hooded cooltubes, when you measure the light at the canopy level. 

Not too long ago I discovered that even with 2 600wHPS in my tents that I wasn't getting the density in my buds that I wanted. I had bought one of the cool tubes that has the hood made to sit above the tube rather than having the small reflector inside. When I installed this fixture, I could see a big difference with my naked eye. I figured it was just the difference in bulbs. At the next change over I swapped bulbs and found that the hooded tube was still brighter, so I checked it with my light meter and found the cooltube was throwing about 26,000Fc and the hooded cool tube was throwing about 39,000Fc. 

I proceded to by 4 more of the hooded cooltube fixtures to replace the standard cooltubes. I have since changed out 2 in one of the 3 tents and tested it with the light meter and found a big difference. This harvest, I am looking for rock hard nuggets the size of my fist


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## ShecallshimThor (May 14, 2013)

Hey hushpuppy, when you say hooded vs standard cool tubes do you mean reflector vs no reflector OR reflector on inside of tube vs reflector outside the tube???


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## 911reagan (May 18, 2013)

The Hemp Goddess said:
			
		

> I am curious why the exact number is so important?  Things like CO2 enhancement can alter how much light the plant can use.  Are you actually thinking of trying 18,000 lumens per sq ft?


 
according to my light meter im receiving 14000- 15000 lumens/sq ft at the top of my canopy and 3500-5000 at the very bottom fan leaves, after some adjustments to the reflectix mylar walls as well as the construction of a parabolic mylar fixture on the underside that attaches to the walls with industrial strength Velcro for easy watering. basically I shaped and molded reflectix mylar as tediously as possible with the help of a light meter attached to a stick with a record mode and a friend so that I shaped the underside reflectix parabola so the bottom leaves of the plants were receiving the max fc they could get. without the fixture it read 200-400 fc average and now im getting 3500-5000 depending on where it is. 

anyway I did some research and found there is a correlation between irradiance, co2 , and photoinhibiton.

at 5000 foot candles 1200 ppm co2 is being utilized to the fullest , at 7000 fc 1600 ppm c02 is optimal. 

so for every 1 foot candle of light your meter measures, 0.22-0.24 ppm of c02 can be used.

how ever I do believe there is a ceiling on this at above 1200 ppm where above that will hurt your plants. 

not exactly the rate at which photoinhibition damages leaves but keeping c02 levels set at the proper ppm with the use of a meter as well as a light meter should eliminate any worry of photoinhibition


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## 911reagan (May 31, 2013)

IMPORTANT UPDATE!!

there is a drop off in effectiveness of light and it is tied to many factors.

the most important being co2. at levels above 1200 ppm rubisco activase is inhibited:

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


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. "

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. "

basically what you can gather from this is people have been wasting co2 for years

the recommended amount of co2 enrichment stands at 1000-1200 ppm
with recommend temps at 85-89 farenheit
rh should be no more than 55%
so the threshold for not exactly photo inhibition, but the rate at which photosynthesis is being utilized to the maximum would be 4500-5000 footcandles, on the bottom leaves.
this insures the entire plant is utilizing all the co2 to maximum efficiency and the only problem now is if over 15000 fc,(which is necessary to ensure light penetration does in fact lead to 5000 fc on the bottom leaves) will in fact cause photo inhibition, bleaching the leaves and destroying chloroplasts.

a simple question turned so complex but ended up needed the answer to the original query

also here is an important conversion:
1,200 ppm CO2 = 0.12% CO2 = 1,200 micromole mol^-1 CO2.

~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.


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## The Hemp Goddess (May 31, 2013)

If you are going to cut and paste, you need to cite your source.


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## shahomy (May 31, 2013)

This all sounds very interesting...any chance of a picture of this reflector/parabola thing your doing...having a bit of a hard time "seeing" it, with your description....congrats on the "light"? increase


> according to my light meter im receiving 14000- 15000 lumens/sq ft at the top of my canopy and 3500-5000 at the very bottom fan leaves, after some adjustments to the reflectix mylar walls as well as the construction of a parabolic mylar fixture on the underside that attaches to the walls with industrial strength Velcro for easy watering. basically I shaped and molded reflectix mylar as tediously as possible with the help of a light meter attached to a stick with a record mode and a friend so that I shaped the underside reflectix parabola so the bottom leaves of the plants were receiving the max fc they could get. without the fixture it read 200-400 fc average and now im getting 3500-5000 depending on where it is.


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## 911reagan (May 31, 2013)

The Hemp Goddess said:
			
		

> If you are going to cut and paste, you need to cite your source.


 
I did. there are sources before each abstract. it lists the page number as well and the authors. I don't see how anyone could miss that


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## 911reagan (May 31, 2013)

shahomy said:
			
		

> This all sounds very interesting...any chance of a picture of this reflector/parabola thing your doing...having a bit of a hard time "seeing" it, with your description....congrats on the "light"? increase


 
look up vertical reflectors and look for a cone type reflector. it looks like that


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## NorCalHal (May 31, 2013)

Fun "facts" from a 16 year old study on wheat production.

Run CO2 yourself PROPERLY and get back to us, duct tape and "recirculating" exhaust included.

You have no clue of what you speak my friend when it comes to cannabis production.


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## WeedHopper (May 31, 2013)

Yummmm,,some good ole wheat buds.


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## 911reagan (Jun 1, 2013)

NorCalHal said:
			
		

> Fun "facts" from a 16 year old study on wheat production.
> 
> Run CO2 yourself PROPERLY and get back to us, duct tape and "recirculating" exhaust included.
> 
> You have no clue of what you speak my friend when it comes to cannabis production.



the co2 saturation point for most if not all c3 plants is ~1000-1200 ppm.

this is simply an alternative to timed exhaust cycles. i have a bit more experience than what i speak but il take a note of that. one more thing you do realize duct tape is called DUCT tape for a reason right? contrary to popular belief it is not for ducks lol


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