No Jive Productions
Join Date: Oct 2006
First, thank you for your time and effort. A lot of work went into your response.
Quotes by Spurr.
“What do you mean by "reset mechanism"? Resetting what? And resetting it when?
I am not sure what you mean by "reset", some so-called "dark reactions" are now more correctly called "light-independent reactions". So some functions that are at optimum under darkness can be carried out under light.
Of interest, plants can tell when it's 'midnight' when the plant is given a stable day/night schedule.
If by "reset" you mean "the point when the plant senses 24 hours have passed", in terms of plants grown under 24 hours of light (24/0), AFAIK it is due to the natural circadian rhythm (24 hours).”
Since beginning this thread I have been studying the plant circadian clock and now see what you mean about my use of the term “reset”. The mechanism is more of an “on/off” than a reset. This on/off cycle is mostly entrained by light although other forces such as temperature can come into play. I also have a better understanding of how the “clock” translates external stimuli into internal reactions by use of the phytochrome and cryptochrome receptors and the oscillator.
“One point about growing 24/0, is the rate of photosynthesis will dip throughout the day; it won't stay high during light like it does when a night time is used.”
i've been using 8 on 4 off 8 on 4 off each 24 hours in veg and the plants are reaching the same size they reached under 24/0, 20/4, or 18/6. this is approx. 30-32” in 4 weeks using a bare vertically oriented 1k hortilux hps at 14” to the nearest part of the plant. I have a quantum meter and it shows 1500 umols at 14” so theoretically I am delivering 43.2 moles per period or 86.4 per day to at least the front of the plant.
“In cannabis "steady-state" Pn is achieved after about 30-40 minutes under bright light (within PAR range), IIRC.”
this I believe I have observed. The device i'm growing in now is fed in part by a sub-irrigation system that has a level controlled by a float valve. When the lights first come on there is virtually no drip occurring. At ten minutes there is a slow drip. At 20 it is faster. At 30 minutes even more volume. By the end of an hour it is dripping at over 100 drips per minute and stable. This is in flower with 8-9 large plants.
“If you are referring to the DLI for Dr. Elsohly, from what I posted in another thread, it's an approximation, a best guess of mine. AFAIK Dr. Elsohly hasn't studied DLI, or has he and I am unaware?”
in figure 1a on page 3 of Dr El Sohly's paper there is a graph and I think I misinterpreted part of it to mean that he used 25 moles per day. Upon rereading it I don't see moles/day clearly indicated anywhere. You might take a look and see what you think.
“Depending upon where the greenhouse is located, DLI (def: mol/meter^2/day) can exceed 30; ex., > 30 mol/area^2/day.”
yes, but rarely does in practice. Those giant hydroponic tomato greenhouses in arizona report 48 moles/day regularly. But they located there for that reason. I think specmeters bases their program on 30 moles/day. All their literature is geared towards this level. I have a set of the “dli 100 lightscout” light meters and you can peg them at 30 moles/day. They measure accumulation only.
I don't believe that 30 moles is the max that can be used, but i'm thinking of a light array that hits each side of the plant in flower with 30 moles at alternating times. 6 hours per side.
One point about PPFD as used below, the definition of PPFD is "Photosynthetic Photon Flux Density". That is a label for the number of photons (particles of light) that hit an area of a meter squared (three-feet squared) in one second; but only photons that are within a range that best drives rate of photosynthesis (Pn), called the PAR range (Photosynthetically Active Radiation) of 400-700 nanometers. However, when one measures PPFD one does not measure the instantaneous irradiance (as umol/area^2/second) over a whole meter squared, to find PPFD. To measure PPFD (i.e., instantaneous irradiance as micromoles per area squared per second) one uses a quantum sensor, but they are only about an inch squared.
When the concept of measuring (quantifying) photons within the PAR range (per second; instantaneous) that best drives Pn, was being developed, it was (mostly) developed to be used for outdoor and greenhouse growing, when used for whole-plant application. AFAIK, the designers did not take irradiance foot-print of a horizontal lamp in a reflector into consideration, only an ideal situation of point-source light from the sun or greenhouse lamps far from the plants. Granted, the scientists who came up with PPFD used single leafs, at times, when measuring effect of irradiance on rate of photosynthesis.
The calculation of PPFD is based upon at least one big assumption: all spots (ex., plants) within the area of a meter squared get the same instantaneous irradiance. Ex., all the plants in a meter squared outside in a field the get the same 'amount' of sun light (photons; if they are no shaded); so one measurement in any spot within the meter^2 will show the PPFD of the meter^2. That means to find PPFD outdoor or in a greenhouse, one measurement is taken within a meter^2 with a quantum sensor and the assumption is made that all other areas within the meter^2 will have the same instantaneous irradiance as the original spot. Thus one measurement of a small area (about an inch or 2 squared) is used to find PPFD for a whole meter^2. But indoors under a reflector with a horizontal lamp, not all plants in a meter squared get the same 'amount' of lamp light (photons). That means using PPFD is not representative of the real-world irradiance per smaller-than-a-meter^2 area, within that meter squared. Also, the PPFD from a horizontal lamp in a reflector can't be found by just a single measurement, many measurements must be taken to account for un-even light distribution over the meter squared (and light side the meter^2).
Along with the reasons above, un-even irradiance over many areas of measurement within a meter^2 is another reason PPFD is not ideal for our use case; IMO anyway. By Definition PPFD is fixed at a meter squared, which works fine for outdoor and greenhouse growing, but indoors canopies are often smaller than 3'x3' or not shaped in a square, as well as the flaws/limits above.
What all the means, in my opinion, is using the term PPFD is not ideal, mostly because the area is fixed. I think it may be better if PPFD was not fixed in area, but required one to report the area measured for irradiance; no assumption made. So PPFD could mean umol/1"^2/second or umol/12"^2/second; as long as area is defined.
Anyway, in the post below when I write PPFD I mean "umol/any-area-^2/second"; not necessarily "umol/meter^2/second". Likewise, DLI means "mol/area^2/day"; not necessarily "mol/meter^2/day".”
so, with sunlight, because of the great distance from the sun to the earth, the photon stream must appear almost as paralell and therefore nearly equal. indoors using fixed emitters close to the plant the stream must appear more like spokes of a wheel and therefore will be unequal applied to a flat surface. My meter shows that just 2” further from the light, for example, means the difference between 1500 umols and 1200 umols in free air. To me this means that foliage “penetration” indoors is all but nonexistant. We are really growing premium bud only on the periphery of the plant canopy.
“DLI is rarely found much higher than 60 mol/m^2/day outside and in a natural greenhouse.
To find a possible goal DLI for flowering cannabis I tried to mimic what would happen in nature (re PPFD bell curve and DLI), using natural PPFD bell curve found in Hawaii that provided peak PPFD of ~1,500-1,800 (because that PPFD has been studied for cannabis). I did my best to create a goal flowering DLI of ~48, using the data from Hawaii and working backwards (ex. for 12 hours) to find PPFD that is both high enough to keep Ph in the 'high' range (ex., > 1,000), and high enough but not too high, to mimic a DLI naturally found during very bright and long days (re Hawaii).
If one used 1,500 PPFD all day (ex. 12 hours, 18 hours, etc) it could be too much DLI, and hinder rate of photosynthesis and cause photoinhibition (ex., midday depression of photosynthesis, (increased) photorespiration, etc.).
Using peak PPFD of 1,500 adjusted with an average PPFD bell-curve (over the whole daylength) from a high irradiance location (ex. Hawaii on relatively cloudless day maxing out at 1,500-2,000 PPFD) to find DLI is the best method I could think of at my disposal to easily guesstimate a possible goal DLI for flowering cannabis. That is, short of using PPFD per wavelength weighted with K.McCree's quantum yield.
1,500 PPFD fits into my DLI model because that irradiance level was found to provided near peak Pn in four different studies on cannabis, with variables such as temp, Co2, etc. And IIRC > 1,000 PPFD could be considered a benchmark for high(est) Pn. However, I didn't want to use 1,500 PPFD to find DLI because IMO 1,500 PPFD is too much instantaneous irradiance for every second over greater than 12 hours. Outside there is a PPFD bell curve, it starts low, gets high, ends low; and outside peak PPFD can be found about and just above 2,000.
I wanted to try and mimic the DLI a plant like cannabis (a 'sun loving' type) could experience outside, naturally, respective to the PPFD found to provide peak Pn for cannabis. So I found a long set of spectroradiometer graphs for PPFD every X minutes over a (nearly) cloudless day in Hawaii in summer; used to find DLI of that spot in Hawaii. I used Hawaii because (a) it has very high irradiance, a good example for what cannabis likes; and (b) I have grown cannabis in Hawaii (halfway up a big mountain, when I had my med card there) and saw first hand the effect of long days and high irradiance.
I think I am making it sound more complicated than it really is, it's quite simple but I think I may be over explaining in an attempt to explain enough.
Basically it's like this:
I used a natural PPFD bell curve from high irradiance location (Hawaii) that peaked at ~1,800 PPFD around noon; about the same PPFD that offers highest rate of photosynthesis for cannabis. Then I worked backward starting with the DLI from Hawaii, using 12 hour daylength (ex., for flowering) to find PPFD. And the PPFD was > 1,000, which is kind a benchmark for high(est) Pn.
My model for flowering (12 hour day) puts DLI at 46-48 (mol/area^2/day); that is achieved using ~1,000-1,100 umol/area^2/second (specifically ~1,065 to ~1,111 umol/area^2/second).
However, in reality most grow rooms will not provide over 1,000-1,100 umol/area^2/second to any plant; unless there is for example one small plant below a good lamp/reflector. For example, under a Blockbuster 8", with 1,000 watt Digilux HPS and Galaxy select-a-watt set to 'super lumens' (supposed to be 10% increase in radiance from 1,000 watt) -- with an Equalizer hot-spot diffuser -- the peak irradiance under about plumb center of lamp (while keeping irradiance high at the range of 3'x3') is ~920-975 umol/6"^2/second (10 second average to account for lamp flicker).
Most grow rooms will probably provide somewhere around 40-44 mol/area^2/day for flowering; those that are 'bright'. Very well designed rooms, esp. with respect to canopy size relative to size of reflector and light mover, can have over 50 mol/area^2/day for 12 hours.”
I gotcha. I use my lights in flower with cooltubes and I can get 1500 umols at 12” but of course that is only part of the plant.
“There are no studies looking at DLI for cannabis, AFAIK. DLI depends upon hours per day and PPFD. However, there are many studies looking at ideal PPFD* for cannabis. We could take the ideal PPFD for cannabis, ~1,500, and simply use that to find DLI for a plant providing 1,500 PPFD all day, but if we did that we would be over-saturating the plant with photons over the whole day, and in turn, cause photoinhibition.”
my thinking exactly. I'm wondering whether the 30 moles per side at the closest point of foliage that I described above can deliver 60 moles/day to the whole plant without causing inhibition. This also has me thinking about how photosynthate partitioning will work in this scenario. I'm wondering whether there is a threshold of light that turns a plant part into a producer instead of a sink. Say 2-300 umols or more. Could you provide links to the studies showing ideal ppfd? I have dr el sohly's.
“Considering once PPFD (for cannabis) drops below ~1,000 the rate of photosynthesis (Pn) drops a bit, using at least ~1,000 PPFD all day seems to be a good (max?) goal. I think using ~1,000 to ~1,200 PPFD all day is good (max?) goal. I plan to test that DLI on Pn and net rate of photosynthesis (Pnnet), etc., for cannabis this coming year.”
I think I can determine the approx max dli relative to my own light and plant arrangement by using the drip rate of my system. As photosynthesis rolls off I should get a noticeable decrease in drip rate. I'll have a test plant set up for this soon.
Thank you for your thoughts and all the number crunching, it all helps.