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Discussion Starter · #1 ·
Ok, if anyone can provide any insight to my theory would be great. Let's say hypothetically, I had a light fixture that provides 35 par and on for 12 hours and all plants are doing well in tank with no algae.

Would doubling the light intensity (70 par) and halving the photoperiod (6 hours) provide the same results? If not, what would most likely change?

Hopefully I didn't word the question too awkwardly. Lol
 

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Cape i hope you do not mind but i would like to expand on your question.

Would increasing the light duration make up for lack of intensity?
 

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Someone would need to do the experimenting to be sure, but I'm 90% sure that duration and intensity are not even close to being equivalent. Plants all require a minimum intensity to grow at all, and no matter how long the lights are on if the intensity is below that minimum, the plants don't grow. Also, plants can only use light if they have all of the nutrients needed to grow at the rate appropriate to that light intensity. Adding more light, without those nutrients, all of them, does not cause the plants to grow faster.

But, it is possible that within a small range of intensities, more time can compensate for lower light.
 

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I would think it would also depend on how u double the par. For example one fixture at 70 par and the difference of two fixtures at 35 par a piece. The Two fixtures side by side would be stronger in the center giving 70 par but the lights would provide more coverage with different intensities than one 70 par fixture, correct?
 

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Someone would need to do the experimenting to be sure, but I'm 90% sure that duration and intensity are not even close to being equivalent. Plants all require a minimum intensity to grow at all, and no matter how long the lights are on if the intensity is below that minimum, the plants don't grow. Also, plants can only use light if they have all of the nutrients needed to grow at the rate appropriate to that light intensity. Adding more light, without those nutrients, all of them, does not cause the plants to grow faster.

But, it is possible that within a small range of intensities, more time can compensate for lower light.
I agree with Hoppy's points. Ultimately several experiments would be needed to figure out exactly the relationship. I'm sure it isn't linear.

If you make your original question more extreme the answer becomes easier to envision; does exposing the plants to a blindingly bright light for 1 second fulfill their daily light needs? Does exposing plants to a candle light for 24 hours a day allow them to grow and survive?

To some extent plants are able to compensate but not by as much as you'd think. In my experience simply keeping the photoperiod the same or reducing it slightly by an hour or two while doubling the intensity will help plants grow more productively as long as they don't run out of nutrients.
 

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There is a ton of data and experimental work on this going back many decades in horticulture because greenhouses want to optimize the conversion of energy to plant growth or crop production. Search on "experiments with DLI and PAR in horticulture" or something along those lines. Sadly, the studies are completely incomprehensible unless one has a sciencey brain. However, it seems likely to me that we aquarists have, by trial and error, found the sweet spot and it is a 6 to 10 hour photoperiod with whatever PAR is needed to mimic the light needs (I think it is the daylight integral) of the plants we grow. Of course, what makes our light needs different from those in a greenhouse monoculture is that we mix plants that evolved at different latitudes in the same tank and those plants have very different light requirements.
 

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Discussion Starter · #7 ·
Some great conversation going on here.

I have thought about the extreme examples but that is the reason I provided the examples I did. Plants are already growing well, so light needs are already met, now we double intensity and half duration from 12 to 6 hours.

Also, assume coverage is same. So same light fixture and placement just different intensity and photoperiod.
 
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