Question About Photosynthesis Over Time

I purchased a CO2 meter several months ago to measure the CO2 levels in my greenhouse. One thing I’ve consistently noticed is that the CO2 levels seems to follow a U shaped pattern where they are initially higher in the morning, dip towards the afternoon, and then rise again in the evening. It’s the evening spike that has me wondering whether plants can reach a photosynthesis exhaustion point in the day where giving them additional light doesn’t really matter because they just need some rest.

So what do you all think, do trees have a daily limit to photosynthesis?

Could it be that photosynthesis is slowing down due to lower light levels, but that generation of CO2 from decomposition of any organic mater and root respiration is continuing?

Hi Marty,

Actually the light is probably the one constant, I use full spectrum LEDs in there in a timer. That being said, you’ve got me thinking about other processes in there that could be generating CO2. Despite my best efforts, there is always algea in the drip trays and who knows what decomposing in the soil.


You did not say you had lights going.
What levels of CO2 are you seeing, and what’s the outside CO2 level.
Is the greenhouse open to the sun? The lumens from th sun would be way above leds. Are the leds proper blue / red for the 2 frequencys for photosynthesys? If not, hey will grow, but poorley.
(Are you suplimenting CO2? Growers sometimes run elevated levels…to push sales.)

That’s a lot of questions I’m happy to answer. The full greenhouse set up is closed to the sun and uses all artificial full spectrum LEDs that put out around 350 PPFD at 18 inches distance from the light source. I use a ventilator to pull in outside air when the temperature heads above 85 or the humidity goes over 60%. I grow tropicals in the north.

I do supplement the CO2 supply with an Exhale bag of organic fungus. What I’m seeing is that the CO2 levels inside the tent jump from a low of 400 ppm around noon up to 2000 ppm around 5:50-6:00 PM. The fungus itself is light activated so I suspect it has a ramp up period.

That being said, even without the CO2 supplement, I still see the spike in the evening. I took measurements for a week without the Exhale bag and found that the spike maxes out around 1400 ppm. The air quality in my home fluctuates between 400-700 ppm so I’m sure the levels within the tent far exceed the levels outside of it even without a supplement.

Another theory I have is that I’m creating a CO2 trap inside the tent because the cooler air comes in from the bottom and hot air is drawn out from the top. The greenhouse is only about 6ft, but that might be enough given that CO2 is heavier than oxygen.

In any case, there are a ton of variables. So do you all know if a plants photosynthetic efficiency has a terminal point at which it says no more?

Sounds like a potgrow…
No, photosynthesis should not lessen over time. Higher CO2 would enhance it, all other things being optimal. Plants do have a diurnal respiration. They will photosynthesis 24/7 if you leave the lights on…
Outside sunlight solar max is about 1700 PPFD. Need to triple the lighting for optimum. That s why potgrows use high intensity lights.
Anything above 1000 ppm of CO2 in the house will cause health problems…
Too many variables, I’m out…

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Yikes, my wife wants me to build us a greenhouse in the garage for my tropicals and her seed starters. This sounds way more complex/expensive than anticipated. :weary:

Sounds like time to grab a lab coat and do some experiments! Tools at your disposal would seem to be turning on/off lights and fans at different times of day.

Speaking of experiments, I’m trying to adjust watering first. My indoor plants have lots of aeration due to the increased humidity of the greenhouse so I’m trying to see if they are losing efficiency as they lose moisture.

Bentley, indoor growing is super complicated, but if you’re doing bonsai, you’re probably going to love it. Building a greenhouse for my tropicals has become a second hobby next to bonsai.

Kurt thinks I’m growing pot because I learned all about indoor growing from the pot community who have a ton of experience with it for better or worse. If you are going to try it out (indoor growing that is), I would advise scaling everything the pot community is doing back by 90%. Marijuana can take extreme amounts of light, water, heat, CO2, and fertilizer, which growers don’t hold back on.

Your average tropical bonsai can’t take anything close to those levels so be careful!

I just need them to last three months. I had my tropicals indoors next to the window and they did fine. It got to be a bit of an eye sore though, so we wanna try having them in a greenhouse. If pot were legal here I’d grow some of that too lol.

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I think your answer is more temperature related. Most bonsai plants are temperate species which are C3 plants. Many of these will close their stomates at temperatures above 90 F. That will effectively stop transpiration and gas exchange. They stop taking in CO2 and releasing O2 as well as water.

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