cannabis is a c3 plant might want to check that out it might help you mr greengenesWell, much more O2 is produced by the plants by photosynthesis during the day than is used back up by them at night, I can tell you that for sure. I think it's on the order of ten times, off the top of my head. The area that nighttime O2 needs pose a problem for plants is in aquaculture. Water has a hard time holding enough O2 for plants to reup at night. While O2 is quite abundant in the atmosphere, CO2 (gonna catch hell from the AGW folks now!) is in short supply. When you consider that a field of corn can use up ALL the available CO2 from the ground up to two meters in about five minutes (without air circulation in midday sun), you can see that cannabis which is also a very high energy 'C4' plant, needs CO2 to grow in sealed rooms.