Regarding my question of:
Now, this bring up an interesting question, what is the lowest level of CO2 in the atmosphere that a plant can survive?"
I look a bit and didn't find anything yet, but thought this was interesting:

"Anyone who has observed the explosive growth of aquarium plants in response to carbon dioxide (CO2) fertilization must be convinced of the usefulness of this system. Certainly, there are thousands of aquarium hobbyists who do not give their plants any sort of special treatment and still end up with a fairly nice display. However, truly luxuriant growth, the sort that you see on the covers of aquarium magazines and in pictures of "Dutch aquariums," can only be achieved by fertilizing with CO2.

During photosynthesis, plants use light energy to capture CO2. This CO2 is used to build the basic carbon structures from which all plant material is made. In a poorly lit aquarium, light is likely to be what limits the rate of plant growth. The amount of CO2 produced by fish- and bacterial respiration is more than enough to allow photosynthesis under these conditions. If on the other hand, you try to make your plants grow faster by adding more light, it is likely that there will not be enough CO2 in your aquarium. The plants simply can not grow as fast as they would like to, given the available light energy. "
http://faq.thekrib.com/plant-co2.html

One way could experiment with this is by growing plants in water- this would allow you to control the environment easier. Another interesting aspect would be determine which plants grow best in low CO2 environments. I also wonder if commerical greenhouse add CO2- if not why not?