I am currently working on a college senior design team that is looking at developing a small modular habitat that could sustain plant growth. These small modules would not include humans, so we only need to design around the requirements for the plants. This allows us to explore lower pressure environments where the structural engineering requirements are relaxes. It also allows us to vary the CO2 and O2 partial pressures to optimize for the plants.
For example, I see in the answers to this question that as long as the partial pressure of O2 is very high, human space suit pressures can go as low as 4 and even possibly 3 psi, or about 0.2 bar.
I understand that the atmospheric pressure and composition requirements may vary significantly between varieties, so for the sake of answering this question, I will ask about two different types of plants; legumes and fruit-bearing plants.
My expectation is that legumes would be less sensitive, but let's say we are trying to grow lima beans in one pot, and tomatoes in another. To what degree can I change the fractions of CO2 and O2 in the atmosphere and its pressure, and still have a reasonable expectation that they can still grow?
I understanding that a typical Earth atmosphere is ~78% N2, ~21% O2, and ~.05% CO2 (according to Wikipedia), that on Earth there are some plants capable of living at altitudes as high as 6,000 meters where the pressure is only about 0.45 bar, but I'd like to have a better understanding of how legumes and fruit-bearing plants (more useful to humans) could be grown in an atmosphere with both variable pressure and composition.