Back in 2016 I did some research into what gas combinations we could use for growing plants in chamber on Mars
The final composition I settled on was “pure Martian air pressurised to 13.1 kPa, supplemented with O2 to give a total pressure of 34.5 kPa and an oxygen partial pressure of 21.4 kPa”
This thread was spun-off from the Science guest suggestion thread, so quoting previous comments here for context:
You totally just pointed out a major flaw in my research - I never accounted for the partial pressure of water vapor. Thanks!
I found a study that showed germination was possible under extremely high CO2.
T.N. McCaig, R.D. Hill, Cyanide-insensitive respiration in wheat: cultivar differences and effects of temperature, carbon dioxide, and oxygen, Canadian Journal of Botany. 55(1977) 549-55.
I didn’t really define what plants would be best, but a lot of my references used lettuce so that might be a safe bet.