Low pressure greenhouses for Mars?

Growing plants on Mars

One of the issues for Martian agriculture is the perceived need to create pressurized greenhouses (Mars has a very tenuous atmosphere - pressures are close to vacuum equivalent). The problem is that as you increase pressure differential, you need more robust structures.

Current plans call for at atmospheric internal pressure of around the equivalent of 10,000 feet on Earth. (around 10PSI) This would require very strong structures as well as being suboptimal for plant growth. The easy answer to the latter is to increase the CO2 content of the greenhouse air to compensate. This should work, but it's never been tried on some crops.

But, why stop there? Plants (except those that can take nitrogen from the air) need just CO2, plus some water vapor, in their atmosphere. So, why not make the greenhouses far lower pressure but almost pure CO2 - just use compressed Martian atmosphere, which is 96% CO2.  

And if we do that, we don't need 10PSI greenhouses. Dalton's law related to partial pressure of gasses shows us that if we have 1/3 CO2 atmosphere at 10PSI, we;d have the same partial pressure of CO2 if we use 100%CO2 at 3.3 PSI.

Plants also need water vapor, so for this and other reasons we can't get too close to vacuum but 1PSI should be possible (and the evaporation of water would be greatly increased, resulting in a CO2/water vapor mix that should work).

1 PSI would make for vastly easier to build, lower-mass greenhouse structures, with far lower potential for explosive decompression - and zero risk of fire.

The problem is that this has never been tried, it's only theoretical. It would, however, be easy and cheap to test on Earth.