One of deficit resources on Mars is nitrogen. There's a plenty of carbon dioxide, and a modest amount of water. There's a deficit of nitrogen, but AFAIK plants use nitrogen in compounds found in soil, not from the atmosphere. This could be provided.
While long-term human habitation would likely necessitate atmosphere similar to Earth, would this be necessary for large biodomes? Could plants, given enough light and water, thrive in atmosphere of oxygen and carbon dioxide, without atmospheric nitrogen? Also importantly, could they live with reduced atmospheric pressure - so that the biodomes wouldn't need to withstand a whole 1bar differential, but say, 0.3 bar, similar to what was used in Apollo missions?