botany - What is the lowest pressure at which plants can survive?

I like your question!

Low surface pressure on Mars (averaging 600 Pa or about 1/170 of Earth's at sea level) is only one difficulty that an organism would have to contend with. In addition, mean surface temperatures are ~210 K (-63 C), the surface ultraviolet flux is extremely high (no ozone layer) and an aridity comparable to the Atacama desert. On the plus side, a 95% CO2 atmosphere might promote photosynthesis.

So, higher plants are extremely unlikely to prosper on Mars. Purely in terms of pressure, experiments suggest that at 1/10 Earth atmospheric pressure vascular plant transpiration increases significantly. So much so that they enter into a drought response which often leads to plant death.

However, lichens may be another matter. These are often adapted to low precipitation/pressure environments such as high mountains. Indeed, this article addresses exactly your question (sadly it is not open access). The authors suggest that the lichen Xanthoria elegans experiences no loss in vitality after 22 days exposure to laboratory Mars-like conditions (low pressure, low temperature etc.)!

Having skimmed this article I am frankly amazed (and slightly dubious) at the result. However, it should be noted that they only ran the experiment for 22 days, applied no ionising radiation (to simulate the high UV flux) and make little mention of the precise metabolic state of the lichen. If it has entered into cryptobiosis (cessation of metabolic processes) then it can't exactly be regarded as prospering.

• de Vera et al., 2010, Astrobiology, 10, 2, Survival Potential and Photosynthetic Activity of Lichens Under Mars-Like Conditions : A Laboratory Study