Earth plants, if taken to Mars, would have to tolerate conditions that would cause them a great deal of stress -- eventually killing them.
7 torr of pressure, sustained temps below 0 C, full solar UV, utter desiccation... yeah, that might put off my blue throatworts.
NASA and others have been screwing around with "Mars jars" for three decades. The research has been no less successful that Project Head Start.
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OK, so NASA cobbles together a Franken-lettuce with genes from lichens, extremophiles, xerophiles, cryophiles, halophiles,
Deinococcus radiodurans, Triffids, crab grass, cholla and such. Why will this be a good thing, as opposed to a fast-growing lignin-rich tree for lumber or lysine-rich corn that are bad things?
You could solve the world's hunger with a Caltech undergrad summer gene-gineering project, BUT IT WOULD BE WRONG.