http://astrobiology.nasa.gov/ask-an-astrobiologist/question/?id=864

Could a photosynthetic plant survive on Mars using the Sun's energy to perform photosynthesis?

There is plenty of sunlight on Mars to support photosynthesis -- the sun is brighter there than on a cloudy day on Earth. The problem that a plant would face is not lack of sunlight but low temperature and lack of liquid water. On Earth, some plants have managed to overcome similar problems by establishing themselves under the translucent skin of rocks. Here they can preserve a tiny bit of liquid water and also protect themselves from extremes of temperature while still obtaining enough sunlight for photosynthesis. Such plant colonies have been found inside rocks in the Antarctic. Whether anything similar could have arisen on Mars, we do not know -- but Mars is lot colder and dryer than the Antarctic on Earth.

David Morrison NAI Senior Scientist