Ermm - Plants also respire too, and require Oxygen. During the day, when photosynthesis is active, they produce more O2 through the process of photosynthesis than their own metabolic process actually needs, and so the excess O2 is dissipated via leaves, bark, and roots into the atmosphere.
During the night,photosynthesis shuts down because they are not receiving the photons from the sun. However, the plants still have a metabolic requirement for O2, which is obtained via the process of gaseous diffusion across all parts of the plant, but especially the leaves.
The net balance of O2 is that, even taking into account their oxygen requirement during the hours of night, they still "exhale" more O2 than they require........
http://wiki.answers.com/Q/Why_do_plants_take_in_oxygen_and_release_CO2_at_night