That's a valid question. Respiration definitely
requires oxygen in plant tissues. Plants can take
up oxygen during the day, however, when they are
photosynthesizing, they generally don't have to.
This is because the photosynthetic reactions split
water (H2O) into hydrogen and oxygen,
forming O2 gas. This oxygen is then
used in respiration inside the plant tissues.
During the day (with plenty of sunlight there
actually becomes too high of an oxygen
concentration inside the plant and a lot of
O2 diffuses out of the plant tissues.
At night, there is no photosynthesis and so
is a net uptake of O2 and release of
CO2 (from respiration). It's as if the
plants are taking one deep breath in during the
day and exhaling at night, but they grew during
the day and incorporated some CO2, so
the breath in is bigger than the breath out.
Check out this image for a basic summary: