Comment by SamBam

Plants respire oxygen continually, day and night. It's a myth that they only respire at night.

Like every other organism except for anaerobes (mostly microbes, some fungi) they need oxygen in order to burn fuel for cellular processes. Plant cells are doing things day and night.

The origin of the myth is simply that they produce more oxygen via photosynthesis than they respire, and so are net producers of oxygen during the day.