Why is oxygen output not an accurate measure of the rate of photosynthesis?

Question

A manometer was set up to measure the oxygen interchange between the plant and its environment at different levels of light intensity. There was an increase of oxygen interchange with an increase of light intensity, but why is it inaccurate?

Answer 1

Because the stoma of the leaf can be closed to prevent water lose. If it is closed, the oxygen cannot escape and thus it would not be an accurate measure of photosynthesis.