During day time, both respiration and photosynthesis occurs in plant. Both respiration & photosynthesis are opposite processes. In respiration, there is an intake of O2 and release of CO2, while in photosynthesis CO2 is taken inside and O2 is released. But the rate of photosynthesis is much higher than the rate of respiration. Due to this reason, release of O2 is also much higher than release of CO2.