Respiration occurs at the same rate in both day & night. But the difference is because of photosynthesis. During day time both respiration & 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. Since there is no net release of CO2, it seems that no respiration occurs in plant.