There will be a certain concentration of CO2 in the atmosphere at which the rate of photosynthesis will be equal to the rate of respiration or in other words photosynthesis will just compensate for respiration and the value of apparant photosynthesis will be zero. This is called as carbon dioxide compensation point. Tidestromia oblongifolia is a C4 plant and does not show photorespiration i.e., law compensation point.