At high oxygen concentration, the rate of photosynthesis decreases due to

At high oxygen concentration, the rate of photosynthesis decreases due to Warburg effect.
The Warburg Effect is defined as an increase in the rate of glucose uptake and preferential production of lactate, even in the presence of oxygen.
This phenomenon, originally known as the “Warburg effect,” was later recognized as the light-dependent release of CO₂ by photosynthetic organisms, or photorespiration, and was the subject of intense investigation and debate for many decades.