In the C3 plant, the Rubisco enzyme is important that fix CO2 into organic molecule by binding with CO2. Rubisco can also bind to oxygen when the oxygen concentration increases in cells which leads to the loss of fixed CO2 and waste of energy which is called photorespiration.
So when stomata are open CO2 comes in and O2 goes out of the cell which maintains the high CO2 concentration in the cell but during hot days C3 plants close stomata to prevent water loss.
So closing of stomata decrease the intake of CO2 and increase the oxygen concentration inside the cell. So in increased temperature and high oxygen concentration, the rubisco enzymes bind to O2 and promote photorespiration.
