C-3 plants

In C3 plants, photorespiration, a type of respiration indirectly stimulated by light, may take place in the light. Ch2O + o2 CO2 + H2O... Photorespiration competes with photosynthesis to decrease the efficiency of carbon fixation in C3 plants. So a mutation that prevents or decreases the efficiency of photorespiration could work. That enzyme could be rubisco (Ribulose-1,5-bisphosphate carboxylase oxygenase). Rubisco favours carbon dioxide to oxygen; photorespiration tends to occur when there is a high concentration of oxygen relative to carbon dioxide.

Have you heard of photorespiration?