Explanation

The external factors would include the availability of sunlight, temperature, $CO_{2}$ concentration and water. As a plant photosynthesises, all these factors will simultaneously affect its rate. Hence, through several factors interact and simultaneously affect photosynthesis or $CO_{2}$ fixation, usually one factor is the major cause or is the one that limits the rate. Hence, at any point the rate will be determined by the factor available at sub-optimal levels