The graph below shows the rate of photosynthesis of a plant measured at a constant carbon dioxide concentration and a constant temperature, as light intensity is gradually increased.



The shaded region on the right of the graph, at the highest light intensities tested, shows the rate of photosynthesis remaining constant even though light intensity continues to increase. Since carbon dioxide concentration and temperature were both kept constant during this experiment, which of the following statements best explains the levelling off of the curve in the shaded region?