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Why does the rate of photosynthesis stop increasing at light saturation?

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Diagram of a tree leaf illustrating photosynthesis, with arrows showing sunlight, water, and carbon dioxide entering the leaf and sugars and oxygen being produced. A person is shown beside the tree to demonstrate the exchange of oxygen and carbon dioxide between plants and animals. — Photosynthesis Diagram of photosynthesis showing how water, light, and carbon dioxide are absorbed by a plant to produce oxygen, sugars, and more carbon dioxide.

At light saturation, the rate of photosynthesis stops increasing because the photochemical, light-harvesting stage is no longer the limiting factor. Instead, the rate becomes constrained by the enzymatic, or carbon-assimilating, stage of the process.1

When light intensity is low to moderate, the rate of photosynthesis is determined by the speed of photochemical reactions. However, as light intensity increases to higher levels, the chemical reactions of the dark stage (the Calvin-Benson cycle) become rate-limiting.1

Additionally, in many land plants, the process of photorespiration can further limit the rate of photosynthesis. This occurs when the enzyme Rubisco reacts with oxygen rather than carbon dioxide, a process that is favored by high temperatures and low internal carbon dioxide concentrations. Under these conditions, photorespiration competes with photosynthesis, effectively capping the plant's ability to increase its photosynthetic output despite higher light availability.123