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The rate of photosynthesis is influenced by several environmental factors. At any given time, the factor that is furthest from its optimum is the one that limits the rate of photosynthesis. This factor is called the limiting factor. Understanding limiting factors is essential for interpreting experimental data and for optimising conditions in agriculture and horticulture (e.g., in greenhouses).
Key Definition: A limiting factor is the factor that is present at the lowest or least favourable level, and which therefore restricts the rate of a process. Increasing the limiting factor will increase the rate of the process, up to a point.
This concept was first described by F.F. Blackman (1905), who proposed the law of limiting factors: "When a process depends on more than one essential condition being favourable, its rate is limited by the factor that is nearest its minimum value."
Exam Tip: When describing the effect of light intensity on the rate of photosynthesis, use precise language: "As light intensity increases, the rate of photosynthesis increases because more light energy is available for the light-dependent reactions, until another factor becomes limiting."
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