13.2.1—Limiting factors

A limiting factor is a variable that restricts the rate of photosynthesis under the stated conditions. It is conditional: light intensity, carbon-dioxide concentration or temperature can limit the rate when that factor is in short effective supply, even if the other requirements are adequate.

• Short supply → constrained rate: if a factor is below the level needed by the photosynthetic reactions, increasing it can increase the rate because the process can proceed closer to its potential.
• Another factor takes over: once the first factor is no longer restricting the rate, further increases have little effect if a different requirement—such as light, carbon dioxide or temperature—is now the limiting factor.
• The conclusion is conditional: the limiting factor is identified for a particular range of conditions, plant system and measured rate. It is not a permanent label attached to that factor.
• Fair evidence: change one independent factor while keeping the important alternatives and sample conditions constant, measure a rate proxy over a comparable time, repeat, and interpret the result only within the tested range.
• Rate boundary: oxygen, carbon-dioxide change, biomass or another supported signal can act as evidence for photosynthetic activity, but a proxy must not automatically be called direct carbohydrate production or the whole plant’s total photosynthesis.

Do not say that the factor with the lowest numerical value is always limiting, that a limiting factor stops photosynthesis completely, or that increasing any one factor must keep increasing the rate. Graph-reading details and specific DCPIP/aquatic-plant methods belong to later cards.