Limiting factor

In the syllabus and the exam

Limiting factor is one of the more analytically demanding ideas in the nutrition in plants topic, since it requires candidates to interpret graphs rather than simply recall facts, and the three factors most commonly tested are light intensity, carbon dioxide concentration, and temperature. The core concept is that whichever of these factors is in shortest supply at a given moment sets the maximum possible rate of photosynthesis, no matter how much of the other factors is available; increasing a factor that is not limiting will not raise the rate, while increasing the limiting factor will. Exam questions typically show a graph where the rate of photosynthesis rises steadily and then plateaus, and candidates are asked to identify which factor is limiting during the rising part of the curve and to explain why the curve flattens once a different factor becomes limiting instead. A frequent error is assuming light intensity is always the limiting factor regardless of what the graph shows, or failing to notice that a plateau specifically means some other factor, not the one being varied on the x-axis, has taken over as the constraint. Candidates should also avoid vague answers such as the plant cannot photosynthesise faster, without naming the specific limiting factor responsible. Candidates should also be able to apply the limiting factor concept beyond a simple graph, for example explaining why a commercial greenhouse might artificially raise carbon dioxide levels and supply extra lighting together, since raising only one factor while another remains scarce would not improve crop yield as effectively. This principle also explains why simply adding more fertiliser to boost mineral supply will not necessarily increase a crop's rate of photosynthesis, since minerals affect plant growth through pathways other than the three factors, light, carbon dioxide, and temperature, that directly limit the photosynthetic reaction itself.

Example

Interpreting a graph, a candidate writes: 'Beyond a certain light intensity, the rate of photosynthesis levels off because carbon dioxide concentration, not light, has become the limiting factor.'

Related terms

PhotosynthesisChloroplastChlorophyll

Source:SRC-DSKP-EN