The rate of photosynthesis responds to light intensity and carbon dioxide supply until another factor becomes limiting. At very low light, extra illumination can raise rate. At higher illumination, a plateau may appear when CO₂ supply, enzyme capacity or another variable limits further increase. The exact limiting factor cannot be inferred from a single curve without a controlled intervention.

Increasing CO₂ at fixed suitable light can improve carbon assimilation up to a plateau. A higher light intensity may allow a larger potential rate when CO₂ is sufficient. A response curve should label whether its vertical axis is gross photosynthesis, net gas exchange, oxygen evolution or carbon fixation.

Blackman’s limiting-factor principle states that the slowest or most limiting required factor constrains the rate under given conditions. Several factors may act jointly; the familiar simplified rule is useful for interpreting controlled experiments rather than assigning a universal limiting factor for all leaves.

When reading a graph, a non-zero plateau does not mean chlorophyll stopped absorbing all light or that photosynthesis ended; it means the measured rate stopped rising within the plotted range. Look for whether adding CO₂ moves the plateau before naming the limitation.

Worked example: A light-response curve levels at 50 units; increasing CO₂ at the same high light raises it to 70. This supports CO₂ limitation in the original high-light region.

NCERT trap: A plateau alone does not prove light is limiting; a factor outside the x-axis may constrain further increase.

NEET method: Read axes, compare experimental curves, and identify the changed variable that restores rate.

Reading estimate; allow another 3–5 minutes for each set of three checks.

Photosynthesis in Higher Plants

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