Biology · Photosynthesis in Higher Plants · NEET
A limiting factor is the one factor, out of several needed for a process, that is present in the smallest amount and so holds back the rate. If a plant has plenty of light and warm temperature but very little CO2, then CO2 is the limiting factor. Increasing any other factor will not raise the rate until you raise the limiting one. This is the core idea NCERT uses to explain why photosynthesis speeds up or slows down.
Blackman (1905) stated: when a chemical process is affected by more than one factor, its rate is decided by the factor that is nearest to its minimum value. In simple words, at any given moment only ONE factor (the one in shortest supply) controls the rate. The other factors, even if plentiful, cannot help until the limiting one is increased.
At low light, light is the limiting factor, so the rate rises in a straight line as you add more light. But after a point (light saturation, near about 10% of full sunlight per NCERT), light is no longer short — now CO2 or temperature becomes the limiting factor, so the curve flattens. Adding still more light does nothing because light has stopped being the limiting factor.
NCERT clearly states that carbon dioxide is the major limiting factor for photosynthesis, because atmospheric CO2 is very low (0.03 to 0.04 per cent). Raising CO2 up to about 0.05 per cent can increase CO2 fixation, which is why greenhouse crops like tomato are grown in CO2-enriched air for higher yield.
For NEET, remember the law as stated. In reality two factors can sometimes limit the rate together, so the sharp 'break point' Blackman drew is a simplification. But NEET questions test the classic version: one factor in shortest supply sets the rate, and the rate-versus-factor graph rises then plateaus.
With reference to factors affecting the rate of photosynthesis, which of the following statements is not correct?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
The British plant physiologist F. F. Blackman proposed the law of limiting factors in 1905, which is why it is called Blackman's law of limiting factors.
External factors are light (intensity, quality, duration), CO2 concentration, temperature and water. Internal (plant) factors include the number, size, age and orientation of leaves and the amount of chlorophyll. Any of these can become the limiting factor.
Because CO2 in air is very low (0.03 to 0.04 per cent). Since it is usually the factor in shortest supply, it most often limits the rate of photosynthesis in normal conditions.
The plateau means the factor on the x-axis is no longer limiting. Some other factor (like CO2 or temperature) has now become the limiting factor, so the rate stays constant even if you keep increasing the first factor.
It tells farmers to raise the factor that is actually limiting yield. In greenhouses, adding CO2 (the common limiting factor) increases photosynthesis and yield, as seen with tomato grown in CO2-enriched air.
