Photosynthesis In Higher Plants

Blackman's law of limiting factors, discussed in NCERT Class 11, Chapter 13 (Photosynthesis in Higher Plants), states that when a process depends on several factors, its rate is governed by the factor in shortest supply. Here light intensity and carbon dioxide are both plentiful, so neither limits the process. Temperature, however, is low, and the carbon-fixing reactions of the Calvin cycle are catalysed by enzymes whose activity falls sharply as temperature drops. Because these enzymatic dark reactions cannot keep pace with the light reactions under cold conditions, temperature becomes the limiting factor. The claim that light limits the rate is wrong, since light is stated to be high and photon absorption is not blocked merely by cold. Carbon dioxide is described as abundant, so its solubility is not the bottleneck in this scenario. Water freezing inside cells is an extreme, unlikely condition not implied by merely low temperature and would damage tissue rather than simply slow photosynthesis. A consistency check confirms the reasoning: with two factors in excess and one in deficit, the deficient factor, temperature, must be the one controlling the rate.