Limiting Factors of Photosynthesis — NEET Biology
📚 Practice ConceptFor example, despite the presence of a green leaf and optimal light and CO₂ conditions, the plant may not photosynthesise if the temperature is very low. This leaf, if given the optimal temperature, will start photosynthesising.
Which of the following statements correctly illustrate the application or principles of Blackman’s Law of Limiting Factors in photosynthesis? S1: If the light intensity is significantly below saturation, an increase in atmospheric CO₂ concentration will lead to a proportional increase in the rate of photosynthesis. S2: The phenomenon where the rate of photosynthesis eventually plateaus despite continuous increases in light intensity exemplifies Blackman's Law. S3: For C₃ plants, increasing temperature beyond their optimum (20-25°C) will always enhance photosynthesis if CO₂ levels are kept high. S4: In conditions of severe water scarcity, providing optimal light and CO₂ will not significantly increase photosynthesis, as water availability becomes the primary limiting factor. S5: Blackman’s Law is only applicable to external environmental factors and does not account for internal plant factors like chlorophyll content.
View solution & NCERT explanation
Correct answer: B — S2 and S4
Let's evaluate each statement in the context of Blackman's Law: S1: This statement is incorrect. According to Blackman's Law, if light intensity is the limiting factor (nearest to its minimal value), increasing CO₂ concentration will not significantly increase the rate of photosynthesis because light remains the bottleneck. S2: This statement is correct. NCERT explains that at higher light intensities, the rate of photosynthesis does not show further increase as other factors (like CO₂ concentration or temperature) become limiting. This is a classic example of Blackman's Law in action, where one factor takes over as the limiting one once another is no longer minimal. S3: This statement is incorrect. C₃ plants have a much lower temperature optimum (20-25°C). Increasing temperature beyond this optimum will lead to a decrease in the rate of photosynthesis due to enzyme denaturation, not enhancement, even with high CO₂. S4: This statement is correct. Water is a reactant in light reactions, but more critically, water stress causes stomatal closure, reducing CO₂ availability and also wilting leaves. If water is severely limiting, it becomes the factor 'nearest to its minimal value,' thus determining the rate, irrespective of other optimal factors. S5: This statement is incorrect. Blackman’s Law applies to 'any chemical process' and considers all factors, both internal (like chlorophyll amount, leaf age, internal CO₂ concentration, number of chloroplasts) and external, that can limit the rate. Therefore, statements S2 and S4 correctly illustrate the application or principles of Blackman’s Law.
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