Blackman's law of limiting factors: graph shows plateau after raising light, so which factor is limiting?
Ananya Reddy11th Sample
18 Sept 2026 · 1 min read
Colour-coded notes for factors affecting photosynthesis: light (yellow), CO₂ (grey), temperature (red), water (blue), and I thought I was done. Then I did a graph-based question.
The graph showed photosynthetic rate vs light intensity at a fixed, low CO₂ concentration. The rate increases at first and then flattens out. The question asked "Which is the limiting factor at the plateau?" I answered "light". Wrong, the key says CO₂ (or another factor other than light).
NCERT mentions Blackman's (1905) Law of Limiting Factors: if a chemical process is affected by more than one factor, its rate is determined by the factor which is nearest to its minimal value. It is the factor that directly affects the process if its quantity is changed.
Where I need clarity:
- At the plateau, why is light not limiting?
- NCERT says the light saturation is at ~10% of full sunlight, except for plants in shade or in dense forests. If so, why do plants need full sunlight?
- For CO₂, C3 and C4 plants saturate at different concentrations. Which is which? I think C4 saturates at a lower level than C3 but I'm not sure.
- Is temperature the same for all plants? I recall tropical plants vs temperate plants.
This would help me for the next test!
8 answers & replies
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[+][–]Meera Patel11thSample· 1mo ago
At the plateau, increasing light no longer raises the rate. That tells you the process is now being held back by something else: with low CO₂ in your question, it's CO₂. Limiting factor = the one in shortest supply relative to what the process needs. On the rising part of the curve, light is limiting; on the flat part, it isn't, so something else is. If you raised CO₂ at that point the curve would shoot up to a higher plateau.
[+][–]Ananya Reddy11thSample· 1mo agoAuthor
Right, I confused "the factor I varied" with "the factor that's limiting". Writing that on a sticky note.
[+][–]Siddharth RaoDropperSample· 1mo ago
CO₂ numbers from NCERT: C4 plants show saturation at about 360 μL L⁻¹, while C3 plants keep responding up to about 450 μL L⁻¹ and beyond. The current atmospheric level is around 0.03–0.04%, i.e. 300–400 μL L⁻¹ or so, so C3 plants are not saturated in ordinary air, which is why greenhouse crops like tomato and bell pepper are given extra CO₂ to increase yield. You had it right: C4 saturates lower than C3.
[+][–]Rahul DubeyDropperSample· 1mo ago
Light: NCERT says light saturation occurs at about 10% of the full sunlight, so beyond that, more light doesn't help (except for plants in shade or in dense forests, which are adapted to very low light). Why do plants need full sunlight, then? Because the leaves at the top shade the ones below. Also, incoming light varies over the day, clouds, and so on, and a plant needs enough capacity at any moment. Also, too much light can damage the chlorophyll (photo-oxidation). So light being saturated at 10% doesn't mean full sunlight is wasted.
[+][–]Ananya Reddy11thSample· 1mo agoAuthor
Photo-oxidation, I didn't know that part. Added to the notes, thanks.
[+][–]Ishita GoyalDropperSample· 1mo ago
Temperature: the dark reactions are enzymatic, so they are temperature controlled, but the light reaction is less affected. C4 plants respond to higher temperatures and show a higher temperature optimum than C3 plants. Likewise, tropical plants have a higher temperature optimum than temperate plants. This was an NCERT line, and it's also what the question writers like to test with "which plant has higher optimum".
[+][–]Kavya NaikDropperSample· 1mo ago
Pretty sure beyond saturation of light, if you give a lot more light, the rate keeps going up slowly, no? Just less steeply?
[+][–]Meera Patel11thSample· 1mo ago
Kavya, in the idealised NCERT-type graph it flattens (plateau). In real leaves, at extremely high intensities the rate can actually fall because of photo-oxidative damage to chlorophyll, but it doesn't keep rising. For exam graphs, the curve plateaus when another factor is limiting. Don't go with "slowly rising" as an answer.
