Q.58 The three plots 𝐏, 𝐐 and R (in different units) in the graph below represent the dependence of
photosynthesis rate (PR), leaf expansion rate (LER) and translocation rate of assimilates (TR) in a plant on
leaf water potential. Which of the following statements is correct in this regard?
(A) P represents LER; Q represents TR; R represents PR
(B) 𝑃 represents TR; 𝑄 represents PR; 𝑅 represents LER
(C) P represents PR; Q represents LER; R represents TR
(D) 𝑃 represents LER; Q represents PR; R represents TR
Introduction
Leaf water potential (Ψleaf) is a critical determinant of plant physiological performance under drought or water stress. Key processes such as leaf expansion, photosynthesis, and translocation of assimilates respond differently as water potential becomes more negative.
In this question, three curves P, Q, and R represent the dependence of:
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Photosynthesis rate (PR)
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Leaf expansion rate (LER)
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Translocation rate (TR)
on leaf water potential. The task is to correctly identify which curve corresponds to which physiological process.
Understanding the Graph
As leaf water potential decreases from 0 MPa (well-watered) to –3.0 MPa (severe water stress):
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Curve P declines very sharply at a relatively high water potential.
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Curve Q shows moderate sensitivity and declines gradually.
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Curve R remains functional even at much lower (more negative) water potentials.
This differential sensitivity forms the basis of the correct answer.
Physiological Basis
1. Leaf Expansion Rate (LER)
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Highly sensitive to water stress
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Depends directly on cell turgor pressure
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Declines sharply even under mild water deficit
➡️ Most sensitive process
2. Photosynthesis Rate (PR)
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Moderately sensitive
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Declines due to stomatal closure and metabolic limitations
➡️ Intermediate sensitivity
3. Translocation Rate (TR)
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Least sensitive
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Phloem transport can continue under relatively severe stress
➡️ Most tolerant process
Identification of Curves
| Curve | Sensitivity to Water Stress | Process |
|---|---|---|
| P | Highest sensitivity | Leaf Expansion Rate (LER) |
| Q | Moderate sensitivity | Photosynthesis Rate (PR) |
| R | Lowest sensitivity | Translocation Rate (TR) |
✅ Correct Answer
(D) P represents LER; Q represents PR; R represents TR
Explanation of All Options
❌ Option (A): P = LER; Q = TR; R = PR
Incorrect. Translocation is not more sensitive than photosynthesis, but this option assumes so.
❌ Option (B): P = PR; R = TR; Q = LER
Incorrect. Photosynthesis does not decline as sharply as leaf expansion.
❌ Option (C): P = PR; Q = LER; R = TR
Incorrect. Leaf expansion should show the earliest and steepest decline, which is curve P, not Q.
✅ Option (D): P = LER; Q = PR; R = TR
Correct. This option correctly matches physiological sensitivity with the graphical trends.
Exam Tip 💡
Mnemonic for water stress sensitivity:
LER > PR > TR
(Most sensitive → Least sensitive)
This sequence is frequently tested in CSIR-NET, GATE, and M.Sc. entrance exams using graphical questions.
Conclusion
The graph clearly demonstrates that leaf expansion is the first process to be inhibited by declining leaf water potential, followed by photosynthesis, while translocation remains functional even under severe stress. Recognizing this hierarchy is key to solving many plant physiology MCQs.
