Q.57 Given below are two statements:
Statement-I: Auxins delay the onset of leaf abscission.
Statement-II: Abscisic acid promotes leaf senescence independently of ethylene hormone.
In the light of the above statements, choose the most appropriate answer
from the options given below:
(A) Both Statement-I and Statement-II are correct
(B) Both Statement-I and Statement-II are incorrect
(C) Statement-I is correct but Statement-II is incorrect
(D) Statement-I is incorrect but Statement-II is incorrect
Statement-I is correct, but Statement-II is incorrect, so the answer is (C).
Statement Verification
Auxins (like IAA) inhibit leaf abscission by maintaining an auxin gradient across the abscission zone; exogenous application delays shedding, as shown in Populus and classic studies. Statement-I is true. Abscisic acid (ABA) promotes leaf senescence, but not independently of ethylene—ABA often acts via ethylene signaling pathways (e.g., through NAP or EIN2), with mutual crosstalk. Statement-II is false.
Option Evaluation
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(A) Both correct: Incorrect; II wrongly claims ethylene independence despite evidence of crosstalk.
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(B) Both incorrect: Incorrect; I is a standard fact in plant physiology.
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(C) I correct, II incorrect: Correct; aligns with hormonal regulation data.
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(D) I incorrect, II correct: Incorrect; confuses roles—auxins delay, not promote abscission.
Introduction: Auxins Delay Leaf Abscission and Plant Hormones
Auxins delay leaf abscission by inhibiting the process at the abscission zone, a key mechanism in plant development. Abscisic acid (ABA) promotes leaf senescence, but interacts with ethylene, not independently. These roles are crucial for NEET and botany exams analyzing plant hormone MCQs like Statement-I and II.
Role of Auxins in Leaf Abscission
Auxins prevent premature leaf drop:
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High auxin from young leaves suppresses enzymes hydrolyzing middle lamella in abscission zone.
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Auxin decline with age triggers abscission; exogenous IAA delays it by 1+ days.
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Classic: Blade removal induces drop, but auxin paste prevents it.
Abscisic Acid in Leaf Senescence
ABA accelerates senescence but relies on ethylene:
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Exogenous ABA upregulates SAGs via OsNAP, connecting to ethylene biosynthesis (ACS/ACO genes).
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NAP mediates ABA-ethylene crosstalk; blocking ethylene delays ABA senescence.
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Not independent: ABA enhances ethylene signaling; mutants like ein2 show partial ABA response.
MCQ Breakdown: Statement-I and Statement-II
Question: Statement-I: Auxins delay leaf abscission onset. Statement-II: ABA promotes senescence independently of ethylene.
| Option | Evaluation |
|---|---|
| (A) Both correct | Wrong; II ignores ABA-ethylene synergy |
| (B) Both incorrect | Wrong; I is textbook fact |
| (C) I correct, II incorrect | Correct; matches physiology evidence |
| (D) I incorrect, II correct | Wrong; reverses auxin role |
Answer: (C)
Practical Implications
These interactions regulate seasonal leaf fall, stress responses, and crop yield (e.g., delaying fruit drop). ABA-ethylene balance fine-tunes senescence for nutrient recycling.
