Practice important
Delayed Senescence
MCQ questions and answers.
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MCQ Questions
1. A detached mature leaf is maintained with its petiole immersed in a cytokinin solution, while an identical leaf is immersed in water. Which observation most specifically demonstrates cytokinin-induced delayed senescence?
A.
The treated leaf retains chlorophyll and protein for longer than the control leaf
B.
The treated leaf develops roots before the control leaf
C.
The treated leaf shows increased cell-wall lignification
D.
The treated leaf undergoes earlier abscission than the control leaf
Correct Answer:
The treated leaf retains chlorophyll and protein for longer than the control leaf
Explanation:
Cytokinins delay senescence by maintaining chlorophyll, proteins, and cellular metabolic activity in leaves, thereby postponing yellowing and functional decline.
2. The most direct physiological basis of cytokinin-mediated delayed senescence in a leaf is that cytokinin treatment:
A.
Promotes nutrient mobilization toward the treated tissue and sustains macromolecule synthesis
B.
Blocks all respiration and prevents ATP consumption
C.
Converts mature mesophyll cells into actively dividing meristematic cells
D.
Induces immediate dehydration of the leaf to reduce metabolic damage
Correct Answer:
Promotes nutrient mobilization toward the treated tissue and sustains macromolecule synthesis
Explanation:
Cytokinins act as sink-promoting signals, directing nutrients toward treated tissues and helping maintain protein synthesis and other metabolic processes during delayed senescence.
3. In an experiment, cytokinin is applied to only one region of a detached leaf. Nutrient accumulation is expected to be greatest near the treated region primarily because cytokinin:
A.
Creates a metabolic sink that attracts assimilates and mineral nutrients
B.
Destroys chloroplasts in untreated regions, forcing nutrients into the treated region
C.
Stops water movement through the leaf veins
D.
Converts the treated region into reproductive tissue
Correct Answer:
Creates a metabolic sink that attracts assimilates and mineral nutrients
Explanation:
A cytokinin-treated region behaves as a sink, attracting nutrients and assimilates. This localized nutrient mobilization helps preserve cellular constituents and delays senescence there.
4. Which combination of changes would provide the strongest evidence that cytokinin has delayed leaf senescence rather than merely increased leaf expansion?
A.
Higher chlorophyll content, lower protein degradation, and prolonged photosynthetic activity
B.
Greater leaf area, earlier chlorophyll loss, and increased membrane breakdown
C.
Reduced chlorophyll content, increased yellowing, and enhanced protease activity
D.
Increased abscission, accelerated lipid degradation, and reduced nutrient retention
Correct Answer:
Higher chlorophyll content, lower protein degradation, and prolonged photosynthetic activity
Explanation:
Delayed senescence is characterized by retention of chlorophyll, reduced degradation of proteins and membranes, and maintenance of photosynthetic function, not simply by increased size.
5. A cytokinin-treated leaf remains green even though its total dry mass changes very little. Which explanation best accounts for this result?
A.
Cytokinin preserves existing chloroplasts and cellular proteins rather than necessarily causing substantial new biomass formation
B.
Cytokinin replaces all leaf proteins with inorganic salts
C.
Cytokinin prevents gas exchange, making the leaf appear green without preserving cells
D.
Cytokinin causes chlorophyll to become permanently independent of chloroplast structure
Correct Answer:
Cytokinin preserves existing chloroplasts and cellular proteins rather than necessarily causing substantial new biomass formation
Explanation:
The delayed-senescence effect mainly maintains existing cellular structures and biochemical functions. It need not produce a large increase in total dry mass.
6. Why can a cytokinin application to an otherwise senescing leaf delay yellowing most effectively when nutrients are available in surrounding tissues?
A.
Cytokinin enhances sink strength, allowing nutrients to be mobilized into the treated leaf
B.
External nutrients chemically convert directly into chlorophyll without cellular metabolism
C.
Cytokinin prevents nutrients from entering the treated tissue, thereby slowing its metabolism
D.
Nutrient availability irreversibly inactivates all senescence-associated enzymes
Correct Answer:
Cytokinin enhances sink strength, allowing nutrients to be mobilized into the treated leaf
Explanation:
Cytokinin-mediated delayed senescence depends partly on nutrient mobilization. Available nutrients can be redirected toward the treated tissue, helping maintain chlorophyll, proteins, and metabolism.
7. A researcher compares two detached leaves: cytokinin is applied to the petiole of one leaf, whereas the same amount is applied to the surface of the other leaf. If uptake is equivalent, which result is most consistent with delayed senescence?
A.
Both leaves retain chlorophyll longer than untreated leaves, because cytokinin reaches living tissues in each case
B.
Only the petiole-treated leaf senesces, because cytokinin cannot act in lamina cells
C.
Neither leaf changes, because cytokinin affects only roots
D.
Both leaves lose chlorophyll faster, because cytokinin accelerates nutrient depletion
Correct Answer:
Both leaves retain chlorophyll longer than untreated leaves, because cytokinin reaches living tissues in each case
Explanation:
When cytokinin is successfully taken up and reaches living leaf tissues, it can delay senescence by maintaining chlorophyll and cellular constituents, regardless of the application site.
8. The commercial use of cytokinins to extend the freshness of harvested leafy vegetables is principally based on their ability to:
A.
Retard chlorophyll loss and preserve cellular proteins after harvest
B.
Increase seed dormancy and prevent germination during storage
C.
Induce rapid fruit ripening and enhance pigment degradation
D.
Cause immediate tissue desiccation that eliminates all microbial metabolism
Correct Answer:
Retard chlorophyll loss and preserve cellular proteins after harvest
Explanation:
After harvest, leafy tissues continue to senesce. Cytokinins can delay this process by preserving chlorophyll, proteins, and metabolic activity, helping maintain freshness.
9. If a cytokinin-treated leaf remains green but shows severe loss of membrane integrity, which conclusion is most appropriate?
A.
Green coloration alone is insufficient evidence that complete senescence has been prevented
B.
Membrane damage proves that cytokinin has fully prevented senescence
C.
Cytokinin delayed senescence only by increasing chlorophyll synthesis in dead cells
D.
Membrane integrity is unrelated to the progression of leaf senescence
Correct Answer:
Green coloration alone is insufficient evidence that complete senescence has been prevented
Explanation:
Delayed senescence involves preservation of several cellular features, including membranes, proteins, and chlorophyll. Retention of green color alone does not establish that all senescence-related deterioration has been prevented.
10. In a leaf experiment, cytokinin treatment preserves chlorophyll only in the treated sector, while the untreated sector yellows normally. Which inference is most justified?
A.
The delayed-senescence response is localized and depends on cytokinin-induced nutrient mobilization toward the treated sector
B.
Cytokinin must have been transported uniformly throughout the entire leaf
C.
The untreated sector is producing more cytokinin but cannot retain chlorophyll
D.
Yellowing in the untreated sector proves that chlorophyll is transported out of the leaf
Correct Answer:
The delayed-senescence response is localized and depends on cytokinin-induced nutrient mobilization toward the treated sector
Explanation:
A localized cytokinin treatment can create a local sink, causing nutrients to accumulate in that region and delaying senescence there without producing the same effect in untreated tissue.
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Delayed Senescence
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