Practice important
Root Growth Inhibition
MCQ questions and answers.
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MCQ Questions
1. A cytokinin concentration that produces little visible effect on shoots causes a marked reduction in primary-root elongation. Which interpretation best explains this result?
A.
Roots are generally more sensitive to cytokinin than shoots with respect to growth inhibition
B.
Cytokinin stimulates root-cell elongation but prevents water uptake
C.
Cytokinin inhibits shoot apical meristem activity before affecting roots
D.
Roots require cytokinin only for chlorophyll formation
Correct Answer:
Roots are generally more sensitive to cytokinin than shoots with respect to growth inhibition
Explanation:
Cytokinin can inhibit primary-root elongation at concentrations that may have weak or different effects on shoots. This reflects the comparatively high sensitivity of root growth to cytokinin-mediated inhibition.
2. Two genetically identical seedlings are grown on media containing equal cytokinin concentrations. Seedling X has cytokinin-receptor activity restricted mainly to the root meristem, whereas Seedling Y lacks this activity there. What is most likely observed?
A.
Seedling X shows stronger primary-root growth inhibition than Seedling Y
B.
Seedling Y shows stronger primary-root growth inhibition than Seedling X
C.
Both seedlings show identical root inhibition because receptors are irrelevant
D.
Neither seedling shows root inhibition because cytokinin acts only in leaves
Correct Answer:
Seedling X shows stronger primary-root growth inhibition than Seedling Y
Explanation:
Cytokinin must be perceived through its receptors to exert its inhibitory effect on root growth. Restricting receptor activity to the root meristem would therefore make Seedling X more responsive to cytokinin inhibition there.
3. A treatment reduces the length of the primary root without causing a proportional decrease in the number of cells produced in the root meristem. Which cytokinin-associated change most plausibly contributes to this phenotype?
A.
Reduced elongation of newly produced root cells
B.
Increased elongation of every root cell
C.
Complete conversion of root cells into leaf cells
D.
Enhanced differentiation of root cells into chloroplast-rich cells
Correct Answer:
Reduced elongation of newly produced root cells
Explanation:
Primary-root length depends strongly on the elongation of cells formed near the meristem. Cytokinin-mediated inhibition can reduce root elongation even when cell production is not proportionally reduced.
4. In an experiment, increasing cytokinin concentration causes primary-root length to decline progressively, while root viability remains high. Which conclusion is most justified?
A.
The cytokinin response is inhibitory and concentration dependent within the tested range
B.
Cytokinin is functioning as a nutrient whose deficiency causes root death
C.
The decline in root length proves that all root cells have died
D.
Cytokinin must be promoting unlimited root elongation at the highest concentration
Correct Answer:
The cytokinin response is inhibitory and concentration dependent within the tested range
Explanation:
A progressive decrease in root length with increasing cytokinin, in the absence of major cell death, indicates a concentration-dependent inhibition of root growth rather than general toxicity.
5. A cytokinin-insensitive mutant develops longer primary roots than the wild type when both are exposed to the same inhibitory cytokinin concentration. Which inference is strongest?
A.
Cytokinin perception is required for normal expression of this root-growth inhibition
B.
The mutant produces more cytokinin because it cannot perceive it
C.
Cytokinin inhibits roots only after being converted into auxin
D.
The wild type lacks any mechanism for responding to cytokinin
Correct Answer:
Cytokinin perception is required for normal expression of this root-growth inhibition
Explanation:
Reduced inhibition in a cytokinin-insensitive mutant indicates that perception and downstream signaling of cytokinin are necessary for the inhibitory response in the root.
6. Which experimental comparison would most directly demonstrate that cytokinin inhibits root elongation rather than merely reducing seedling water content?
A.
Measure primary-root length and root-cell viability at several cytokinin concentrations
B.
Measure only the green color of the cotyledons after treatment
C.
Compare leaf number without recording any root characteristic
D.
Record the medium volume before treatment but not root growth
Correct Answer:
Measure primary-root length and root-cell viability at several cytokinin concentrations
Explanation:
Measuring root length together with viability distinguishes a specific growth-inhibitory response from nonspecific dehydration or tissue death. A shorter but viable root supports inhibition of growth.
7. A root is treated locally with cytokinin while the shoot and the remainder of the root receive hormone-free medium. The treated region shows reduced elongation compared with an untreated region. What does this result primarily establish?
A.
Cytokinin can inhibit root growth through a local action
B.
Cytokinin inhibits roots only after transport from leaves
C.
Cytokinin has no direct effect on root tissues
D.
Root inhibition requires cytokinin exposure of the entire plant
Correct Answer:
Cytokinin can inhibit root growth through a local action
Explanation:
A reduction in elongation specifically in the cytokinin-treated root region demonstrates that cytokinin can act locally to inhibit root growth, without requiring treatment of the whole plant.
8. When cytokinin is supplied to seedlings, primary-root elongation decreases most strongly near the root tip. Which explanation is most consistent with this pattern?
A.
Root-tip tissues contain growth-regulatory zones that are particularly sensitive to cytokinin
B.
Cytokinin is active only in mature leaves and reaches the root tip by evaporation
C.
The root tip is unaffected by hormones but always dies first from lack of light
D.
Cytokinin increases root-tip elongation, but the apparent decrease is caused by leaf growth
Correct Answer:
Root-tip tissues contain growth-regulatory zones that are particularly sensitive to cytokinin
Explanation:
The root tip contains the meristematic and elongation regions that determine primary-root extension. Cytokinin sensitivity in these zones can therefore produce especially strong inhibition near the tip.
9. A researcher finds that a cytokinin treatment shortens primary roots, but removing cytokinin from the medium allows elongation to resume. Which property of cytokinin action does this observation support?
A.
Root-growth inhibition can be reversible when the cytokinin signal is removed
B.
Cytokinin permanently destroys the root meristem in every treatment
C.
Cytokinin inhibition is unrelated to the concentration surrounding the root
D.
Cytokinin causes irreversible genetic conversion of roots into shoots
Correct Answer:
Root-growth inhibition can be reversible when the cytokinin signal is removed
Explanation:
Recovery after cytokinin removal indicates that the inhibition was not necessarily permanent tissue destruction. The root can resume growth when the inhibitory hormonal signal is withdrawn.
10. In a screening assay for compounds that counteract cytokinin-mediated root inhibition, which result identifies the most effective candidate, assuming the compounds do not alter viability?
A.
The treated seedlings retain the greatest primary-root elongation in the presence of cytokinin
B.
The treated seedlings have the lowest viability in the absence of cytokinin
C.
The treated seedlings show the strongest cytokinin-induced root shortening
D.
The treated seedlings produce the fewest roots regardless of cytokinin exposure
Correct Answer:
The treated seedlings retain the greatest primary-root elongation in the presence of cytokinin
Explanation:
A compound that counteracts cytokinin-mediated inhibition should preserve primary-root elongation despite cytokinin exposure. Root viability must remain comparable so that the result reflects growth protection rather than survival differences.
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