Practice important
Gravitropism
MCQ questions and answers.
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10
MCQ Questions
1. A horizontally placed stem develops upward curvature because gravity-induced lateral redistribution of auxin causes
A.
greater auxin concentration on the upper side, inhibiting upper-side elongation
B.
greater auxin concentration on the lower side, promoting lower-side elongation
C.
equal auxin concentration on both sides, producing identical elongation
D.
complete destruction of auxin on the lower side, preventing curvature
Correct Answer:
greater auxin concentration on the lower side, promoting lower-side elongation
Explanation:
In shoots, increased auxin concentration generally promotes cell elongation. When a stem is placed horizontally, auxin accumulates on its lower side, causing greater elongation there and bending the shoot upward, away from gravity.
2. In a horizontally placed root, downward curvature occurs because the higher auxin concentration on the lower side
A.
stimulates greater elongation of lower-side cells than upper-side cells
B.
inhibits elongation of lower-side cells relative to upper-side cells
C.
promotes cell division only in the root cap
D.
causes the upper side to lose its statoliths
Correct Answer:
inhibits elongation of lower-side cells relative to upper-side cells
Explanation:
Roots are generally more sensitive to auxin than shoots. Excess auxin on the lower side inhibits elongation there, while the upper side elongates more, producing downward curvature toward gravity.
3. Which sequence best represents the early events in gravitropism of a horizontally placed shoot?
A.
Perception of gravity, lateral auxin redistribution, differential elongation, curvature
B.
Differential elongation, gravity perception, auxin synthesis, curvature
C.
Lateral auxin redistribution, loss of gravity perception, uniform elongation, curvature
D.
Curvature, gravity perception, auxin degradation, lateral cell division
Explanation:
Gravitropism begins with gravity perception, commonly involving sedimentation of statoliths. This leads to lateral redistribution of auxin, unequal elongation on opposite sides, and finally organ curvature.
4. A root is placed horizontally, but its root cap is removed before the experiment. Which result is most likely?
A.
Normal downward curvature because the root cap is unrelated to gravity perception
B.
Reduced or absent gravitropic curvature because gravity perception is impaired
C.
Upward curvature because auxin becomes permanently concentrated on the upper side
D.
Immediate uniform elongation because the root cap produces all root auxin
Correct Answer:
Reduced or absent gravitropic curvature because gravity perception is impaired
Explanation:
The root cap, particularly its columella cells, contains gravity-sensing statoliths. Removing it disrupts gravity perception and therefore interferes with the auxin redistribution required for root gravitropism.
5. In a rotating clinostat experiment, a young seedling shows little or no persistent gravitropic curvature primarily because
A.
auxin synthesis stops completely during rotation
B.
the direction of the gravity stimulus is continuously averaged
C.
the root cap is removed by centrifugal force
D.
auxin becomes permanently confined to the organ's upper side
Correct Answer:
the direction of the gravity stimulus is continuously averaged
Explanation:
A slowly rotating clinostat continually changes the organ's orientation relative to gravity. As a result, no single side receives a sustained gravitational stimulus, preventing stable auxin asymmetry and persistent gravitropic curvature.
6. A mutant plant has defective statolith sedimentation but normal auxin transport and cell elongation. Which gravitropic response is most directly affected?
A.
Detection of the direction of gravity
B.
Auxin-mediated elongation after gravity perception
C.
Cellular response to auxin on the two sides
D.
Conversion of differential elongation into organ curvature
Correct Answer:
Detection of the direction of gravity
Explanation:
Statoliths act as gravity-sensing structures. If they cannot sediment properly, the plant may retain auxin responsiveness and transport capacity but fail to determine which side should receive the gravitropic signal.
7. If lateral auxin transport in a horizontally placed shoot is experimentally blocked while gravity perception remains intact, the shoot would most likely
A.
curve normally because auxin need not move laterally
B.
show reduced curvature because unequal auxin distribution cannot develop
C.
curve more strongly because auxin remains concentrated at the sensing site
D.
show root-like downward curvature because all shoot auxin is inhibited
Correct Answer:
show reduced curvature because unequal auxin distribution cannot develop
Explanation:
Gravity perception must be followed by lateral auxin redistribution for differential growth. Blocking this movement prevents the lower and upper sides of the shoot from receiving different auxin concentrations, reducing gravitropic bending.
8. A vertically growing primary root is displaced horizontally and then restored to its original vertical position before visible curvature develops. What is the most likely outcome?
A.
It must curve downward permanently because any gravity stimulus is irreversible
B.
It may show little net curvature if the transient auxin asymmetry is removed before differential growth becomes substantial
C.
It must curve upward because auxin redistribution reverses immediately
D.
It will stop growing because gravity perception permanently inhibits the root meristem
Correct Answer:
It may show little net curvature if the transient auxin asymmetry is removed before differential growth becomes substantial
Explanation:
Gravitropic curvature requires sustained differential growth following auxin redistribution. If the organ is returned to its original orientation early, the asymmetry may dissipate before enough unequal elongation occurs to produce visible bending.
9. Why can the same gravity-induced auxin redistribution produce upward curvature in a shoot but downward curvature in a root?
A.
Auxin moves upward in shoots but downward in roots
B.
Shoots and roots differ in their elongation responses to the local auxin concentration
C.
Gravity acts upward on shoots and downward on roots
D.
Roots lack auxin receptors, whereas shoots possess them
Correct Answer:
Shoots and roots differ in their elongation responses to the local auxin concentration
Explanation:
In shoots, the higher auxin concentration on the lower side usually promotes elongation, causing upward bending. In roots, the same relative auxin accumulation inhibits lower-side elongation, causing downward bending.
10. A horizontally placed shoot is treated with an auxin transport inhibitor only on its lower side. Which effect would most directly reduce its normal upward gravitropic curvature?
A.
Lower-side auxin accumulation would be reduced, decreasing differential elongation
B.
Upper-side auxin accumulation would increase, causing stronger upward curvature
C.
Gravity perception would become stronger because the inhibitor stimulates statolith sedimentation
D.
The lower side would elongate less because auxin inhibition is stronger in shoots
Correct Answer:
Lower-side auxin accumulation would be reduced, decreasing differential elongation
Explanation:
Normal shoot gravitropism depends on increased auxin on the lower side. Inhibiting auxin transport there weakens the auxin difference between the two sides and therefore reduces the differential elongation responsible for upward curvature.
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