Practice important
Root elongation inhibition
MCQ questions and answers.
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MCQ Questions
1. A set of genetically identical seedlings is exposed to increasing concentrations of auxin. Root elongation is 92%, 76%, 41%, and 8% of the untreated control at concentrations of 10^-9 M, 10^-7 M, 10^-5 M, and 10^-3 M, respectively. Which conclusion is best supported?
A.
Auxin inhibits root elongation progressively as its concentration increases over this range
B.
Auxin promotes root elongation at all concentrations but damages roots at 10^-3 M
C.
Root elongation is independent of auxin concentration below 10^-3 M
D.
The highest concentration specifically stimulates cell division in the root elongation zone
Correct Answer:
Auxin inhibits root elongation progressively as its concentration increases over this range
Explanation:
The decreasing percentage of control elongation with increasing auxin concentration indicates concentration-dependent inhibition of root elongation.
2. Roots and shoots of the same plant are treated with the same auxin concentration. Root elongation decreases markedly, whereas shoot elongation increases. Which interpretation most accurately explains this result?
A.
Roots are generally more sensitive to auxin than shoots, so a concentration inhibitory to roots may stimulate shoots
B.
Auxin is transported only into roots, but not into shoots
C.
Auxin always inhibits cell elongation in roots and shoots equally
D.
Roots lack auxin receptors, causing their elongation to stop at low concentrations
Correct Answer:
Roots are generally more sensitive to auxin than shoots, so a concentration inhibitory to roots may stimulate shoots
Explanation:
The concentration-response range differs between organs; roots commonly show inhibition at concentrations that can promote shoot elongation.
3. In an experiment, the root elongation rates at four auxin concentrations are as follows: 0 M: 4.8 mm/day, 10^-8 M: 5.6 mm/day, 10^-6 M: 2.9 mm/day, and 10^-4 M: 0.4 mm/day. Which concentration is closest to the transition between stimulation and inhibition of root elongation?
A.
0 M
B.
10^-8 M
C.
10^-6 M
D.
10^-4 M
Correct Answer:
10^-8 M
Explanation:
Elongation is stimulated above the untreated value at 10^-8 M but is reduced at 10^-6 M, so the transition lies between these concentrations and is closest to 10^-8 M among the choices.
4. A researcher applies an auxin concentration to one side of a vertically growing root. The treated side elongates less than the untreated side. What immediate structural consequence is most likely?
A.
The root bends toward the side receiving auxin
B.
The root bends away from the side receiving auxin
C.
The root remains straight because unequal elongation cannot alter growth direction
D.
The root bends randomly because auxin inhibits only cell division
Correct Answer:
The root bends away from the side receiving auxin
Explanation:
Greater inhibition of elongation on the treated side causes the opposite side to extend more, producing curvature away from the auxin-treated side.
5. Two roots receive identical auxin concentrations, but Root X shows stronger inhibition of elongation than Root Y. Which explanation is most consistent with the known response of roots?
A.
Root X has greater sensitivity to auxin or a lower threshold concentration for inhibition
B.
Root X must have a higher rate of photosynthesis than Root Y
C.
Root Y cannot contain any cells capable of elongation
D.
Root X must be receiving less auxin because inhibition requires auxin deficiency
Correct Answer:
Root X has greater sensitivity to auxin or a lower threshold concentration for inhibition
Explanation:
Different roots can show different response thresholds; stronger inhibition at the same concentration indicates greater sensitivity or a lower inhibitory threshold.
6. A root is exposed to an auxin concentration that inhibits elongation. After the auxin is removed and the root is transferred to an auxin-free medium, elongation gradually resumes. Which conclusion is most justified?
A.
The inhibition was reversible and depended on continued exposure or persistence of auxin
B.
Auxin permanently destroyed all cells in the root elongation zone
C.
The root had become genetically incapable of responding to auxin
D.
Auxin had permanently converted the root into a shoot
Correct Answer:
The inhibition was reversible and depended on continued exposure or persistence of auxin
Explanation:
Recovery after removal indicates that the inhibitory effect on root elongation was not necessarily permanent and can decline when auxin exposure ends.
7. A student concludes that a high auxin concentration promotes root elongation because roots treated with that concentration are shorter and thicker than untreated roots. Why is this conclusion incorrect?
A.
Shorter roots indicate reduced longitudinal elongation, even if radial growth or tissue swelling increases
B.
Root length cannot be used to assess elongation under any experimental condition
C.
Auxin promotes only root elongation and never affects root diameter
D.
A shorter root necessarily indicates faster longitudinal elongation
Correct Answer:
Shorter roots indicate reduced longitudinal elongation, even if radial growth or tissue swelling increases
Explanation:
Root elongation refers to increase in length. A root may become thicker while its longitudinal extension is inhibited by high auxin.
8. In a dose-response experiment, untreated roots elongate 10 mm in 24 hours. At a particular auxin concentration, they elongate 6 mm. What is the percentage inhibition of root elongation relative to the control?
9. Four auxin treatments produce the following root elongation values after 48 hours: 0 M: 12 mm, 10^-9 M: 14 mm, 10^-7 M: 8 mm, and 10^-5 M: 2 mm. Which treatment demonstrates the strongest inhibition relative to the untreated control?
A.
0 M
B.
10^-9 M
C.
10^-7 M
D.
10^-5 M
Correct Answer:
10^-5 M
Explanation:
The 10^-5 M treatment produces the smallest elongation, 2 mm compared with 12 mm in the control, representing the greatest inhibition.
10. A researcher wants to identify the lowest auxin concentration that inhibits root elongation rather than stimulates it. Which experimental design is most appropriate?
A.
Measure root length in untreated seedlings and in seedlings exposed to several closely spaced auxin concentrations below and above the suspected transition
B.
Compare only seedlings treated with the highest auxin concentration to seedlings treated with water
C.
Measure shoot height after applying one auxin concentration to the roots
D.
Measure root color without recording changes in root length
Correct Answer:
Measure root length in untreated seedlings and in seedlings exposed to several closely spaced auxin concentrations below and above the suspected transition
Explanation:
A concentration series with an untreated control allows the threshold between stimulation and inhibition of root elongation to be located accurately.
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