Apical dominance in plants, in which the main shoot suppresses the growth of lateral buds, is caused by which hormone?
- A.Cytokinin
- B.Auxin
- C.Abscisic acid
- D.Ethylene
Correct answer
Explanation
The correct answer is B, auxin. Auxin made at the shoot tip moves downwards and keeps the lateral buds below it inactive, which is why a gardener who nips off the tip of a hedge plant gets a bushy growth of side branches. Option A, cytokinin, does the opposite: it promotes the growth of lateral buds and so breaks apical dominance, and the balance of the two hormones decides the shape of the plant. Option C, abscisic acid, induces dormancy and closes stomata under stress but is not the agent of apical dominance. Option D, ethylene, causes ripening, senescence and the fall of leaves and fruits. Auxin is also the hormone of phototropism, the bending of a shoot towards light, which papers often ask in the same set.
Read the full article: Plant Hormones and Economic Botany: Notes and MCQs
Practice Questions
View allWhich of the following plant hormones is responsible for the ripening of fruits?
- A.Auxin
- B.Gibberellin
- C.Ethylene
- D.Cytokinin
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Ethylene is the only gaseous plant hormone, and it raises the rate of respiration in climacteric fruits such as banana, mango and tomato, so a single ripe fruit speeds up the ripening of those around it; the compound ethephon releases ethylene and is used in orchards for uniform ripening.
Read the full explanation →Which plant hormone is known as the stress hormone?
- A.Abscisic acid
- B.Ethylene
- C.Indole-3-acetic acid
- D.Zeatin
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It is produced when a plant faces drought, salinity or extreme temperature; it closes the stomata so that water is not lost, for which reason it is called an antitranspirant, and it induces dormancy in seeds and buds so that growth waits for better conditions.
Read the full explanation →Gibberellin, the plant hormone, was first obtained from which organism?
- A.A bacterium of the genus Rhizobium
- B.The fungus Gibberella fujikuroi
- C.The alga Chlorella
- D.Oat seedling tips
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Japanese rice plants attacked by this fungus grew abnormally tall and spindly, a condition called the foolish seedling or bakanae disease, and the substance responsible was isolated from the fungus and named gibberellin; gibberellic acid is the best known member of the group.
Read the full explanation →The chemical 2,4-D is widely used in agriculture as a
- A.fungicide
- B.weedicide
- C.chemical fertiliser
- D.rat poison
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2,4-dichlorophenoxyacetic acid is a synthetic auxin, and when it is sprayed in a heavy dose it throws broad-leaved dicot weeds into uncontrolled growth that kills them, while narrow-leaved cereals such as wheat and rice are far less affected, so it clears weeds from a standing crop.
Read the full explanation →Saffron, the costliest spice in the world, is obtained from which part of the plant?
- A.Dried flower bud
- B.Dried stigma of the flower
- C.Inner bark
- D.Rhizome
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Saffron is made from the stigmas of the flower of Crocus sativus, and since each flower carries only three stigmas which must be picked by hand, the spice is the most expensive in the world; in India it is grown mainly in the Kashmir valley.
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