Which plant hormone is chiefly responsible for cell division and for delaying the ageing of leaves?
- A.Auxin
- B.Cytokinin
- C.Abscisic acid
- D.Ethylene
Correct answer
Explanation
The correct answer is B, cytokinin. Cytokinins promote cytokinesis, that is cell division; kinetin was the first of them to be identified, from the degraded DNA of herring sperm, and zeatin, the natural form, was obtained from maize grains. They delay the ageing of leaves, keep cut flowers fresh and encourage lateral buds, and in tissue culture a high cytokinin to auxin ratio produces shoots. Option A, auxin, works mainly by cell elongation rather than division and maintains apical dominance. Option C, abscisic acid, is the stress hormone: it closes stomata and induces dormancy, so it restrains growth. Option D, ethylene, has the opposite effect to cytokinin on leaves, since it hastens senescence and causes leaves and fruits to fall from the plant. The auxin to cytokinin ratio in tissue culture is itself a frequently asked one-mark question.
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 →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
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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.
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.
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