Which one of the following plant hormones promotes stomatal closure during drought stress?
- A.Auxin
- B.Cytokinin
- C.Gibberellin
- D.Abscisic acid
Correct answer
Explanation
The correct answer is D, Abscisic acid. When water is short, leaves make more abscisic acid (ABA), which makes the guard cells lose water and close the stomata, so the plant loses less water through transpiration.
ABA is a growth-inhibiting hormone and is often called the stress hormone because it helps plants survive drought, cold and salinity. It also keeps seeds and buds dormant and promotes the shedding of leaves and fruits. In guard cells it causes potassium ions to leave, water follows by osmosis, the cells go limp and the pore shuts.
A is wrong because auxin promotes cell elongation, apical dominance and rooting of cuttings, not stomatal closure. B is wrong because cytokinin promotes cell division, delays leaf ageing and tends to keep stomata open. C is wrong because gibberellin causes stem elongation, breaks seed dormancy and is used to enlarge grapes, but it does not close stomata.
Exam tip: ABA = stress hormone, closes stomata, induces dormancy; gibberellin breaks dormancy, the opposite of ABA.
Practice Questions
View allThe potential difference between the terminals of an electrical heating device is 40 V, when it draws a current of 2 A from the source. Which one of the following is the correct value of current drawn by the heating device if the potential is increased to 140 V?
- A.7 A
- B.5 A
- C.4 A
- D.2 A
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By Ohm's law the heater's resistance is 40 V divided by 2 A, which is 20 ohms, so at 140 V it draws 140 divided by 20, or 7 A. Ohm's law states that, at constant temperature, the current through a conductor is directly proportional to the potential difference across…
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- A.1 only
- B.2 only
- C.Both 1 and 2
- D.Neither 1 nor 2
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Read the full explanation →Which one among the following statements related to human eye is correct?
- A.For a person suffering from short-sight defect, the image of a distant object is focused behind the retina.
- B.Short-sight defect can be corrected by a converging spectacle lens.
- C.For a person suffering from long-sight defect, the image of a near object is focused behind the retina.
- D.Long-sight defect can be corrected by a diverging spectacle lens.
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Read the full explanation →When sunlight (white light) falls on a triangular glass prism, a band of colours called a spectrum is obtained. Which one among the following represents this effect appropriately?
- A.Reflection
- B.Diffraction
- C.Refraction
- D.Dispersion
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Read the full explanation →Which one of the following statements is correct?
- A.The number of electrons in alpha particle is 2.
- B.The coordination number of EDTA is 6.
- C.The charge of beta (β) particle is +2.
- D.Both carbon and silicon form ionic compounds.
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EDTA is a hexadentate ligand: one EDTA ion binds a metal ion through six donor atoms, so it forms complexes with coordination number 6.
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