CDS (II) 2024 General Knowledge · Question 3 of 120

Correct — C, Abscisic acid. Of the five classical plant hormones, four promote growth and one restrains it, and abscisic acid is that one. It is the hormone of stress and slowdown: it closes stomata when a plant starts to lose water, it imposes and maintains dormancy in buds and seeds, and it hastens the ageing and shedding of leaves. Auxin, cytokinin and gibberellin, the other three names offered here, each drive growth forward — elongation, division and shoot extension respectively — so none of them can be the inhibitor. Abscisic acid is often called the stress hormone for exactly this reason, and its effect is antagonistic to gibberellin at almost every point where the two meet, most visibly in the control of seed germination.

• (a)Auxins — Auxin is a promoter, not an inhibitor. It drives cell elongation, and through that it produces phototropism, the bending of a shoot towards light. Its one restraining effect, apical dominance, is a suppression of side buds by a growing tip, not a slowing of growth as such.
• (b)Cytokinins — Cytokinins promote cell division — the name is built on cytokinesis. They are made largely in the roots and in other actively dividing tissue, and they delay leaf ageing rather than hasten it, which is the opposite of what abscisic acid does.
• (d)Gibberellins — Gibberellins drive stem elongation, bolting in rosette plants and the breaking of seed dormancy. On seed germination gibberellin and abscisic acid pull in opposite directions, and it is abscisic acid that holds the seed dormant.

Plant hormones are chemical messengers made in one part of a plant and active in very small amounts, often somewhere else. Five groups make up the classical list: auxins, gibberellins, cytokinins, ethylene and abscisic acid. The first three are growth promoters. Abscisic acid is the growth inhibitor, and ethylene sits partly in each camp because it ripens fruit and drives leaf fall while also promoting growth in some water-logged plants.

The item only asks the candidate to split a four-name list into promoters and one inhibitor, so the fastest route is to attach one verb to each hormone — auxin elongates, cytokinin divides, gibberellin lengthens the stem, abscisic acid stops. The trap is that abscisic acid was named for a job it turned out not to do: it was isolated from cotton fruit thought to trigger abscission, the shedding of leaves and fruit, and the name stuck even though ethylene is the hormone chiefly responsible for shedding. Candidates who remember only the name and not the function sometimes reject abscisic acid because they associate it with fruit fall rather than with growth inhibition, and pick gibberellin instead on the strength of the word dwarf.

• Abscisic acid closes stomata under water stress, which is why it is called the stress hormone.
• It imposes bud and seed dormancy, and gibberellin acts against it to break that dormancy.
• Cytokinins promote cell division; kinetin was the first one named, in 1954, and zeatin, from maize, was the first found occurring naturally.
• Auxin drives cell elongation and therefore phototropism, and also enforces apical dominance.
• Gibberellin drives stem elongation and bolting, the sudden lengthening of a stem before flowering.

• Reading the name abscisic acid as the abscission hormone; ethylene, not abscisic acid, is the main driver of leaf and fruit fall.
• Confusing cytokinin with gibberellin — one divides cells, the other lengthens the stem.
• Assuming every hormone effect is growth; the same molecule can promote one process and inhibit another, so learn the process, not just the hormone.

Usually as a one-hormone-to-one-function match, or as an odd-one-out on the promoter-inhibitor split; CDS and NDA have both asked which hormone to use to raise cell division.

• practice — not a real PYQ

  Which one among the following plant hormones is chiefly responsible for the closing of stomata when a plant is short of water?

  • (a)Auxin
  • (b)Gibberellin
  • (c)Abscisic acid
  • (d)Cytokinin

  Answer(c) Abscisic acid — it accumulates under water stress and closes the stomata, which is why it is known as the stress hormone.

• practice — not a real PYQ

  The bending of a shoot towards a source of light is caused mainly by which one of the following hormones?

  • (a)Abscisic acid
  • (b)Auxin
  • (c)Ethylene
  • (d)Cytokinin

  Answer(b) Auxin — it collects on the shaded side of the shoot and lengthens those cells more, so the shoot curves towards the light.