MPSC 2026 Prelims Paper-I · Question 64 of 100

Correct — option (3), Methylene. The syllabus recognises five groups of natural plant growth regulators, also called phytohormones: auxins, gibberellins, cytokinins, abscisic acid and ethylene. Three of the four names printed here belong to that list. Methylene does not, and it is not a plant substance at all — the word names a chemical group, the divalent -CH2- unit, which survives in everyday chemistry in compound names such as methylene chloride (dichloromethane, a solvent) and methylene blue (a dye and stain). Nothing called simply 'methylene' circulates in a plant or regulates its growth. The question is built on the near-collision between 'ethylene', which is a genuine gaseous plant hormone, and 'methylene', which is one carbon down the homologous series and is not a hormone at all; a candidate reading quickly sees two similar words, assumes both are of a kind, and looks elsewhere for the answer. Because the stem asks which is NOT a hormone — and prints the word 'not' in bold, so the negation is deliberate and flagged — the odd one out is the answer, and it is option (3). The disciplined method on a negative stem is to positively confirm the three that belong rather than to hunt for the one that does not: auxins, ethylene and abscisic acid can each be placed in the standard list with a named function, methylene cannot be placed anywhere.

• (1)Auxins — Auxins are natural plant hormones and the first group to be discovered — the name comes from the Greek auxein, to grow. Indole-3-acetic acid (IAA) and indole butyric acid (IBA) are the naturally occurring members, isolated from plant tissue; naphthalene acetic acid (NAA) and 2,4-dichlorophenoxyacetic acid (2,4-D) are synthetic analogues used in agriculture. Auxins maintain apical dominance, initiate roots on stem cuttings, control the shedding of leaves and fruits, and can induce parthenocarpy, which is how seedless tomatoes are produced; 2,4-D is used to kill dicot weeds. Since the stem asks for the item that is not a natural hormone, auxins are excluded.
• (2)Ethylene — Ethylene is a genuine natural plant hormone and the only one that is a gas. Plants produce it in quantity in ripening fruits and in ageing tissues. It ripens fruit and raises the rate of respiration during ripening, promotes senescence and the shedding of leaves and flowers, breaks the dormancy of seeds and buds, initiates sprouting in potato tubers, promotes root hair formation, and induces flowering in pineapple. Farmers apply it indirectly through ethephon, a compound that releases ethylene — but the release of a natural hormone by a synthetic carrier does not make the hormone itself synthetic. This is the option the question is really trying to sell, because it differs from the keyed answer by a single syllable.
• (4)Abscissic acid — Abscisic acid, printed here as 'Abscissic acid', is a natural plant hormone and one of the two classical growth inhibitors, the other being ethylene. It acts against the growth promoters: it inhibits growth and metabolism, closes the stomata so the plant loses less water, helps seeds withstand desiccation and enforces seed dormancy so that germination waits for favourable conditions. Because it is the hormone that carries the plant through drought, cold and other adverse conditions, it is commonly called the stress hormone. Its presence in the standard list of five phytohormones is not in doubt, so it cannot be the odd one out.

Plant growth regulators fall into two functional camps. The promoters — auxins, gibberellins and cytokinins — drive cell division, cell enlargement, the growth of buds and shoots, flowering, fruiting and seed formation. The inhibitors — abscisic acid and, in most of its roles, ethylene — work in the opposite direction, imposing dormancy, closing stomata, ageing tissue and shedding leaves, flowers and fruits. Real plant behaviour is almost never the work of a single hormone: apical dominance, for example, is an auxin effect that cytokinins counteract, and a seed germinates when the balance shifts from abscisic acid towards gibberellins. That is why the modern term is 'plant growth regulator' rather than 'hormone' — these molecules regulate by ratio and context rather than by simple command.

The three remaining members of the list are worth carrying alongside the three that appear in this question. Gibberellins, of which GA3 is the most studied, cause bolting — the sudden elongation of the stem in rosette plants before flowering — lengthen grape stalks, delay senescence in fruit so it can be left longer on the tree, and speed up the malting process in brewing. Cytokinins promote cell division, encourage the growth of lateral shoots and delay the ageing of leaves; the natural member is zeatin, first obtained from maize kernels, while kinetin, though the compound that gave the group its name in the laboratory, is not a naturally occurring plant substance. Between them the five groups cover the whole of a plant's development, which is why agricultural applications of them — rooting hormones, ripening agents, weedkillers, sprouting inhibitors for stored potatoes — appear so often in general-studies papers.

• The five natural plant growth regulators (phytohormones) are auxins, gibberellins, cytokinins, abscisic acid and ethylene. Methylene is not among them and is not a plant substance — it names the -CH2- group, as in methylene chloride or methylene blue.
• Auxins: natural members IAA and IBA; synthetic analogues NAA and 2,4-D. They maintain apical dominance, initiate roots on cuttings, regulate the shedding of leaves and fruits, and can induce parthenocarpy, giving seedless fruit such as tomatoes.
• Ethylene is the only gaseous plant hormone. It ripens fruit and raises respiration during ripening, hastens senescence and abscission, breaks seed and bud dormancy, sprouts potato tubers and induces flowering in pineapple. Ethephon is the synthetic compound used to release it in the field.
• Abscisic acid is the classical growth inhibitor and the 'stress hormone': it closes stomata to conserve water, inhibits growth and metabolic activity, and enforces seed dormancy so that germination waits for favourable conditions.
• The two remaining groups: gibberellins (GA3) cause bolting in rosette plants, elongate grape stalks, delay fruit senescence and speed up malting; cytokinins (natural member zeatin, from maize kernels) promote cell division and lateral shoot growth and delay leaf ageing.

• Missing the bolded 'not' in the stem and answering with a hormone that genuinely belongs to the list. On a negative stem, confirm the three that fit before naming the one that does not.
• Reading 'methylene' as a variant spelling of 'ethylene'. They differ by one carbon and by everything else: ethylene is a plant hormone, methylene is a chemical group with no biological role of this kind.
• Calling ethylene synthetic because farmers apply it through ethephon, or calling auxins synthetic because NAA and 2,4-D exist. Each group has natural members and synthetic analogues; the group itself is natural.

Plant hormones are examined in four recurring shapes. The odd-one-out, as here, where a plausible non-hormone is slipped into a list of real ones. The function match, where a hormone must be paired with an effect — ethylene with ripening, abscisic acid with stomatal closure, gibberellin with bolting, cytokinin with delayed senescence. The application question, which reaches the paper through agriculture: what makes a cutting root, what ripens a mango, what kills a dicot weed in a wheat field. And the discovery or naming question, which asks who worked on coleoptiles or where zeatin was first found. All four are answerable from one well-made table, and none of them from a vague sense that a word 'sounds biological'.

• practice — not a real PYQ

  Which one of the following plant growth regulators is a gas and is chiefly responsible for the ripening of fruits?

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

  Answer(c) Ethylene — the only gaseous phytohormone. It is produced in quantity by ripening fruit and ageing tissue, raises the rate of respiration during ripening, and also promotes senescence, abscission, potato sprouting and flowering in pineapple. In the field it is applied as ethephon, a compound that releases ethylene.

• practice — not a real PYQ

  Which plant hormone closes the stomata during water stress, inhibits growth and enforces seed dormancy, and is for these reasons called the stress hormone?

  • (a)Auxin
  • (b)Abscisic acid
  • (c)Zeatin
  • (d)Gibberellic acid

  Answer(b) Abscisic acid — the classical growth inhibitor, which acts against the promoters by closing stomata to conserve water, suppressing growth and metabolism, and keeping seeds dormant until conditions favour germination. Zeatin is a natural cytokinin and gibberellic acid a growth promoter, both on the opposite side of the balance.