Which of the following is not a natural plant growth hormone ?

Question

UnlockIAS · Accepted Answer

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.

Why the other options are wrong — (a) 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.  (b) 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.  (d) 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.

Concept: 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.

State PCS 2026 Q64 — Science & Technology PYQ with Answer

State PCS 2026 Q64 — Science & Technology PYQ with Answer