MPSC 2022 Prelims Paper-I · Question 42 of 100

The Commission cancelled this question. It carries no answer in the final key, and this card therefore names none — nothing here should be read as an indication of what the withdrawn answer was. What the question was reaching for is nevertheless a standard part of the plant physiology syllabus, and it is worth learning properly. Senescence is the ordered, age-related deterioration of a plant or of one of its organs, and it is a programmed process rather than mere decay: chlorophyll is broken down, which is why a senescing leaf yellows; proteins and nucleic acids are dismantled; and the nitrogen, phosphorus and other nutrients released are withdrawn from the dying organ and moved to the parts of the plant that are still growing, to developing seeds and fruits above all. Only when that salvage is complete does abscission follow, the leaf or fruit being shed at a special layer of cells at the base of its stalk. Senescence is under hormonal control, and the point that a student has to hold is that it is not the work of a single hormone acting alone. Of the five classical groups of plant growth regulators, two are described in the standard accounts as promoting senescence and two as delaying it. Ethylene, the only one of the five that is a gas, is associated with the ripening of fruits, with abscission, and with the senescence of leaves and flowers; commercial preparations that release it are used to ripen fruit after harvest. Abscisic acid, long known as the stress hormone, inhibits growth, closes the stomata when the plant is short of water, imposes dormancy on seeds and buds, and is likewise described as promoting senescence and abscission. On the other side, cytokinins delay the senescence of a detached leaf, keeping it green and drawing nutrients towards the treated tissue, an effect demonstrated in classical experiments on leaf discs; and auxins delay the abscission of young leaves and fruits, although they hasten the fall of older ones. Because two of the four hormones offered are promoters of the process and the other two are best known for retarding it, a question that asks for the single hormone 'responsible for senescence' is asking for something the physiology does not supply cleanly, and the safe preparation is to learn what each of the five groups does rather than to attach one label to one name. The wider list is worth carrying whole: auxins for cell elongation, apical dominance, rooting of cuttings and parthenocarpy; gibberellins for stem elongation, bolting, breaking seed dormancy and the malting of barley; cytokinins for cell division, lateral bud growth and nutrient mobilisation; ethylene for ripening, the triple response of seedlings and the breaking of dormancy in potato tubers; and abscisic acid for stomatal closure, dormancy and the general inhibition of growth.

Plant growth regulators fall into five classical groups, and the syllabus expects each to be known by its discovery, its chemical identity and its principal effects. Auxins, of which indole-3-acetic acid is the natural example, emerged from the coleoptile experiments begun by Charles Darwin and his son Francis and completed by F. W. Went; they promote cell elongation, maintain apical dominance so that the terminal bud suppresses the lateral ones, induce rooting in stem cuttings, and can produce seedless fruit. Gibberellins were traced to the fungus that causes the foolish-seedling disease of rice and produce dramatic stem elongation, bolting in rosette plants, the breaking of seed dormancy and the speeding of malting in the brewing industry. Cytokinins were isolated as kinetin from degraded herring sperm DNA and as zeatin from maize kernels, and they promote cell division, encourage lateral buds against apical dominance, mobilise nutrients towards the tissue where they are applied, and delay the senescence of detached leaves. Ethylene is the one gaseous regulator, responsible for the climacteric ripening of fruit, the triple response of dark-grown seedlings, epinasty, the breaking of dormancy in potato tubers, and the induction of flowering in pineapple and mango. Abscisic acid, first described under the names inhibitor-B and dormin, works largely against the growth promoters: it closes stomata under water stress, enforces bud and seed dormancy, and is the hormone most often called the stress hormone. Senescence, the process this question names, is the programmed dismantling of an organ with the recovery of its nutrients, and it sits at the junction of the promoting and the retarding hormones rather than under any single one of them.

Three questions in this paper were withdrawn by the Commission after the examination, and this is the first of them. A cancelled question is not a licence to skip the topic; the material it came from remains in the syllabus and returns in later papers in better-drafted forms, so the sensible response is to learn the ground it stood on rather than to note the cancellation and move past it. Plant hormones are a favourite of MPSC's science section because they can be asked in several shapes from one small body of material — the hormone behind a named effect, the effect produced by a named hormone, the scientist or the organism associated with a discovery, the commercial use to which a regulator is put — and all of those are answerable from a single well-made table of five rows. What this question also illustrates is a drafting hazard worth recognising in the hall: when a biological process is known to involve more than one agent, a stem that demands the single agent 'responsible' for it is unstable, and a candidate who finds two defensible options in such a stem is usually reading the physiology correctly rather than misremembering it. The right response under time pressure is to mark the item, answer it on the balance of what the syllabus emphasises, and move on, since the Commission alone decides afterwards whether such an item stands.

• The Commission cancelled this question and published no answer for it in the final key; the topic it was set on, plant growth regulators, remains part of the syllabus and is examined regularly in other forms.
• Senescence is the programmed, age-related breakdown of a plant organ in which chlorophyll, proteins and nucleic acids are dismantled and the released nutrients are moved to the growing parts of the plant, with abscission following once the salvage is complete.
• The five classical groups of plant growth regulators are the auxins, the gibberellins, the cytokinins, ethylene and abscisic acid; ethylene is the only one that acts as a gas, and abscisic acid is the group commonly called the stress hormone.
• Cytokinins were isolated as kinetin from degraded herring sperm DNA and as zeatin from maize, and they promote cell division, encourage lateral bud growth against apical dominance, and delay the senescence of a detached leaf.
• Auxins were traced through the coleoptile experiments of Charles and Francis Darwin and of F. W. Went, and they govern cell elongation, apical dominance, the rooting of stem cuttings and the setting of seedless fruit.

• Treating a cancelled question as a topic that can be skipped, when the syllabus material behind it returns in later papers in better-drafted form
• Learning one effect per hormone, when the examiner asks about several effects of each and most of the five regulators act on more than one process
• Confusing the hormone that delays the fall of young leaves and fruits with the one that hastens the fall of older ones, since auxin does both depending on the age of the organ
• Mixing up abscisic acid and ethylene, which are frequently offered together in the same option set and share more than one of their listed effects

Plant hormones reach MPSC papers as one-line recall items — which regulator produces a named effect, which is a gas, which is called the stress hormone, which is used to ripen fruit — and as multi-statement questions in which four claims about different regulators have to be sorted into true and false. The discovery stories are asked as often as the effects, so the fungus behind the gibberellins, the herring sperm DNA behind kinetin and the coleoptile experiments behind the auxins are all worth holding. The efficient preparation is a five-row table carrying, for each group, its natural example, the experiment or organism that revealed it, three or four established effects and one commercial use, because nearly every question the Commission can build from this topic is answerable from those columns.

No directly related past PYQ was found.

• practice — not a real PYQ

  Which plant growth regulator is responsible for the closure of stomata when a plant is under water stress ?

  • (a)Gibberellic acid
  • (b)Abscisic acid
  • (c)Indole-3-acetic acid
  • (d)Zeatin

  Answer(b) Abscisic acid — it is the regulator commonly called the stress hormone, and its closing of the stomata is the plant's principal short-term defence against water loss during drought. It also enforces dormancy in seeds and buds and acts generally against the growth-promoting regulators. Gibberellic acid and indole-3-acetic acid are growth promoters, and zeatin is a naturally occurring cytokinin isolated from maize.

• practice — not a real PYQ

  The suppression of the growth of lateral buds by the terminal bud of a shoot is known as apical dominance. Which regulator is chiefly responsible for it ?

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

  Answer(c) Auxin — produced at the shoot apex and transported downwards, it suppresses the growth of the lateral buds below, which is why removing the terminal bud of a hedge or a tea bush makes the plant grow bushy. Cytokinins act in the opposite direction on this particular effect, encouraging lateral buds to grow, and the two together illustrate that plant development is governed by the balance between regulators rather than by any one of them alone.