Plant Hormones: Auxins, Gibberellins, Cytokinins, ABA and Ethylene

Five Hormones, Five Accidents

By Akanksha Kumari · Science faculty, The Cavalier · B.Tech, 15 years teaching Science

Plants have no glands and no nerves, yet they coordinate growth over metres of stem. They use hormones (plant growth regulators): small molecules made in one place that act in tiny amounts elsewhere. All five major groups were found by accident, while people were explaining a sick rice field, a bending seedling or a basket of fruit. Unlike the animal hormones on our endocrine glands page, they come from no special gland.

Rows one, two and four form one detective story. Darwin located the sensor (the tip), Boysen-Jensen proved the message was chemical, and Went caught it in agar. Placed off-centre on a headless seedling in the dark, the block makes it curve away from the side that carries it, because that side grows faster.

What Each Hormone Does, and What It Is Sold For

Learn this table. The right-hand column is where "which hormone would you use to..." questions come from.

Auxin. A dab of rooting powder on a rose or hibiscus cutting is IBA or NAA. The same hormone, in a synthetic form, is also a weapon. 2,4-D kills broad-leaved (dicot) weeds but spares grasses and cereals, which is why it is used for weed-free lawns and wheat fields. Auxin also holds apical dominance: the growing tip suppresses the side buds. Snip the tip, as gardeners trimming a hedge do, and the side buds burst into branches. Cytokinin also pushes the side buds to grow.

Gibberellin. The stretching hormone is also a dormancy-breaker. In rosette plants such as cabbage and beet, a burst of it makes the stem shoot up, called bolting, just before flowering. Growers use it to lengthen grape-bunch stalks and sugarcane stems, which store the sugar, and brewers use GA₃ to speed up malting. Gibberellin and auxin can both trigger parthenocarpy, fruit that forms without fertilisation and so has no seeds.

The tissue-culture rule. In a laboratory dish the ratio of two hormones decides what a lump of cells (a callus) becomes. High auxin and low cytokinin makes roots. High cytokinin and low auxin makes shoots. This is how thousands of identical banana or orchid plantlets are grown from a few cells.

Abscisic acid is the stress hormone. In a drought it closes the stomata, saving water, and it keeps seeds dormant until conditions suit them, acting against gibberellin. When a question asks which hormone inhibits growth, this is the answer. ABA is the textbook inhibitor.

Ethylene, Ripening and the Carbide Ban

Ethylene is the only plant hormone that is a gas, which is how one ripe fruit can ripen its neighbours. Banana, mango, apple, papaya and tomato are climacteric fruit: as they ripen, respiration surges (the "respiratory climacteric") and they release a pulse of ethylene. Grapes, oranges and strawberries ripen only on the plant. This is why one rotten fruit spoils the basket.

Ethylene also breaks the dormancy of seeds and potato tubers and lifts deepwater rice leaves above a flood. Its usual source is ethephon, a liquid that the plant absorbs and that releases ethylene slowly. It is used to ripen tomato and to bring pineapple and mango into flower together.

FSSAI allows ethephon only as a source of ethylene gas inside a ripening chamber, run to the prescribed procedure. Dipping fruit in ethephon solution is not allowed, the practice its 2026 advisory targeted. In short, carbide gives acetylene (banned), and chambers give ethylene (allowed). Acetylene imitates ethylene because they are close relatives. Both are hydrocarbons, as our page on hydrocarbons and polymers explains: ethylene has a C=C double bond and acetylene a C≡C triple bond.

Tropisms and Nastic Movements: Telling Them Apart

Plants cannot walk, but they can move. A tropism is a directional growth movement: the plant part bends towards (positive) or away from (negative) the stimulus, and the direction depends on where the stimulus comes from. A nastic movement does not depend on the direction of the stimulus. Auxin is the usual messenger. In the Cholodny-Went model, light pushes auxin to the shaded side of the shoot, where it lengthens the cells, so the stem leans into the light. Our page on xylem and phloem shows the cells that carry out that elongation.

Photoperiodism: How a Plant Reads the Calendar

Photoperiodism is a plant's response to the length of day and night, and flowering is its best-known result. Garner and Allard showed in 1920 that Maryland Mammoth tobacco flowered only when days shortened. Plants fall into three groups.

The surprise is that the plant is really counting the dark hours. Interrupt a long night of an SDP with a few minutes of light, a "night break", and it will not flower. The sensor is the pigment phytochrome, found by US Department of Agriculture scientists in the 1950s. It exists in two interchangeable forms: P_r absorbs red light and turns into P_fr, and P_fr absorbs far-red light and turns back. In darkness P_fr slowly reverts, and that reversal is, in effect, the clock.

The leaf measures the day, but the flower appears at the shoot tip, so a messenger called florigen must travel between them. Textbooks still call it hypothetical, though since 2007 experiments point to a protein called FT as its main component. Growers exploit all this: chrysanthemum beds are covered with black cloth to force early flowering, and lit at night to delay it.

Vernalisation: Flowering After Winter

Some plants need a spell of cold before they will flower. Vernalisation is the promotion of flowering by exposing a plant, or its soaked seeds, to low temperature, typically 1 to 10 °C for a few weeks. The word was popularised by the Soviet agronomist T.D. Lysenko in 1928. Winter wheat and biennials such as cabbage and beet depend on it. A biennial grows leaves in its first year, meets winter, and flowers in its second. Chilled seed lets it flower in one season.

In some biennials a gibberellin spray can stand in for the cold. Most wheat grown in India is a spring type, sown in the cool Rabi season, and needs no pre-chilled seed, which links back to the Rabi sowing in our lead. True winter wheat of colder countries is sown in autumn and must feel the winter first.

How UPSC Has Asked It: Past Papers Solved

These come from CDS and NDA papers (2021 to 2025) and the CAPF (AC) paper, which tests the same science.

Practice MCQs (with answers)

1. Which hormone is used to induce roots on stem cuttings?
   1. Abscisic acid
   2. Gibberellic acid
   3. Indole-3-butyric acid
   4. Ethylene
2. An experiment in which agar blocks that had held oat coleoptile tips were placed on headless seedlings is associated with
   1. Charles Darwin
   2. Frits Went
   3. E. Kurosawa
   4. Miller
3. Calcium carbide is banned for ripening fruit because it releases a gas that imitates ethylene. The gas is
   1. acetylene
   2. methane
   3. ethane
   4. carbon monoxide
4. A short-day plant will flower when
   1. it receives more than 14 hours of light every day
   2. the temperature falls below 10 °C
   3. far-red light is given in the middle of the day
   4. its uninterrupted dark period is longer than its critical length
5. Spraying sugarcane with gibberellin is done mainly to
   1. increase the number of leaves
   2. lengthen the sugar-storing stems
   3. prevent root hair formation
   4. deepen seed dormancy
6. The folding of the leaves of a touch-me-not plant when touched is an example of
   1. phototropism
   2. geotropism
   3. a nastic (turgor) movement
   4. chemotropism
7. In a tissue-culture medium, a high auxin to cytokinin ratio mainly encourages
   1. shoot buds
   2. seed germination
   3. death of the callus
   4. root formation
8. Vernalisation is
   1. promotion of flowering by exposure to low temperature
   2. promotion of flowering by short days
   3. ripening of fruit by ethylene
   4. removal of the shoot tip to induce branching

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