🎄Plant Growth Hormones — Auxins, Gibberellins, Cytokinins, ABA, and Ethylene

From Field to Lab — The Invisible Architects of Every Crop

In the previous lesson, we defined plant growth — its phases, measurement, and rate patterns. But growth does not happen on its own. It is orchestrated by chemical messengers called plant hormones (phytohormones) that tell cells when to divide, elongate, differentiate, ripen, or die. Without hormones, the growth machinery we studied would have no coordination.

When a tea planter prunes the growing tips to make bushes denser, the hidden logic is auxin — removing the auxin-rich apical bud releases lateral buds from dormancy. When a farmer applies gibberellic acid to seedless grapes, the berries grow larger without pollination. When a fruit seller places unripe bananas alongside a ripe apple in a closed box, the ethylene released by the apple triggers ripening in the bananas. Every agricultural practice from rooting of cuttings to weed control to fruit ripening involves plant hormones.

This lesson covers:

1. Auxins — discovery, IAA, synthetic auxins (2,4-D), apical dominance, rooting, weed control
2. Gibberellins — Bakanae disease, GA₃, bolting, malting, seedless grapes
3. Cytokinins — kinetin, zeatin, cell division, tissue culture ratios
4. Abscisic acid (ABA) — stress hormone, stomatal closure, dormancy
5. Ethylene — gaseous hormone, fruit ripening, ethephon, triple response
6. Growth retardants — Cycocel, Maleic Hydrazide, Paclobutrazol

This is one of the highest-yield topics in plant physiology for competitive exams.

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What Are Plant Hormones?

Plant hormones (phytohormones) bridge the gap between the cellular growth processes of the previous lesson and the whole-plant developmental responses we see in the field. They are produced in minute quantities but control every aspect of plant life.

Plant hormones (phytohormones) are organic substances produced in meristematic tissues, translocated to target sites, and active in extremely minute quantities (parts per million).

• Term “Phytohormone” suggested by Thimann (1948)
• Plant Growth Regulators (PGR) include both natural hormones and synthetic compounds

Classification

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1. Auxins — The “Growth Hormone”

Auxin was the first plant hormone discovered and remains the most studied. It controls cell elongation, root initiation, apical dominance, and tropisms. Its name comes from the Greek auxein (“to grow”). Auxin is unique among plant hormones because it exhibits polar transport — it moves in only one direction (apex to base).

Discovery

• Avena Curvature Test (Went’s bioassay): Agar block with auxin placed on decapitated oat coleoptile → degree of curvature proportional to auxin concentration

Key Facts

Synthetic Auxins

Free vs Bound Auxins

Physiological Effects and Agricultural Applications

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2. Gibberellins (GA)

Gibberellins are the stem elongation hormones. While auxin primarily causes cell elongation in coleoptiles, gibberellins dramatically increase internodal length in intact plants. Their discovery came from a plant disease — one of the best stories in plant science.

Agricultural Applications

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3. Cytokinins

Cytokinins promote cell division (cytokinesis) and work in concert with auxins. The ratio of auxin to cytokinin is the master switch that determines whether a tissue produces roots, shoots, or undifferentiated callus — a principle that underpins all of modern plant tissue culture.

Agricultural Applications

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4. Abscisic Acid (ABA) — The “Stress Hormone”

The first three hormones (auxin, GA, cytokinin) are growth promoters. ABA is the first growth inhibitor — it acts as the plant’s “emergency brake,” shutting down growth and conserving resources when conditions turn unfavourable. It is central to drought tolerance and seed dormancy.

Effects

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5. Ethylene — The “Ripening Hormone”

Ethylene is unique among all plant hormones — it is a gas (C₂H₄). Despite being classified as a growth inhibitor, it has both promotive and inhibitive roles. Its most famous effect is fruit ripening, which is why a single ripe fruit can trigger ripening in an entire batch.

Effects and Applications

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6. Growth Retardants and Inhibitors

Beyond the five natural hormones, agriculture uses synthetic growth retardants that manipulate plant architecture. These chemicals typically work by blocking gibberellin biosynthesis, reducing stem elongation while allowing root growth and flowering to proceed normally.

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Exam Mnemonics

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Master Comparison — Five Plant Hormones

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Summary Table — Key Facts at a Glance

Plant Growth Regulators: Practical Field Applications

How farmers and horticulturists actually USE plant hormones:

Exam-critical mnemonic — hormone precursors “T-K-A-M-M”:

• Tryptophan → Auxin (IAA)
• Kaurene → Gibberellin (GA₃)
• Adenine → Cytokinin
• Methionine → Ethylene (via SAM → ACC)
• Mevalonic acid → ABA

The 5 classical hormones as a team analogy: Auxin = the builder (cell elongation, root initiation). Gibberellin = the stretcher (stem elongation, seed germination). Cytokinin = the divider (cell division, delay senescence). Ethylene = the ageing signal (ripening, abscission). ABA = the brake (dormancy, stomatal closure under stress). They don’t work alone — it’s the RATIO between them that determines plant behaviour.

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Summary Cheat Sheet