Plant Hormones and Tropisms

Plants cannot run from predators, seek shade on a hot day or move towards better soil. Instead, they respond to their environment through growth, coordinated by a small set of plant hormones (often called plant growth regulators). These molecules allow a plant to orient itself towards light, cope with drought, shed leaves in autumn and time the sprouting of seeds. OCR A-Level Biology A specification 5.1.5(a)–(d) requires you to know the roles of several hormones and the responses they control.

Key Definitions:

• Tropism — a directional growth response to an external stimulus.
• Phototropism — growth in response to light (positive towards, negative away).
• Geotropism (gravitropism) — growth in response to gravity.
• Apical dominance — inhibition of lateral buds by the apical bud, maintaining a single main stem.
• Abscission — the shedding of leaves, flowers or fruits.

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The Five Main Plant Hormones

Plant hormones are produced in very small amounts and have dramatic effects on growth and development. OCR expects you to know five:

Hormone	Where produced	Main effects
Auxin (IAA)	Shoot tips, young leaves, developing seeds	Cell elongation, phototropism, apical dominance, root initiation
Gibberellins (GA)	Young leaves, germinating seeds, roots	Stem elongation, seed germination, flowering
Cytokinins	Roots, developing fruits, seeds	Cell division, delay of senescence, release of lateral buds
Abscisic acid (ABA)	Mature leaves, roots under water stress	Stomatal closure, seed dormancy, stress responses
Ethene (ethylene)	Ageing tissues, ripening fruits	Fruit ripening, leaf abscission, flower senescence

Unlike animal hormones, plant hormones typically affect multiple processes. Their effects depend heavily on context: concentration, combination with other hormones, tissue sensitivity and developmental stage. This is why plant biology can feel slippery at A-level: the same hormone does different things in different places.

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Auxins and Phototropism

The best-known plant hormone is indole-3-acetic acid (IAA), a natural auxin made in the shoot tips and young leaves. It was discovered in the classic experiments of Charles Darwin and his son Francis in the 1880s, who showed that the growing tip of a grass seedling could sense light. Frits Went in 1926 demonstrated that a chemical substance — later identified as IAA — could diffuse out of the tip into a block of agar and reproduce the effect.

How Phototropism Works

When a shoot is illuminated from one side, IAA is redistributed from the lit side to the shaded side. This is due to lateral transport driven by membrane PIN proteins (not required in detail for OCR, but worth knowing). Higher IAA on the shaded side causes the cells there to elongate more than those on the lit side, so the shoot curves towards the light.

How does IAA cause elongation?

• IAA binds to a receptor in the cell membrane.
• This activates the cell's H⁺ pump (proton pump), lowering pH in the cell wall.
• Low pH activates enzymes called expansins that loosen cross-links between cellulose microfibrils.
• The softened cell wall stretches as the cell takes up water by osmosis.
• The cell elongates, and lignin is later deposited to fix the new shape.

This model is known as the acid growth hypothesis and is one of the few times an exam question might push you towards molecular detail of auxin action.

flowchart TB
    L[Light from one side] --> T[Shoot tip senses light]
    T --> R[IAA moves to shaded side]
    R --> E[More IAA on shaded side]
    E --> EL[Cells on shaded side elongate more]
    EL --> CURV[Shoot curves towards light]

Geotropism

The same hormone controls the response to gravity, though it affects shoots and roots differently:

• Shoot (negative geotropism) — cells on the lower side accumulate more IAA, elongate more, causing the shoot to bend upwards.
• Root (positive geotropism) — cells on the lower side accumulate more IAA, but in roots high IAA inhibits elongation, so the upper side grows more and the root bends downwards.

Why the opposite effect? Roots are much more sensitive to IAA than shoots. At the same concentration that stimulates shoot cells, root cells are already past their optimum and are inhibited.

Apical Dominance

An actively growing shoot tip (the apical bud) produces IAA that inhibits the growth of lateral buds lower down. This keeps the plant growing tall and straight. If you remove the apical bud (e.g. by pinching out the top of a tomato plant), the lateral buds are released from inhibition and the plant becomes bushier. Cytokinins, meanwhile, promote lateral bud growth, so the balance between IAA (inhibitory) and cytokinins (stimulatory) determines branching.

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Gibberellins and Seed Germination

Gibberellins were discovered in a fungus (Gibberella fujikuroi) that caused "foolish seedling disease" in rice, in which young rice plants grew abnormally tall. The fungus was producing gibberellins, which the plants absorbed. Plants make these hormones naturally in smaller amounts.

Functions

• Stem elongation — gibberellins cause internode elongation, making stems taller. Genetic dwarf varieties of plants often have defective gibberellin pathways (e.g. dwarf peas studied by Mendel).
• Seed germination — crucial role described below.
• Flowering — gibberellins induce flowering in some long-day plants.
• Fruit development — used commercially to increase size of seedless grapes.

Mechanism of Seed Germination

OCR wants you to know the classical mechanism by which gibberellins trigger seed germination. In a cereal seed such as barley: