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Spec Mapping — OCR H420 Module 5.1.5 — Plant and animal responses, content statements covering plant hormones (auxin/IAA, gibberellins, cytokinins, abscisic acid, ethene), their effects on growth and development, the mechanisms of phototropism and gravitropism, apical dominance, seed germination via gibberellin → α-amylase, and the commercial applications of plant growth regulators (refer to the official OCR H420 specification document for exact wording). This is the canonical worked example of "responses without nerves" — a parallel signalling system that achieves coordination in the absence of a nervous system.
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.
The scientific lineage of plant-hormone research is well-documented. Charles Darwin and his son Francis published The Power of Movement in Plants (1880, paraphrase), describing pioneering experiments on grass-seedling coleoptiles. They showed that the tip of the coleoptile is required for phototropism — covering the tip with an opaque cap prevented the seedling from bending towards light, but covering the basal region did not. They concluded that "some influence" transmitted from the tip down to the lower elongation zone caused the bending response. Peter Boysen-Jensen (1913, paraphrase) demonstrated that the "influence" was chemical, not electrical, by interposing a gelatin block between the tip and the base — the response was preserved, but a mica sheet abolished it. Frits Went (1928, paraphrase) isolated the chemical by allowing it to diffuse from coleoptile tips into agar blocks, then placing the agar blocks on decapitated coleoptiles and reproducing the bending response. The chemical was named auxin (Greek auxein, "to grow") and later identified as indole-3-acetic acid (IAA). The Cholodny-Went hypothesis (1927, paraphrase) proposed that auxin redistributes laterally — from the lit to the shaded side of a coleoptile — to produce phototropic bending; this remains the textbook account at A-Level, although modern molecular work has refined it by identifying the PIN auxin efflux transporters that physically move auxin laterally across cells. Plant hormone biology is one of the cleanest examples of how careful experimental design (Darwin → Boysen-Jensen → Went) builds a mechanistic understanding from simple physiological observations.
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.
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.
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.
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?
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]
The same hormone controls the response to gravity, though it affects shoots and roots differently:
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. This is the key A* point: auxin has opposite effects on shoot and root elongation at the same concentration. A given IAA concentration that promotes shoot elongation will inhibit root elongation; the dose-response curves are different shapes for the two tissues.
flowchart TB
G[Gravity sensed by statoliths<br/>in root cap columella] --> IAA[IAA redistributed to lower side]
IAA --> ROOT["In roots: high IAA INHIBITS elongation"]
IAA --> SHOOT["In shoots: high IAA PROMOTES elongation"]
ROOT --> RBEND[Lower side elongates less → root bends DOWN]
SHOOT --> SBEND[Lower side elongates more → shoot bends UP]
The mechanism by which plants sense gravity is one of the more elegant pieces of plant cell biology. In the columella cells of the root cap (and in the shoot endodermis), specialised starch-filled organelles called statoliths (or amyloplasts) settle to the bottom of the cell under gravity. The statoliths physically displace the cell's cytoplasm and trigger a redistribution of auxin transport proteins (PIN proteins) in the plasma membrane, which in turn redistributes IAA to the lower side of the root or shoot. Statoliths can be visualised under a light microscope by staining starch with iodine — a useful classroom demonstration. The classic experiment is to invert a germinating seedling: within hours, statoliths re-settle on the new "down" side, IAA redistributes accordingly, and the root and shoot reorient.
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.
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.
OCR wants you to know the classical mechanism by which gibberellins trigger seed germination. In a cereal seed such as barley:
This is an elegant example of gene activation by a plant hormone. The knock-on effect is that seedlings gain energy to fuel their early growth before they develop leaves capable of photosynthesis.
Cytokinins — discovered in the 1950s as substances that promoted cell division in tissue culture — are produced mainly in root tips and transported upwards in the xylem. Their main effects:
Abscisic acid (ABA) is often called the "stress hormone" of plants. Its key roles:
When soil water is low, roots synthesise ABA and transport it in the xylem to the leaves. ABA binds to receptors on guard cells, triggering a complex signalling cascade:
This is one of the fastest plant responses — it can be measured within minutes of a drought stress.
ABA maintains seed dormancy by preventing germination. The ratio of ABA to gibberellin determines whether a seed stays dormant or germinates. Cold winter conditions often deplete ABA, preparing the seed for spring germination (this is why gardeners "stratify" some seeds by chilling them).
ABA plays a role in leaf senescence, though ethene is more important.
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