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This lesson is mapped to AQA 7402 Section 3.6.1 — survival and response in plants; tropisms; IAA / auxin (refer to the official AQA specification document for exact wording). Plants do not have nervous systems, but they nonetheless detect environmental stimuli and respond in coordinated, directional ways. The mechanism is chemical, not electrical — slower and less directional than animal neural signalling but no less remarkable in its capacity to direct growth toward favourable conditions and away from unfavourable ones.
This lesson covers the four tropisms (phototropism, gravitropism, hydrotropism, thigmotropism), the role of the plant hormone indole-3-acetic acid (IAA) — the predominant auxin — as master regulator of tropic responses, the classical experimental tradition that established the auxin paradigm (paraphrasing Darwin's coleoptile experiments, Boysen-Jensen's gelatin-block work, and Went's quantitative agar-block assay), the modern molecular model of polar auxin transport via PIN efflux carriers, the acid growth hypothesis for cell-wall extension, apical dominance, leaf abscission, and commercial applications including synthetic herbicides.
Key Definition: A tropism is a directional growth response of a plant to a directional environmental stimulus. The direction of the response is determined by the direction of the stimulus, distinguishing tropisms from non-directional nastic movements (e.g. petal opening at dawn).
| Tropism | Stimulus | Example |
|---|---|---|
| Phototropism | Light direction | Shoot grows toward light (positive phototropism); root grows away (negative phototropism) |
| Gravitropism (geotropism) | Gravity | Root grows downward (positive gravitropism); shoot grows upward (negative gravitropism) |
| Hydrotropism | Water concentration gradient | Roots grow toward moisture |
| Thigmotropism | Touch / contact | Tendrils coil around supports (positive thigmotropism) |
The sign convention is critical: positive means growth toward the stimulus; negative means growth away from the stimulus.
All these responses arise from a single mechanism: differential elongation of cells on opposite sides of the responding organ, driven by differential distribution of IAA.
Indole-3-acetic acid (IAA) is the principal naturally-occurring auxin (a class of plant hormones). It is:
The dose–response curve of cell elongation against IAA concentration is bell-shaped: low IAA stimulates elongation, high IAA inhibits it, and the optimal concentration is lower for roots than for shoots. Practical consequence: a concentration that promotes shoot growth simultaneously inhibits root growth.
graph TD
A["IAA synthesised at shoot tip"] --> B["Polar transport down shoot<br/>via PIN efflux carriers"]
B --> C["Light from one side<br/>causes IAA redistribution<br/>to shaded side"]
C --> D["Higher IAA on shaded side"]
D --> E["H⁺ pumped into cell wall<br/>acid growth hypothesis"]
E --> F["Cell wall loosens<br/>expansins activated"]
F --> G["Cells elongate on shaded side"]
G --> H["Shoot bends toward light<br/>positive phototropism"]
style C fill:#27ae60,color:#fff
style E fill:#3498db,color:#fff
style H fill:#e74c3c,color:#fff
The auxin hypothesis was built by a sequence of cleverly-designed experiments on grass coleoptiles (the protective sheath that emerges first when a cereal seed germinates). All these experiments are paraphrased here — the original papers should be consulted for verbatim wording, which is not reproduced.
Charles Darwin and his son Francis worked with canary-grass coleoptiles. Their framework (paraphrased — not a verbatim quotation) was that some influence is transmitted from the tip of the coleoptile to the elongation zone below, and this influence causes the asymmetric growth that produces the phototropic bend.
They demonstrated:
Darwin concluded that the tip detects light and transmits a chemical signal to the elongation zone below.
Peter Boysen-Jensen extended the Darwin paradigm by inserting a gelatin block (porous to diffusing chemicals) between the cut tip and the stump of a decapitated coleoptile. The coleoptile still responded to light — the signal had crossed the block. He next inserted a mica strip (impermeable to diffusing chemicals). The response was abolished. He concluded that a chemical signal was transmitted from tip to elongation zone, paraphrased here as his interpretation that an inhibition or stimulation chemical message moves down through the tissue.
Arpad Paál cut off coleoptile tips and replaced them off-centre. In darkness, the coleoptile bent away from the side where the tip was replaced. This established that the chemical signal flows from the tip into the underlying tissue and asymmetric distribution alone — without light — causes asymmetric growth.
Frits Went placed cut coleoptile tips on agar blocks for several hours, allowing the diffusing chemical to accumulate. He then placed these agar blocks (without any tip) on decapitated coleoptiles in darkness. The coleoptiles bent toward the side opposite the block. The bend angle was proportional to the time the tip had sat on the agar — providing the first quantitative assay for the chemical, which Went named "auxin" from the Greek auxein, to increase. Went's quantitative work made it possible to purify auxin chemically; IAA was identified shortly afterwards.
| Experiment | Question Answered |
|---|---|
| Darwin (intact vs decapitated coleoptile) | Tip is required for phototropic response |
| Darwin (opaque cap on tip) | Tip is the light detector |
| Boysen-Jensen (gelatin block) | The signal can cross a porous barrier — it is chemical |
| Boysen-Jensen (mica strip) | An impermeable barrier abolishes the response |
| Paál (asymmetric tip) | Asymmetric distribution alone causes the bend; light is needed only to create the asymmetry |
| Went (agar block) | The chemical is diffusible and the response is quantifiable |
The cumulative case for a single diffusible chemical messenger (later identified as IAA) was overwhelming by 1930.
The classical work establishes that IAA is the messenger; molecular biology has established how.
IAA is moved from cell to cell by a combination of passive diffusion (the protonated form readily crosses the membrane) and active efflux via PIN proteins. PIN proteins are integral membrane proteins located on one face of the plant cell — they pump IAA out of the cell into the apoplast on that face. The asymmetric placement of PIN proteins on the basal (downward-facing) cell membrane drives polar IAA flow from shoot tip toward the base. Light or gravity can redistribute the PIN proteins to redirect the flow:
IAA promotes elongation by the following sequence:
This acid growth hypothesis explains why IAA-induced growth begins within minutes of IAA application — the response is biochemical, not transcriptional. IAA also has slower transcriptional effects that take hours and reshape patterns of organ development.
The actively-growing apical bud of a shoot suppresses the growth of the lateral (axillary) buds below it. Removing the apical bud (by pinching or pruning) releases the lateral buds to grow, producing bushier plants. The proposed mechanism — though still debated — is that apical-bud-derived IAA travels down the stem and indirectly inhibits cytokinin signalling in axillary buds, keeping them dormant.
Practical applications:
Leaf fall (abscission) in deciduous plants is regulated by the antagonism between auxin (inhibits abscission) and ethylene (promotes abscission). Young, photosynthetically-active leaves produce high levels of IAA, keeping themselves attached. As autumn approaches, IAA production declines, ethylene production rises, and an abscission layer of weak parenchyma cells develops at the base of the petiole. Eventually the layer breaks under wind or weight, and the leaf falls. This is synoptic with course 7 — homeostasis (the broader theme of regulated antagonism) and with course 5 — photosynthesis (chlorophyll breakdown reveals carotenoids and produces autumn colours).
| Use | Hormone or analogue | Mechanism |
|---|---|---|
| Rooting powders for cuttings | Synthetic IAA analogues (IBA, NAA) | Stimulates adventitious root formation on cut stems |
| Selective broadleaf herbicides | 2,4-D (2,4-dichlorophenoxyacetic acid) | Synthetic auxin analogue; broadleaf weeds cannot metabolise it and over-elongate to death. Grasses tolerate it because of differential metabolism. Used in cereal crops to control dicot weeds |
| Fruit set without pollination | NAA, GA | Auxin sprays induce parthenocarpy (seedless fruit) |
| Delaying fruit drop | NAA | Sprayed on apple trees to retain fruit until harvest |
| Tissue culture / micropropagation | IAA, BAP | Auxin / cytokinin ratio controls root vs shoot formation in callus |
2,4-D was one of the first commercial selective herbicides and revolutionised cereal agriculture in the 1940s. Its synoptic relevance to course 9 — ecosystems / agriculture is that selective herbicides reduce biodiversity in arable systems by eliminating broadleaf wildflowers — a contemporary conservation concern.
This content sits in AQA 7402 Section 3.6.1 — survival and response in plants, IAA, tropisms (refer to the official AQA specification document for exact wording). Examined on Paper 2 and synoptically on Paper 3.
This lesson connects to:
Tropism / IAA questions split AO marks predictably:
| AO | Typical share | Earned by |
|---|---|---|
| AO1 (knowledge) | 40–50% | Naming the tropism, defining IAA, listing experiments |
| AO2 (application) | 30–40% | Interpreting experimental setups; predicting growth direction from IAA distribution |
| AO3 (analysis / evaluation) | 15–25% | Evaluating Boysen-Jensen vs Went; explaining why same [IAA] promotes shoot but inhibits root growth |
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