Plant hormones
| Hormone | Main effects | Uses by people |
|---|
| Auxins | Cell elongation (tropisms); apical dominance; root formation; fruit development | Rooting powder for cuttings; selective weedkillers; seedless fruit |
| Gibberellins | Stem elongation; break seed dormancy so seeds germinate; flowering; fruit growth | Larger seedless grapes; speeding up germination in brewing (malting) |
| Abscisic acid (ABA) | Closes stomata during water stress; keeps seeds and buds dormant; inhibits growth | Helping plants survive drought; controlling dormancy in storage |
Apical dominance: auxin made in the shoot tip inhibits the growth of side buds, so the plant grows tall. Removing the tip (pruning) removes the auxin source and side branches grow, making the plant bushier.
The coleoptile experiments
The discovery of auxin came from experiments on grass seedlings (coleoptiles) lit from one side:
- Tip covered with an opaque cap: no bending, so the tip detects light.
- Tip removed: no growth or bending, so the tip produces something that causes growth.
- Tip placed on an agar block, then the block placed on a cut coleoptile: it grows, so a chemical diffuses from the tip into the agar.
- Agar block placed on one side of a cut coleoptile in the dark: the coleoptile bends away from that side, so the chemical makes cells on that side elongate more.
Investigation skills: each test needs a control (an untreated coleoptile under the same conditions) and several seedlings per test so the results are reliable.
Plant defences
Plants cannot run from herbivores and pathogens, so they defend themselves:
- Physical — thorns, spines and prickles; tough leaves; a thick waxy cuticle; bark; hairs that make leaves hard to eat.
- Chemical — toxins and bitter compounds (alkaloids, tannins); irritants such as the stinging hairs of nettles; sticky resins that trap insects.
- Responses to attack — some plants produce more toxins after being eaten, or release chemicals that attract the predators of the insects eating them.
Worked example: interpreting an auxin investigation
✏️ Rooting cuttings
1Twenty cuttings are dipped in rooting powder (auxin) and twenty are not. After three weeks, 18 treated cuttings and 7 untreated cuttings have roots.
2Independent variable: whether auxin is applied. Dependent variable: number of cuttings that form roots.
3Conclusion: auxin increases root formation in cuttings (90% compared with 35%).