Plant hormones: auxins and their discovery
Auxins are plant hormones made at the shoot tip that make cells grow longer. Darwin's and Went's experiments showed that a chemical from the tip causes bending towards light.
Darwin's experiment (1880)
Charles Darwin and his son Francis Darwin used seedlings of **canary grass** (Phalaris canariensis). The young shoot of a grass is covered by a sheath called the **coleoptile**.
- 1Light from one side: the coleoptile bent towards the light.
- 2Tip cut off: no bending.
- 3Tip covered with an opaque (light-proof) cap: no bending.
- 4Tip left open but the lower part covered: the coleoptile still bent.
- 5★Conclusion: the tip senses light and sends some influence down that makes the lower part bend.
★ key step
Went's experiment
F.W. Went, a Dutch scientist, used **oat (Avena) coleoptiles** and small **agar blocks**. Agar is a jelly that can soak up chemicals.
- 1He cut off coleoptile tips and kept them on an agar block for some time.
- 2He removed the tips. A chemical from the tips had passed into the agar.
- 3He placed the agar block on one side of a cut coleoptile stump. The stump bent away from that side, even in the dark.
- 4A plain agar block, never touched by a tip, gave no bending.
- 5★Conclusion: a chemical from the tip makes cells grow. This chemical is auxin. The amount of bending shows how much auxin is present (the Avena curvature test).
★ key step
Effects and uses of auxins
- Cell elongation: shoot cells grow longer, so stems grow taller.
- Phototropism: auxin collects on the shaded side; cells there grow more, so the shoot bends towards light.
- Apical dominance: auxin from the shoot tip stops side buds growing. Pinch off the tip and the plant becomes bushy.
- Rooting: dipping stem cuttings of rose or hibiscus in auxin helps roots form.
- Seedless fruits (parthenocarpy): auxin can make fruit form without fertilisation, e.g. tomato.
- Stops early fruit drop: sprays keep young fruits of mango and citrus from falling.
- Weed killer: 2,4-D kills broad-leaved weeds in paddy and wheat fields but does not harm the narrow-leaved crop.
Why does a shoot bend towards light?
Light falls on one side of the shoot. Auxin moves away from the light to the shaded side. Cells on the shaded side grow longer than cells on the lit side. So the shoot bends towards the light. This is positive phototropism.
How to draw Went's experiment
- 1Draw a coleoptile with its tip cut off and placed on a small rectangle. Label coleoptile tip and agar block.
- 2Draw the agar block placed on one side of a cut stump. Draw the stump curving away from the block. Label bending away from the block.
- 3Draw a plain agar block on another stump that stays straight. Label control: no auxin, no bending.
- 4Write a heading under each drawing and an arrow from step to step.
Gardeners dip rose and hibiscus cuttings in rooting powder, which contains an auxin, before planting. Farmers spray 2,4-D to kill broad-leaved weeds in paddy fields without harming the rice.
2 marks: “What is apical dominance?” Auxin from the shoot tip stops side (lateral) buds from growing. Removing the tip makes the plant bushy.
2 marks: “Write any two uses of auxins in agriculture.” Rooting of stem cuttings; seedless fruits; preventing early fruit drop; 2,4-D as a weedicide (any two).
7 marks: “Describe the experiments that led to the discovery of auxins.” Darwin's canary grass experiment (one-sided light, tip removed, tip covered) and its conclusion; Went's oat coleoptile and agar block experiment and its conclusion; diagrams; uses of auxins.
Auxin runs from the light: it gathers on the shady side, so the shoot bends to the bright side.