Plant Hormones¶

Part of 4.5 Homeostasis and Response.

Plant hormones coordinate growth rather than rapid movement. Auxin is the main example and explains how plants respond to light and gravity.

Learning Objectives¶

Control and Coordination¶

Plants cannot move from place to place, but they respond to their environment through directed growth, called tropisms. These responses are coordinated by plant hormones — particularly auxin (also called IAA, Indole Acetic Acid).

• Tropisms can be positive (growth towards the stimulus) or negative (growth away from the stimulus).
• Auxin is produced in the tips of shoots and roots, and moves through the plant by diffusion.
• Auxin promotes cell elongation in shoots, but inhibits elongation in roots.

Phototropism (Response to Light)¶

Phototropism is a growth response to light.

• Shoots show positive phototropism (grow towards light).
• Roots show negative phototropism (grow away from light).

Mechanism in the shoot: When light hits from one side, auxin migrates to the shaded side of the shoot tip. The higher auxin concentration on the shaded side causes greater cell elongation there. The shaded side grows faster → the shoot bends towards the light.

In roots: Auxin also accumulates on the shaded side, but high auxin concentration inhibits elongation in roots (the opposite of what happens in shoots). The illuminated side grows slightly more → the root bends away from the light.

Gravitropism (Response to Gravity)¶

Gravitropism (also called geotropism) is a growth response to gravity.

• Roots show positive gravitropism (grow downwards, towards gravity).
• Shoots show negative gravitropism (grow upwards, away from gravity).

Mechanism: When a plant is on its side, auxin accumulates in the lower part of both the shoot and root due to gravity. In the shoot, greater elongation at the bottom causes it to bend upward. In the root, greater auxin at the bottom inhibits elongation there, causing the root to grow downward towards gravity.

Practical Investigations: Darwin's Experiments¶

Charles Darwin first demonstrated that auxin is produced in the shoot tip (coleoptile):

• Control plant (tip intact, lit from one side): bends towards the light.
• Tip removed: grows straight upwards — no tropism because there is no auxin source.
• Tip covered with black paper: grows straight upwards — light cannot reach the tip so no auxin redistribution occurs.
• Tip removed, placed on agar block for 8 hours, then replaced: bends towards the light — auxin diffused into the agar and was re-applied, restoring the response.

This series of experiments proved that: 1. Auxin is produced in the tip. 2. The tip must detect light for phototropism to occur. 3. Auxin moves downwards from the tip.

Gravitropism experiment: place seedlings with roots pointing in different directions on damp cotton wool in a dark petri dish. After a week, all roots will have turned to grow downward — regardless of starting orientation — showing gravitropism is independent of light.

Commercial Uses of Plant Hormones¶

Auxins¶

• Rooting powders — plant cuttings are dipped in auxin powder before being planted. The auxin encourages root formation, making propagation faster and more reliable.
• Selective weedkillers — synthetic auxins (e.g. 2,4-D) cause susceptible weeds to grow at an uncontrolled rate, which kills them. Crop plants are not affected. This increases crop yield by eliminating competition from weeds.
• Tissue culture — auxins are added to the growth medium to stimulate cell growth and root development in cultured plant cells.

Gibberellins¶

• Breaking dormancy — seeds have a dormancy period in which they cannot germinate. Gibberellins end dormancy by stimulating production of amylase enzymes that break down starch stores.
• Increasing fruit size — cell elongation caused by gibberellins makes fruits larger.
• Seedless fruit — when applied to unpollinated flowers, gibberellins cause fruit to develop without fertilisation (called parthenocarpy). This produces seedless fruits such as some citrus varieties.
• Promoting flowering — gibberellins promote flowering in some plants.

Ethene¶

• Fruit ripening — ethene gas is used commercially to ripen fruits that are transported unripe. This allows large-scale shipping of undamaged fruit that is then ripened on demand before sale.
• Leaf shedding — ageing leaves produce ethene, which causes the cells at the base of the leaf stalk to weaken and break down, causing leaves to fall.

Common Confusions¶

• Auxin in shoots vs roots: auxin promotes elongation in shoots but inhibits elongation in roots. This means the same hormone causes opposite bending effects in different tissues.
• Positive vs negative tropism: positive = growth towards the stimulus; negative = growth away. Roots are positively gravitropic (grow down) and negatively phototropic (grow away from light).
• Weedkillers and auxin: selective weedkillers work by overstimulating growth (not by inhibiting it). Too much auxin causes weed stems to grow uncontrollably until the plant collapses.

Key Terms¶

• Auxin: a plant hormone (e.g. IAA) produced in shoot and root tips that controls directional growth by affecting cell elongation.
• Phototropism: a growth response to light; positive = towards light, negative = away from light.
• Gravitropism: a growth response to gravity; positive = towards gravity (downward), negative = away from gravity (upward).
• Rooting powder: a product containing auxins that encourages root development on plant cuttings.
• Selective weedkiller: a herbicide containing synthetic auxin that causes rapid uncontrolled growth and death in target weed species.
• Gibberellin: a plant hormone that breaks seed dormancy, promotes cell elongation, and can stimulate flowering and fruit development.
• Ethene: a gaseous plant hormone that promotes fruit ripening and causes leaf shedding.
• Parthenocarpy: the development of fruit without fertilisation; can be induced by gibberellins to produce seedless fruit.
• Coleoptile: the protective sheath covering the shoot tip in cereal seedlings; used in classic auxin experiments.
• Tropism: a directional growth response of a plant to an environmental stimulus.

Connected Pages¶

• 4.5.3 Hormonal Coordination in Humans
• GCSE Biology Key Terms Glossary
• Required Practical Reference