Plant Hormones¶

Part of 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.

What You Need to Learn¶

Explain why uneven growth bends a plant towards light or changes its direction relative to gravity. Connect these responses to practical investigations. Higher Tier study also covers the roles of gibberellins and ethene and the use of plant hormones in growing crops and ripening fruit, without needing detailed molecular mechanisms.

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 from the tips to regions where growth occurs.
Auxin promotes cell elongation in shoots. Roots are more sensitive: concentrations that stimulate shoots can inhibit root elongation.

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 and Francis Darwin’s seedling experiments in 1880 showed that the shoot tip detects light and sends a signal to the growing region below it. They did not identify auxin itself.

Compare intact seedlings lit from one side with seedlings whose tips are removed or covered by opaque caps. Intact seedlings bend towards light; an opaque cap prevents the bending response. A transparent cap is a useful control because light can still reach the tip. Tip removal also reduces normal elongation, so it should not be described as producing normal straight growth.

For a light or gravity investigation, use seedlings of the same species and similar age, keep moisture and temperature consistent, and repeat each treatment. Record changes in direction and shoot or root length, together with careful labelled drawings. Seedlings placed horizontally in darkness isolate the gravity response from the effect of directional light.

Other Growth Hormones — Higher Tier¶

Gibberellins help initiate seed germination. Ethene helps regulate cell division and fruit ripening. Their roles and uses are needed here, rather than the detailed biochemical mechanisms by which they act.

Commercial Uses of Plant Hormones — Higher Tier¶

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. Suitable crop species are less susceptible at the recommended dose, so the product must be matched to the crop. 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 — Higher Tier¶

Breaking dormancy — gibberellins can trigger dormant seeds to germinate when conditions are suitable.
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 — Higher Tier¶

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, while higher concentrations inhibit elongation in the more sensitive 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.