SPI Biology · Plant Biology

Plant Hormones and Movements

Plants have no nerves, so they control growth with chemicals called plant hormones, and they respond to light, gravity, water and chemicals by growing towards or away from them.

All Plant Biology notes

Why this matters

Two questions in the bank, both EASY: the hormone that makes mature leaves and fruits fall, and the stimulus a sunflower follows. Five hormones and four tropisms cover every option either question can print.

Concept 1 of 2: The five plant hormones

Four plant hormones push growth forward in different ways. One, abscisic acid, does the opposite: it slows growth down and makes old leaves and ripe fruit drop.

Definition

Auxin: made at the shoot tip; makes cells grow longer; bends the shoot towards light.
Gibberellin: makes the stem grow long.
Cytokinin: promotes cell division; found where cells divide fast, as in fruits and seeds.
Abscisic acid: stops growth; makes mature leaves and fruits fall (abscission) and leaves wilt.
Ethylene: a gas that ripens fruit.

Hormone	Main job	Promotes or stops growth
Auxin	Cell lengthening, bending towards light	Promotes
Gibberellin	Stem lengthening	Promotes
Cytokinin	Cell division	Promotes
Abscisic acid	Fall of leaves and fruits, wilting	Stops Abscisic acid is the one growth stopper among them.
Ethylene	Ripening of fruit	Ripens

Practice this conceptself-check · 4 quick reps

The same idea in a practice question:

Practice questionEasy

Example 1 · Plant Biology · Plant Hormones and Movements

The substance that triggers the fall of mature leaves and fruits from plants is due to

Thinking every plant hormone promotes growth

Abscisic acid stops growth and makes leaves and fruits fall. It is the inhibitor; auxin, gibberellin and cytokinin all promote growth.

Concept 2 of 2: Tropisms: growing towards or away

A tropism is a slow bending of a plant part towards a stimulus or away from it. The name of the tropism is the name of the stimulus.

Definition

Phototropism: response to light. Shoots bend towards light; sunflowers turn to follow the sun through the day.
Geotropism: response to gravity. Roots grow down, shoots grow up.
Hydrotropism: response to water. Roots grow towards water.
Chemotropism: response to chemicals. The pollen tube grows towards the ovule.
Touch-me-not (Mimosa) folding its leaves when touched is a quick movement, not growth, and not a tropism.

Tropism	Stimulus	Example
Phototropism	Light	A shoot bending to a window; a sunflower following the sun A question may call the stimulus 'day or night': it means light.
Geotropism	Gravity	Roots growing downwards
Hydrotropism	Water	Roots growing towards wet soil
Chemotropism	Chemicals	Pollen tube growing to the ovule

Practice this conceptself-check · 3 quick reps

The same idea in a practice question:

Practice questionEasy

Example 2 · Plant Biology · Plant Hormones and Movements

The movement of sunflowers is in response to a stimulus such as :

Touch-me-not as a tropism

The leaves of touch-me-not fold in seconds when touched, by a change of water in their cells. It is a quick movement without growth, so it is not a tropism.

Shoots and roots answer gravity the same way

Roots grow towards gravity (downwards) and shoots grow away from it (upwards). Both are geotropism, in opposite directions.

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Reference tables (2)

The five plant hormones5 rows

Hormone	Main job	Promotes or stops growth
Auxin	Cell lengthening, bending towards light	Promotes
Gibberellin	Stem lengthening	Promotes
Cytokinin	Cell division	Promotes
Abscisic acid	Fall of leaves and fruits, wilting	Stops Abscisic acid is the one growth stopper among them.
Ethylene	Ripening of fruit	Ripens

Tropisms: growing towards or away4 rows

Tropism	Stimulus	Example
Phototropism	Light	A shoot bending to a window; a sunflower following the sun A question may call the stimulus 'day or night': it means light.
Geotropism	Gravity	Roots growing downwards
Hydrotropism	Water	Roots growing towards wet soil
Chemotropism	Chemicals	Pollen tube growing to the ovule

Watch out for (3)

Thinking every plant hormone promotes growth
→ The five plant hormones
Touch-me-not as a tropism
→ Tropisms: growing towards or away
Shoots and roots answer gravity the same way
→ Tropisms: growing towards or away

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