Plant Hormones

Plant unit (Section 14.2)

Auxin (IAA) - Effects

CELL ELONGATION (mostly in stems):

1. Controls cell division and differentiation

2. Apical dominance

Gibberellins - types

over 100 different gibberellins identified

Gibberellins - effects

• stem elongation

• fruit growth

• seed germination (process by which a plant grows from the seed)

Example of Gibberellins in real life

plump grapes in grocery stores have been treated with gibberellin hormones while on the vine

Abscisic acid (ABA) - Effects

• Stops growth

• Causes seed dormancy (prevents germination)

• Causes stomata to close during drought (prevents water loss)

• Helps plants survive stress (cold, drought)

• high concentrations of abscisic acid 

  • germination only after ABA is inactivated or leeched out

• survival value: seed will germinate only under optimal conditions

  • light, temperature, moisture

Ethylene - What is it?

Hormone gas released by plant cells

Ethylene - Effects

Fruit ripening
Ethylene makes fruit ripen by:

• Softening the fruit

• Making it sweeter

• Changing color (green → red/yellow)

• Example: bananas ripen faster when ethylene builds up.

Leaf drop (like in autumn)
Ethylene causes leaves to fall off trees.

• In fall, ethylene levels increase

• Leaves detach from the stem

• This helps plants survive winter

Apoptosis

programmed cell death

• Cells are told to die on purpose

• It’s controlled and normal

• Used for leaf drop, fruit ripening, and aging

Ethylene: Fruit Ripening (ADAPTATION)

• Plants want to protect seeds while they’re developing → hard, tart fruit prevents animals from eating immature seeds.

• When seeds are ready → fruit becomes soft, sweet, and ripe → attracts animals to eat it.

• Animals then disperse seeds (via droppings), helping the plant reproduce.

Key idea: Fruit changes over time to protect seeds first, then help disperse them.

Ethylene: Fruit Ripening (MECHANISM)

• Ethylene hormone triggers the ripening process.

• Cell wall breakdown → fruit softens.

• Starch converted to sugar → fruit sweetens.

• Positive feedback:

  • Ethylene triggers ripening.

  • Ripening stimulates more ethylene production, speeding up the process.

Key idea: once ripening starts, it accelerates itself through ethylene production.

positive feedback system

• ethylene triggers ripening

• ripening stimulates more ethylene production

Cytokinins

(Think Cytokenisis)

• Promote Cell Division

• Delays aging of leaves by maintaining chlorophyll content

• Reverses apical dominance by auxin

Reactions to stimuli

1. Tropism

2. Phototropism

3. Gravitropism

Tropism

Phototropism

Gravitropism

• Plant’s response to gravity

• Roots grow down (positive gravitropism), stems grow up (negative gravitropism).

hormone

a chemical compound produced in one part of the plant that controls growth activity in another part of the plant

nastic response

a plant’s movement in response to a stimulus that is not associated with the direction of the stimulus

Thigmotropism

A plant’s growth response to touch or contact