Plant hormones

HomeostasisThe human nervous systemReaction time required practicalThe human brainThe human eyeControl of body temperatureThe endocrine systemControl of blood glucose concentrationControl of nitrogen and water levelsHormones in human reproductionControl of fertility and treatment of infertilityAdrenaline, thyroxine and negative feedbackPlant hormonesGrowth of seedlings required practical

This topic is for students studying the full biology course only

Auxin and tropisms

Plants produce hormones to control growth and respond to environmental factors like light (phototropism) and gravity (gravitropism or geotropism). These hormones help plants grow in the right direction.

Auxin is a plant hormone that influences the growth of shoots and roots. Auxin is produced in the tips of shoots and roots and spreads backward to stimulate cell growth. An uneven distribution of auxin causes unequal growth rates, resulting in bending toward light or gravity.

In Shoots: Auxin causes cells to grow (elongate) more on the side that is shaded or facing away from light, causing the shoot to bend toward the light. This is how shoots exhibit positive phototropism (grow towards light).
In Roots: Auxin inhibits cell elongation, making the root bend downwards, which is positive gravitropism (grow towards gravity).

Commercial use of plant hormones (Higher tier only)

Plant hormones have various applications in agriculture and horticulture to control growth:

Auxins:
- As Weed Killers: Selective weed killers containing auxins kill broad-leaved plants (like weeds) but leave crops like grasses unaffected.
- As Rooting Powders: Auxins are used to encourage root growth in plant cuttings.
- In Tissue Culture: Auxins promote cell division and the growth of roots and shoots in tissue culture.
Gibberellins are another type of plant hormone that help to:
- End Seed Dormancy: They trigger seeds to start germinating.
- Promote Flowering: They stimulate flowering, sometimes without needing changes in the environment.
- Increase Fruit Size: Gibberellins can be used to grow larger fruits.
Ethene is another type of plant hormone.
- Ethene gas regulates fruit ripening and cell division.
- In the food industry, ethene is used to control the ripening of fruits during storage and transport, allowing fruits to ripen just before reaching consumers.

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