Plant hormones are chemical signals that control how plants grow, develop, and respond to their environment. This reference helps students compare the major hormone classes, where they are produced, and what effects they cause. It is useful for studying plant physiology, tropisms, seed germination, fruit ripening, and stress responses.
Knowing these hormones also helps explain agriculture practices such as pruning, rooting cuttings, and controlling fruit ripening.
The main plant hormones are auxins, gibberellins, cytokinins, abscisic acid, and ethylene. Auxins often promote cell elongation and directional growth, while gibberellins promote stem growth and seed germination. Cytokinins promote cell division, abscisic acid helps close stomata and maintain dormancy, and ethylene promotes fruit ripening and leaf drop.
A plant response depends on hormone concentration, target tissue, timing, and interactions with other hormones.
Key Facts
- Auxins are produced mainly in shoot tips and young leaves, and they promote cell elongation, apical dominance, root formation, and phototropism.
- Gibberellins are produced in young leaves, embryos, and roots, and they stimulate stem elongation, seed germination, and enzyme production in seeds.
- Cytokinins are produced mainly in roots and developing tissues, and they stimulate cell division, shoot growth, and delayed leaf aging.
- Abscisic acid is produced in stressed leaves, roots, and seeds, and it promotes seed dormancy and stomatal closure during water stress.
- Ethylene is a gaseous hormone produced by ripening fruits, aging tissues, and stressed plants, and it promotes fruit ripening, leaf abscission, and the triple response.
- Phototropism occurs when uneven auxin distribution causes cells on the shaded side of a shoot to elongate more, bending the shoot toward light.
- Gravitropism occurs when roots grow downward and shoots grow upward in response to gravity, partly through changes in auxin distribution.
- Hormone response = hormone signal + receptor detection + target cell response, so the same hormone can cause different effects in different tissues.
Vocabulary
- Plant hormone
- A chemical messenger made by a plant that affects growth, development, or responses in target cells.
- Auxin
- A plant hormone class that commonly promotes cell elongation, apical dominance, root initiation, and directional growth responses.
- Gibberellin
- A plant hormone class that promotes stem elongation, seed germination, and growth of young tissues.
- Cytokinin
- A plant hormone class that promotes cell division, shoot formation, and delayed aging in leaves.
- Abscisic acid
- A plant hormone that helps plants respond to drought, closes stomata, and maintains seed dormancy.
- Tropism
- A directional growth response in which a plant grows toward or away from a stimulus such as light, gravity, or touch.
Common Mistakes to Avoid
- Thinking each hormone has only one job is wrong because plant hormones often have multiple effects depending on tissue type, concentration, and stage of development.
- Confusing auxin and cytokinin is wrong because auxin is strongly linked to cell elongation and root initiation, while cytokinin is strongly linked to cell division and shoot formation.
- Saying abscisic acid causes growth in all situations is wrong because it usually slows growth, maintains dormancy, and helps plants survive stress.
- Forgetting that ethylene is a gas is wrong because its gaseous form lets it spread through air and influence nearby fruits or tissues.
- Assuming tropisms are simple movement is wrong because tropisms are growth responses, meaning the plant changes its growth pattern over time.
Practice Questions
- 1 A seedling grows 12 cm in 4 days after treatment with gibberellin. What is its average growth rate in cm per day?
- 2 A plant cutting has 30 cuttings treated with auxin, and 24 form roots. What percent of the cuttings formed roots?
- 3 A batch of bananas ripens faster when stored near ripe apples. Which plant hormone most likely caused this effect?
- 4 A shoot bends toward a window while its roots continue growing downward in the soil. Explain how these responses show that plant hormones can produce different effects in different tissues.