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.
Understanding Plant Hormones Reference
Plants do not have a brain or nerves, yet they can coordinate growth across the whole organism. Their chemical messages move in several ways. Some travel through the xylem with water from roots.
Others move through the phloem with sugars from leaves. Auxin can move cell to cell in a controlled direction because transport proteins place it on particular sides of cells. This directional transport creates uneven amounts of hormone across a stem or root.
That difference is more important than the total amount. Cells compare their local signal with signals received by nearby cells, then change which genes they use and how their cell walls behave.
A hormone works only when a cell has the right receptor. A receptor is a protein that recognizes a particular chemical signal. After binding, the receptor begins a chain of events inside the cell.
It may switch genes on or off, change ion movement across a membrane, or alter enzyme activity. These steps explain why plant responses are often slower than animal reflexes. Some responses take minutes, such as stomatal closure during drought.
Others take days, such as a bud growing into a side branch. The same hormone can produce opposite results in different organs because receptor types, hormone amount, and cell conditions differ.
Growth responses depend on balance rather than one hormone acting alone. A growing shoot tip can limit the growth of buds below it. When the tip is removed, the balance changes and side buds may grow.
This is why cutting back a hedge often makes it bushier. In tissue culture, scientists adjust the relative amounts of auxin and cytokinin to encourage roots or shoots from a small piece of plant tissue. Seeds provide another useful example.
Dormant seeds are protected from sprouting at the wrong time. Water, temperature, light, and internal hormone balance can shift the seed toward germination. Germination begins only when stored food can be released to support the young plant.
Ethylene shows that plant communication can spread through air. A ripening fruit releases this gas, which can speed ripening in nearby fruit. Commercial storage sometimes uses cool temperatures and careful air control to slow this process.
Plants use ethylene during flooding, injury, and aging, so it is not simply a ripening signal. For exams and lab work, pay close attention to the direction of movement, the tissue receiving the message, and the conditions around the plant.
Do not memorize a hormone as having one fixed effect. Trace the full pathway from environmental cue to hormone distribution, receptor action, cell change, then visible response.