What Are Plant Hormones?
Plant hormones (phytohormones) are chemical messengers produced in tiny amounts that coordinate growth, development and responses to the environment. Unlike animal hormones, they are not made in specialized glands but in various tissues, and they often act by diffusing to nearby or distant cells.
Plant hormones are naturally occurring organic compounds that regulate plant growth, development, and responses to stimuli such as light, gravity, and stress, even at very low concentrations.
- •Auxins — stem elongation, phototropism, apical dominance
- •Gibberellins — stem growth, seed germination, fruit growth
- •Cytokinins — cell division, delays leaf senescence
- •Abscisic acid (ABA) — stomatal closure, seed and bud dormancy
- •Ethylene — fruit ripening, leaf and flower abscission
- •Both often act antagonistically against auxins and gibberellins
Step-by-step worked examples
A potted plant on a windowsill bends toward the light. Which hormone is responsible and how does it work?
Light hits the stem unevenly, stronger on the side facing the window Auxin migrates to the shaded side of the stem Higher auxin concentration causes cells on the shaded side to elongate faster Uneven elongation bends the stem toward the light (positive phototropism)
A farmer sprays gibberellin on grapevines. What effect would you expect and why?
Gibberellins promote cell elongation and fruit growth Applied to grapevines, they increase internode length and grape (berry) size They are also used commercially to produce seedless, larger grapes Result: bigger grape clusters with elongated stems
Ripe bananas are placed next to unripe green bananas in a closed bag. The green ones ripen faster. Explain using a plant hormone.
Ripe bananas release ethylene gas as they ripen In a closed bag, ethylene accumulates around the fruit Ethylene triggers ripening enzymes in the unripe bananas (autocatalytic effect) Result: nearby green bananas ripen faster than if left alone
Flashcards
Quick quiz
Q1.Which plant hormone is mainly responsible for phototropism?
Q2.Which hormone is a gas and triggers fruit ripening?
Q3.Which hormone helps plants survive drought by closing stomata?
Q4.Which hormone class is most associated with promoting cell division?
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Common mistakes
Thinking plant hormones are made in dedicated glands like in animals. — Correct: Plant hormones are produced in various tissues (shoot tips, roots, fruits) without specialized glands.
Assuming all hormones promote growth. — Correct: ABA and, in many contexts, ethylene inhibit growth or promote dormancy/senescence.
Believing auxin is only involved in phototropism. — Correct: Auxin also controls apical dominance, root initiation, and fruit development.
Confusing gibberellins with cytokinins. — Correct: Gibberellins mainly drive stem elongation and germination; cytokinins mainly drive cell division.
FAQ
What are plant hormones?
They are chemical signals produced in small amounts by plant cells that regulate growth, development, and environmental responses.
What are the types of plant hormones?
The five classical types are auxins, gibberellins, cytokinins, abscisic acid, and ethylene.
What are some plant hormone examples in everyday life?
Ethylene ripening bananas, auxin causing a plant to bend toward light, and ABA closing stomata during drought are common examples.
How do plant hormones differ from animal hormones?
Plant hormones are made in ordinary tissues rather than specialized glands and often act locally as well as at a distance.