A phytohormone is a plant hormone— a chemical substance produced in plants that controls growth and responses to stimuli, even when present in very small amounts.
Definitions [30]
Definition: Hormones
A hormone is a chemical messenger produced in one part of an organism and transported to other parts, where it regulates growth, development, and responses.
Definition: Phytohormones
Define the following term:
Phytohormones
Plant hormones are known as phytohormones. These are organic molecules that serve as mediators, similar to animal hormones, to facilitate the coordination of a variety of cellular activities within a plant.
Definition: Stimuli
Stimuli are the changes in the external or internal environment of an organism that trigger a response or movement.
Definition: Responses
Responses are the actions or movements shown by an organism as a result of the stimuli.
Definition: Parthenocarpy
Parthenocarpy is the process of fruit development without fertilization.
Definition: Parthenocarpic Fruits
Parthenocarpic fruits are those that develop without fertilization. Auxins can induce such fruit formation in plants like apples, tomatoes, and bananas.
Define the following term:
Parthenocarpy
Parthenocarpy is the process by which fruits develop naturally or artificially without the necessity of fertilizing ovules, resulting in seedless fruits.
Define the following term:
Apical dominance
In the majority of vascular plants, apical buds dominate over lateral buds. The growth of lateral buds occurs only after the removal of the apical buds. This phenomenon of the suppression of the growth of lateral buds by apical buds is called apical dominance.
Definition: Auxin
Auxin is the first discovered plant growth hormone, coined by F.W. Went in 1928. Derived from the Greek word auxein meaning "to grow," auxins are powerful growth stimulants effective even in low concentrations. They are mainly found in actively growing regions like shoot apex, root apex, and meristems. The main natural auxin is IAA (Indole-3-acetic acid).
Definition: Apical dominance
Apical dominance is the phenomenon where the apical (terminal) bud of a plant suppresses the growth of lateral buds. This is mainly due to the action of auxins produced in the apical bud.
Definition: Gibberellin
Gibberellin is a plant growth hormone found mainly in meristematic tissues like stem apex, root apex, buds, and seeds. It promotes internode elongation, breaks seed dormancy, and supports fruit development. Gibberellic acid (GA₃) is the most studied form of gibberellin.
Definition: Cytokinin
Cytokinin is a plant hormone discovered in the 1950s by Skoog and Miller. It is primarily produced in root tips and transported via xylem. Cytokinin promotes plant growth by stimulating cell division, especially in germinating seeds, developing fruits, and embryos.
Definition: Ethylene
Ethylene is a plant hormone that exists as a gas at room temperature. It is produced in fruits and acts at the same place where it is made. It was discovered by R. Gane and is formed in higher plants, fungi, and meristematic tissues.
Definition: Abscisic Acid (ABA)
Abscisic acid is a plant hormone found in chloroplasts of leaves and in high amounts in fruits and seeds. It acts as a growth inhibitor and is present in angiosperms, gymnosperms, pteridophytes, and some mosses.
Define the following term:
Abscission
Abscission is the process by which plants shed or detach various portions that are no longer necessary for them, such as leaves, fruits, flowers, and seeds.
Definition: Tropism
The direction of the response is related to the direction from which the stimulus comes. Such a response is known as tropism.
Definition: Tropic
The term "tropic" comes from a Greek word "tropos" which means to "turn'.
Definition: Tropic Movements
Growth movements occurring in response to unidirectional external stimuli in a plant part are called tropic movements.
Definition: Phototropism
Phototropism is the directional growth of a plant in response to light.
Definition: Positive Phototropism
Positive phototropism is when a plant part, like a shoot, grows towards a light source.
Definition: Negative Phototropism
Negative phototropism is when a plant part, like a root, grows away from a light source.
Definition: Geotropism / Gravitropism
Geotropism (also called gravitropism) is the directional growth of a plant in response to Earth's gravity.
Definition: Positive Geotropism
Positive geotropism is when a plant part, such as a root, grows in the direction of gravity (downward).
Definition: Negative Geotropism
Negative geotropism is when a plant part, such as a shoot, grows against the direction of gravity (upward).
Definition: Hydrotropism
Hydrotropism is the directional growth of a plant part in response to water or moisture.
Definition: Positive Hydrotropism
Positive hydrotropism is when a plant part, such as a root, grows towards the source of moisture.
Definition: Negative Hydrotropism
Negative hydrotropism is when a plant part, such as a shoot, grows away from the source of moisture.
Definition: Thigmotropism
Thigmotropism is the directional growth of a plant part in response to touch or contact with a solid object.
Definition: Chemotropism
Chemotropism is the growth movement of plant parts in response to chemicals.
Key Points
Key Points: Auxins
- Auxins stimulate stem, root, and fruit growth by enhancing cell elongation, especially in growing regions like shoot and root apices.
- They delay leaf senescence and help prolong foliage life.
- Auxins promote apical bud growth while suppressing lateral bud development (apical dominance).
- They aid in root initiation in plant cuttings like roses and bougainvillea.
- Auxins can trigger parthenocarpic fruit development in crops like apple, tomato, and banana.
Key Points: Gibberellins
- Gibberellins promote internode elongation by enhancing cell growth, especially in meristematic regions.
- They break seed dormancy and stimulate germination.
- These hormones aid fruit growth and can induce parthenocarpy.
- Gibberellins delay ageing in plants, prolonging functional life.
- Widely used in horticulture and industry—for example, to enlarge grapes and apples, and to accelerate malting in brewing.
Key Points: Cytokinins
- Cytokinins promote plant growth by stimulating cell division, even in non-meristematic tissues.
- They help expand cotyledons during seed development.
- Cytokinins break seed dormancy and encourage germination.
- They enhance chlorophyll synthesis and delay leaf senescence.
- Cytokinins inhibit apical dominance, promoting lateral bud growth.
Key Points: Ethylene
- Ethylene is a gaseous plant hormone produced at the site of action, mainly in fruits and meristematic tissues.
- It promotes fruit ripening and is widely used in agriculture for this purpose.
- It reduces stem elongation and accelerates plant senescence.
- Ethylene initiates seed germination (e.g., in peanuts) and sprouting in potato tubers.
- It enhances root growth, root hair formation, and induces flowering in crops like mango.
Key Points: Abscisic Acid (ABA)
- ABA is a growth-inhibiting hormone that slows down plant metabolism.
- It induces seed dormancy and prevents germination under unfavorable conditions.
- ABA promotes senescence and abscission of leaves, flowers, buds, and fruits.
- It causes stomatal closure, helping the plant conserve water.
- ABA enhances stress tolerance in plants, earning it the name "stress hormone."
