Revision: Plant Physiology >> Chemical Coordination in Plants Biology (English Medium) ICSE Class 10 CISCE

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.

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.

Responses are the actions or movements shown by an organism as a result of the stimuli.

Stimuli are the changes in the external or internal environment of an organism that trigger a response or movement.

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.

Parthenocarpy is the process by which fruits develop naturally or artificially without the necessity of fertilizing ovules, resulting in seedless fruits.

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.

Parthenocarpy is the process of fruit development without fertilization.

Parthenocarpic fruits are those that develop without fertilization. Auxins can induce such fruit formation in plants like apples, tomatoes, and bananas.

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.

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.

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.

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.

The direction of the response is related to the direction from which the stimulus comes. Such a response is known as tropism.

The term "tropic" comes from a Greek word "tropos" which means to "turn'.

Growth movements occurring in response to unidirectional external stimuli in a plant part are called tropic movements.

Positive phototropism is when a plant part, like a shoot, grows towards a light source.

Phototropism is the directional growth of a plant in response to light.

Geotropism (also called gravitropism) is the directional growth of a plant in response to Earth's gravity.

Positive geotropism is when a plant part, such as a root, grows in the direction of gravity (downward).

Negative geotropism is when a plant part, such as a shoot, grows against the direction of gravity (upward).

Hydrotropism is the directional growth of a plant part in response to water or moisture.

Positive hydrotropism is when a plant part, such as a root, grows towards the source of moisture.

Thigmotropism is the directional growth of a plant part in response to touch or contact with a solid object.

Chemotropism is the growth movement of plant parts in response to chemicals.

• Meaning — Phytohormones regulate growth and physiological functions at a site remote from production. Coined by Thimann & Pincus (1948).
• Types — Promoters: Auxin, Gibberellin, Cytokinin. Inhibitors: ABA, Ethylene.
• Discovery — Darwin (1880) observed growth stimulus at the coleoptile tip, causing bending towards light. Auxin = first plant hormone discovered.
• Key Scientists — Boysen-Jensen (1910): chemical stimulus; Paal (1919): bending in dark; Went (1928): isolated auxin from Avena using agar blocks.
• Transport — Active in minute amounts; transported through phloem parenchyma (Phillips, 1971).

• Discovery — First discovered by Kurosawa (1926) from Gibberella fujikuroi, causing Bakanae disease in rice. Crystalline form isolated by Yabuta & Sumiki (1938). Active form: GA₃ (Gibberellic acid).
• Synthesis & Transport — Synthesised in young leaves, seeds, roots, and stem tips from mevalonic acid. Transport is non-polar. Over 150 types are known.
• Stem Elongation & Bolting — Elongates internodes; converts dwarf plants to tall (e.g. maize, pea). Causes bolting in rosette plants (beet, cabbage).
• Seed Germination & Dormancy — Breaks seed and bud dormancy; stimulates amylase and protease for germination in cereals.
• Parthenocarpy & Fruit Growth — Produces seedless fruits (tomato, apple, pear); increases grape bunch length. Pomalin (GA₄ + GA₇ + cytokinin) = apple enlarger.
• Other Effects — Causes maleness in some plants; delays citrus ripening; inhibits root growth; delays senescence; prevents abscission.
• Vernalization Substitute — Overcomes the need for cold treatment and induces flowering in long-day plants.

• Discovery — Term coined by Letham. The first cytokinin (kinetin) was discovered by Skoog & Miller (1954) from tobacco callus culture. First natural cytokinin: Zeatin (from unripe maize by Letham, 1963). Synthetic: 6-benzyl adenine.
• Nature & Site — Purine (adenine) derivative. Found at root apices and immature fruits. Coconut milk is a rich source.
• Cell Division & Growth — Promotes cell division and enlargement. High cytokinin: auxin ratio → shoots; low ratio → roots.
• Lateral Bud & Apical Dominance — Promotes lateral bud growth and reverses apical dominance.
• Senescence & Dormancy — Delays senescence and abscission (Richmond & Lang, 1957). Breaks seed dormancy and promotes germination.
• Other Effects — Induces flowering (Lemna, Wolffia); promotes chloroplast development; causes femaleness; favours phloem transport.
• Cytokinin + Auxin — Balanced combination induces organogenesis and controls morphogenic differentiation.

• Introduction — Only a gaseous phytohormone. Denny (1924) reported the fruit ripening role; Gane (1934) confirmed natural synthesis. Produced from methionine. Source: Ethephon.
• Site of Synthesis — Roots, shoot apical meristems, ripening fruits. Acts as both a promoter and an inhibitor.
• Fruit Ripening — Promotes the ripening of bananas, apples, mangoes, and tomatoes. Increases respiratory climacteric. Used in post-harvest technology.
• Abscission & Senescence — Enhances abscission of leaves, flowers, and fruits. Promotes senescence. Causes degreening in bananas and citrus.
• Dormancy & Roots — Breaks seed dormancy. In low concentrations, it promotes root growth and lateral root initiation.
• Inhibitory Effects — Inhibits flowering (except pineapple); causes apical dominance; causes epinasty (drooping of leaves/flowers).
• Other Effects — Promotes horizontal seedling growth; may cause ageotropism (roots lose sensitivity to gravity).

• Discovery — Carns & Addicott (1961–65): abscisin from cotton; Wareing (1963): dormin from Acer buds. Both are named ABA. Chemically: 15-carbon sesquiterpenoid from mevalonic acid.
• Nature & Transport — Natural growth inhibitor and stress hormone. Also called anti-gibberellin. Transport is non-polar. Found in leaves, fruits, roots, and seeds.
• Stomatal Closure — Causes K⁺ efflux from guard cells → stomata close during stress. Known as an antitranspirant.
• Dormancy & Abscission — Induces bud and seed dormancy. Causes abscission of leaves, flowers, and fruits. Regulates fruit drop.
• Senescence & Stress — Accelerates senescence. Helps plants tolerate drought, salinity, cold and frost (stress hormone).
• Inhibitory Effects — Inhibits cell division, elongation, and cambium activity. Inhibits flowering in long-day plants; promotes in short-day plants.
• Other Effects — Induces carotenoid synthesis; turns green oranges yellow; causes geotropism when applied to roots.