byPlant Growth Regulators (PGRs) are complex organic substances (other than nutrients) that are naturally produced in specific parts of a plant. They are transported to various sites where they regulate growth, development, and metabolism.
In simple terms, these hormones control processes such as cell division, elongation, and differentiation. Below is an organized review of the major types of PGRs along with their discovery, synthesis, functions, and applications.
Auxins
Definition & Discovery
- Meaning: Derived from a Greek word meaning “to increase.”
- Discovery: First noted by F. W. Went (1928) using the oat ovule curvature test; later the term was proposed by Kogel and Hegel-Smith (1931).
- Synthesis: Produced in meristematic tissues (e.g., bud tips, apical buds, new leaves) and transported in a polar (top-to-bottom) direction. Sunlight greatly affects their synthesis.
Functions
- Apical Dominance: Inhibits lateral bud formation.
- Tropism: Controls phototropism and geotropism.
- Growth Regulation:
- More auxin promotes shoot growth.
- Lower levels favor root growth.
- Xylem Differentiation & Callus Formation: Aids in tissue regeneration.
- Nucleic Acid Activity: Increases cellular metabolic activity.
Types
- Natural Auxins:
- Example: Indole Acetic Acid (IAA)
- Functions: Stimulates growth at the apical stem and root tips.
- Artificial Auxins:
- Indole Butyric Acid (IBA): Enhances root development (used in cuttings; trade names include Serdox, Rapid-root, etc.).
- Naphthalene Acetic Acid (NAA): Prevents flower/fruit drop (used in crops like guava, pomegranate, mango, and tomato).
- 2,4-D: Controls premature fruit drop and acts as a broadleaf herbicide.
- 2,3,5-Triiodobenzoic Acid (TIBA): Increases female flower numbers (e.g., in cucumber and watermelon) and boosts seed production in certain crops.
- Other derivatives (e.g., 2,4,5-trichlorophenoxy acetic acid and 2-Methyl-4-Chloro Phenoxy Acetic Acid) are also used mainly for herbicidal purposes.
Gibberellins
Definition & Discovery
- Discovery: Identified by Kurosawa (1926) from the fungus Gibberella fujikorai (a causal agent of the rice “bakane” disease).
- Key Milestones:
- Naming by Hayashi, Yabuta, and Conway.
- Isolation by Yabuta and Sumiki (1935).
- GA₃ (the major gibberellin) discovered by Best and Finney (1956) and isolated by Macmillan and Suter (1958).
Synthesis & Precursor
- Synthesized in young leaves, roots, immature shoots, xylem, phloem, and even in flowers, seeds, and pollen.
- Precursor: Terpenoid compounds (via the mevalonate pathway).
Functions
- Overcoming Dwarfism: Used to remove genetic or physical dwarfism (e.g., in peas).
- Breaking Dormancy: Example—potato tubers treated with 25 ppm GA₃.
- Enhancing Germination: Improves seed germination (varying doses for papaya, potato, and ornamental plants).
- Fruit & Flower Management:
- Produces seedless fruits (e.g., in guava using 1000–8000 ppm).
- Increases fruit size (e.g., grapes and citrus with 30–50 ppm).
- In cucurbits, high concentrations (1500–2000 ppm) increase the number of male flowers.
- Used for fruit thinning and extending shelf life in certain crops.
Cytokinins (Phytokinins)
Definition & Discovery
- Discovery: First identified by Miller and Skoog (1955) with nomenclature standardized by Leatham (1963).
- Alternate Name: Sometimes called “Juneline hormone.”
Synthesis & Precursor
- Synthesized primarily at root tips.
- Precursor: Adenine or 5 AMP with an isopentenyl side chain.
Types & Examples
- Natural Cytokinins:
- Zeatin (discovered in maize) and isopentenyl adenine (IPA).
- Synthetic Cytokinins:
- Kinetin and Benzyl Adenine (BA).
Functions
- Cell Division: Promotes cell division in tissues.
- Delayed Senescence: Reduces aging and delays leaf yellowing.
- Tissue Culture: Crucial for callus formation in plant tissue culture.
- Dormancy: Helps remove dormancy from seeds and buds.
- Metabolic Effects: Increases oil content in oilseed crops and counteracts apical dominance (acting as an anti-auxin).
Abscisic Acid (ABA)
Definition & Discovery
- Discovery: Identified in cotton by Lue and Carn.
- Alternate Names: Known as the “stress hormone” or “anti-gibberellin hormone.”
Synthesis & Precursor
- Synthesized in the terminal bud.
- Precursor: Mevalonic acid via the sesquiterpenoid pathway.
Functions
- Stomatal Closure: Helps in closing stomata to reduce water loss.
- Dormancy: Increases dormancy in seeds and buds.
- Transpiration: Decreases transpiration rate, which is beneficial in dry farming.
- Disease Resistance: Enhances resistance to diseases.
- Cell Protection: Protects cells from dehydration.
- Additional Uses: Applied in cold treatments and mechanical harvesting improvements.
Cycocel (CCC)
Alternate Names & Active Ingredient
- Also known as Chloromequat or Lihocin.
- Active Ingredient: 2-chloroethyl (trimethyl ammonium chloride).
Functions
- Growth Inhibition: Inhibits excessive leafy growth.
- Root Growth: Promotes better root development.
- Dry Farming: Particularly useful in dry farming conditions.
- Dwarfing: Used for dwarfing in wheat varieties.
- Additional Roles: Acts as a lodging reducer and pigment former.
Ethylene
Definition & Discovery
- Early Work: First described in 1934 as a natural product associated with fruit ripening.
- Further Discovery: Detailed by Berg (1962) and officially included as a plant hormone in 1966.
- Precursor: Methionine.
Forms & Functions
- Forms:
- Liquid Form (Ethephon): 500–1000 ppm used for ripening fruits like banana, dates, plum, citrus, sapota, and mango.
- Gaseous Form: Ethephon gas is used for ripening in crops like sugarcane and grapes.
- Additional Functions:
- Ethrel (150–200 ppm) increases female flower numbers in cucumber and watermelon.
- Enhances flower production in pineapple.
- Stimulates shoot and root growth (triple response).
- Note: Calcium carbide, once used for ripening, is banned in India.
Maleic Hydrazide (MH)
Definition & Discovery
- Discovery: Introduced in 1950.
- Trade Names: MH 30, Sprout Stop, Suckers Out.
- Active Ingredient: 1,2 dihydro-3,6 pyridazinedione.
Functions
- Growth Inhibition: Acts as a growth inhibitor.
- Storage Use: Prevents germination in onions and garlic during storage (1500–2000 ppm).
- Flower Regulation: Increases the number of female flowers in crops like bottle gourd and bitter gourd.
- Regulatory Note: This compound has been banned by the Government of India and is replaced by CIPC (Isopropyl N-3 chlorophenol carbamate).
Silver Nitrate (AgNO₃)
Function
- At concentrations of 200–300 ppm, silver nitrate is used to increase the number of male flowers in cucumber and watermelon.
Dominozide
Details
- Trade Names: Alar, B-nine, Kyalar.
- Active Ingredient: Succinic acid – 2,2 dimethyl hydrazine.
- Function: Acts as a germination inhibitor.
Glyphosate
Details
- Trade Name: Round-up.
- Active Ingredient: Phosphonomethyl glycine.
- Function: Although commonly known as an herbicide, it is used in certain contexts for ripening sugarcane.
Mepiquat Chloride
- Trade Names & Function
- Trade Names: White Gold, Marvel, Chamatkar.
- Functions: Enhances tuber and bulb growth. Improves pod set and fruit set in crops such as groundnut, cotton, and pigeon pea.
