PLANT GROWTH REGULATORS- CLASSIFICATION & FUNCTIONS

PLANT GROWTH REGULATORS- CLASSIFICATION & FUNCTIONS

The plant growth regulators are the organic chemical compounds, other than nutrients and vitamins, which modify or regulate physiological processes in an appreciable measure in the plant when used in small concentration. They are known to influence fruit set, retention, yield and quality of horticultural crops.

Contents:

• Introduction
• Growth & Development
• Phytohormones and Plant Growth Regulators
• Classification and Role of PGR
• Methods of Application
• FAQ

Introduction

Plant growth regulators (PGRs) are natural or synthetic compounds that affect developmental or metabolic processes in higher plants, mostly at low dosages. They do not possess a nutritive value. They are used to regulate the growth and yield of plants.

PGRs are employed in horticulture mainly in floriculture, olericulture, pomology and viticulture to obtain specific advantages, such as decreased susceptibility towards biotic and abiotic stress, improved morphological structure, facilitation of harvesting, quantitative and qualitative increases in yield and modification of plant constituents.

Salicylic acid and jasmonic acid induce defence reactions in fruits and vegetables that prevent fungal infection and induce resistance to low temperature injury.

Growth and Development?

Plant growth refers to the quantitative increase in plant body such as increase in the length of stem and root, the number of leaves etc. whereas,

Development refers to the qualitative changes such as germination of seed, formation of leaves, flowers and fruits, falling of leaves and fruits.

The two sets of internal factors i.e., nutrition and hormone control the growth and development of the plant. The raw material required for growth is supplied by nutritional factors which include the minerals, organic substances, protein and carbohydrates etc. Utilization of these substances for proper development of the plant is regulated by certain “chemical messengers” called plant growth substances or plant growth regulators, which in minute amounts increase or decrease or modifies the physiological process in plants.

Plant hormones and Plant growth regulators

Plant hormones or phytohormones are the organic compounds produced by plants which in low concentrations regulate plant physiological process. These usually move within the plants from a site of production to a site of action. It means they synthesized in one part of the plant and translocated to another part, where in very low concentrations causes a physiological response.

The plant hormones are identified as promoter (Auxin, Gibberellins and Cytokinin), inhibitors (Abscisic acid and Ethylene) and other hypothetical growth substance (florigen, flowering hormone etc.)

Growth regulators work similar to phytohormones but synthetic in nature. These are organic compounds which are active at low concentrations (1-10 ml) in promoting, inhibiting or modifying growth and development in plants.

The naturally occurring (endogenous) growth substances are commonly known as plant hormones, while the synthetic ones are called growth regulators.

Classification of Plant Growth Regulators

1. Auxin
2. Gibberellins
3. Cytokinin
4. Ethylene
5. Abscisic Acid

Auxins

• Auxins were discovered by Charles Darwin and Francis Darwin.
• Auxin produced in the apex bud inhibits lateral buds from growing. Auxin is a ‘Greek’ word which means to It is a generic term for chemicals that typically stimulate cell elongation by loosening cell wall but also influence a wide range of growth and development response.
• The chemical isolation and characterization were done by Kogl et al. (1934).
• Auxins are the first identified hormones of which IAA seems to be the major naturally occurring endogenous auxin in plants and crops. Besides IAA, plants contain three other compounds which are structurally similar and elicit any of the same response as that of IAA, 4, Chloro-indole acetic acid (CIAA), Phenyl acetic acid (PAA), Indole butyric acid (IBA).
• Site of Auxin synthesis: Auxins are synthesized in stem tips region and in young tissues and move mainly downward (Basipetal movement) i.e., from shoot tip to root.
• Synthetic compounds are: Indole acids, Naphthalene acids, Chlorophenoxy acid, Picolinic acid, Derivatives.

Role of Auxin:

• Cell division and enlargement: IAA + GA, example – cambial growth in diameter.
• Tissue culture: Shoot multiplications (IBA and BAP), callus growth (2, 4-D), root multiplication IAA and IBA (1-2 mg).
• Breaking dormancy and apical dominance (inhibition of lateral buds):
• Shortening internodes: Apple trees (NAA) dwarf branch fruit.
• Rooting of cutting: (10-1000 ppm-NAA, IAA, Phenyl acetic acid, IBA).
• Prevent lodging: NAA develop woody and erect stem.
• Prevent abscission: premature leaf, fruit and flower fall (NAA, IAA and 2,4-D).
• Parthenocarpic fruit: Grapes, Banana and Orange (IAA).
• Flower initiations: Pineapple uniform flowering and fruit ripening (NAA) and delay flowering (2, 4-D).
• Weed eradication: 2, 4-D.
• Fruit thinning: Application of NAA at post bloom for thinning in Apple.

Gibberellins

• Gibberellin was first recognized in 1926 by a Japanese scientist, Eiichi Kurosawa, studying bakanae, the “foolish seedling” disease in rice.
• Gibberellins are plant hormones that regulate growth and influence various developmental processes, including stem elongation germination, dormancy, flowering, enzyme induction and leaf and fruit senescence.
• It is the active principle isolated from the soil borne fungus Gibberella fujikuroi.
• The concentration of GA3 is usually highest in immature seeds, reaching up to 18 mg/kg fresh weight in Phaseolus species, but it decreases rapidly as the seeds mature.
• In general, roots contain higher amounts of GA3 than shoots.
• Gibberellins have also been found effective in overcoming both kinds of dormancy in buds as well as seeds.

Role of Gibberellins:

• GA: Synthesis in leaf and induce shoot elongation (IAA + GA3), by effecting cell elongation or cell division or both.
• Enhance metabolic activity: Mobilization of reserved food material, promote growth and height, increase root activity and kinetin production in root- translocate to growing bud.
• Shoot elongation: GA3 spray increases height of seedlings.
• Delay senescence: Increase photosynthetic and protein synthesis so decrease abscission.
• Increase cambial growth and differentiation: Induce flower and fruit set (IAA+GA3).
• Dwarf plant (genetically) to normal height:
• Promote flowering in Long Day Plants: Substitute for long day condition and cold treatment (vernalization).
• Induction of parthenocarpy in grapes: Three physiological events: Rachis cell elongation, flower thinning and berry enlargement.
• Breaking dormancy and leaf expansion.

Cytokinin

• Cytokinins were discovered by Skoog, C. Miller and co-workers during the 1950s as factors that promote cell division (Cytokinesis).
• The first cytokinin discovered was an adenine (aminopurine) derivative named kinetin (6-furfuryl- aminopurine), which was isolated as a DNA degradation product.
• Cytokinin produced in root meristems, young leaves, fruits, seeds.
• First endogenous cytokinin was isolated from maize kernels named as
• Germinating seeds, roots, sap streams, developing fruits and tumour tissues are rich in cytokinins.
• Cytokinins imbibed seeds germinate better in dark than unimbibed lettuce seeds. Similarly, cytokinins together with gibberellins effectively breaks the photo-dormancy of Celery (Apium graveolens) seeds.
• Synthetic cytokinins are: Kinetin, Benzyladenine, Ethoxy ethyladenine, Zeatinriboside, Isopentenyladenine, Isopentenyladenosine, 6-benzylaminopurine and Thidiazuron.

Role of Cytokinins:

• Cell division, elongation and enlargement.
• Tissue culture morphogenesis.
• Induction of flowering and fruit development.
• Apical dominance overcoming.
• Breaking dormancy.
• Delay senescence.
• Improves N2 metabolism.

Ethylene

• Ethylene was discovered by Glaston & davis in 1970.
• Ethylene is a gas that forms through the breakdown of methionine, which is in all cells.
• Ethylene (IUPAC name: ethene) is a hydrocarbon which has the formula H₂C=CH₂. Ethylene is produced at a faster rate in rapidly growing and dividing cells, especially in darkness.
• Ethylene is known as the “ripening hormone”. Manipulations of fruits and vegetables are done by either exogenous ethylene or inhibitors of ethylene production.
• Ethylene is the only gaseous hormone which stimulates growth.
• Ethylene is formed naturally in plants in amounts sufficient to bring about regulatory effect and it might be considered as plant hormones.
• Recently a synthetic chemical known as ethrel, ethephon, chloroethyl phosphonic acid (CEPA) has been reported to release ethylene when applied to plants.

Role of Ethylene:

• Breaking dormancy: Ethylene plays a key role in dormancy release in numerous species.
• Induce fruit ripening: Ethral are used in ripening in Banana and Mango and for uniform ripening and early fruit maturity in Apple.
• Induce abscission of leaves: Ethylene promotes abscission and senescence of both leaves and flowers.
• Induce flowering: Ethylene and ethephon used for flowering in pineapple.
• Inhibit elongation and lateral bud growth: Ethylene is involved in various developmental processes and responses to biotic and abiotic stresses in plants.

Abscisic acid (ABA)

• It is a plant growth regulator which plays a variety of important roles throughout a plant’s life cycle. These roles include seed development and dormancy, plant response to environmental stresses and fruit ripening.
• ABA concentration is very low in unripe fruit, but it increases as a fruit ripens, so it is therefore believed that ABA plays an important role in regulating the rate of fruit ripening.
• A plant hormone C₁₅H₂₀O₄that is a sesquiterpene widespread in nature and that typically promotes leaf abscission and dormancy and has an inhibitory effect on cell elongation.
• It is also called stress hormone.
• Abscisic acid (ABA), also known as abscisin II and dormin, is best known as a plant hormone. Abscisic acid owes its names to its role in the abscission of plant leaves.
• In preparation for winter, ABA is produced in terminal buds. It is thermostable but light sensitive.

Role of Abscisic acid (ABA):

• To stop elongation.
• Induce dormancy.
• Delay germination.
• Inhibit growth process.
• Adaptation of plants to various environmental stresses,

Others hormone

Jasmonic acid: Inhibits growth and promote senescence

Brassinosteroids: Same function of auxins and Gibberellins 

Oligosaccharins: Stimulated elongation of cell, regenerated tobacco tissue, inhibit roots and stimulate flowers

Methods of Application of Plant Growth Regulators

Growth regulators can be applied in different ways like:

1. Spraying method.
2. Injection of solution into internal tissues.
3. Root feeding method.
4. Powder form.
5. Dipping of cuttings in solution.
6. Soaking in dilute aqueous solution.

Frequently Asked Questions (FAQ)

The term “Phytohormone” for plant hormones was used by?

Thimann.

What is the difference between Plant hormones and Plant growth regulators?

The naturally occurring (endogenous) growth substances are commonly known as plant hormones, while the synthetic ones are called growth regulators.

Plant hormones are identified as promoter?

Auxin, Gibberellins and Cytokinin.

Plant hormones identified as inhibitors?

Abscisic acid and Ethylene.

The major naturally occurring endogenous auxin is?

IAA.

First endogenous cytokinin was isolated from maize kernels known as?

Zeatin.