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Sunday, 26 April 2020

Role of PGR's in Horticultural Crops

Plant Hormones and Their Role in Horticultural Crops.

Planthormones or phytohormones are signal molecules produced within plant in extremely low concentrations. Plant hormones control all aspects of plant growth and development from embrayogenesis to reproductive development.


Different hormones are sorted into different classes based on their chemical structures. Initial research into plant hormones identified five major classes namely.

• Auxin
• Gibberellin
• Cytokinin
• Abscisic acid &
• Ethylene

This list was later expanded and following hormones where added to the major hormones class:

• Brassinosteroids
• Jasmonates
• Salicylic acid &
• Strigalonates 

Role of PGR's in Horticultural Crops:

1. Auxin:

    ♠ Auxins are produced in meristimatic              tissue such as root tips, shoot tips,                    apical buds, young leaves and flowers.
    ♠ The name auxin was given by Kogl and      bioassay for auxin is Avena curvature            test, oat coleoptile test.

Precursor - Tryptophan

Types of Auxins

⚜ Naturally occuring auxin - Indole-3-acetic      acid (IAA)
⚜ Synthetic auxins - IBA, NAA , 2, 4-D                  (systemic herbicide)

Role of Auxin in Plants:

⚜ Promotes apical dominance.
⚜ Apical dominance: The main central stem of plant is dominant over other stems.
⚜ Inhibition of lateral shoot growth.
⚜ Phototropism- the bending of growing point of plant towards light.
⚜ Rooting hormone to induce rooting in cuttings
⚜ Stimulate cell elongation
⚜ Promotes or inhibits growth of adventitious roots.
⚜ High amount of auxin induce root formation in callus.

2.Gibberellins (GA):

    ♠ Gibberellins are produced in young            leaves.
    ♠ They were first discovered by Japanese     researchers (Eiichi Kursowa) when he            noticed a chemical produced by fungus          called Gibberella fujikuroi that produced      abnormal growth in rice plants.
Gibberellin structure

Precursor - Terpenoids

Role of Gibberellin in Plants:

⚜ Stimulate the stem elongation, cell division (internode elongation)
⚜ Promotes the seed germination (breaking seed dormancy)
⚜ Tolerance to chilling.
⚜ Induction of flowering in long day plants.
⚜ Breaking dormancy (seed, tuber and shoot)
⚜ Induces seedlessness in fruit crops.
⚜ GA stimulates bolting in rosette plants.
⚜ Prevention of genetic and physiological dwarfism in plants.

3. Cytokinin:

     ♠ Cytokinin occur in embrayonic or                    meristimatic organs.
     ♠ Natural cytokinin - Isopentyl adenine            (IPA), and zeatin (Z)
     ♠ Synthetic cytokinin - Kinetin, BA                     (Benzyl adenine)
     ♠ Cytokinins are produced in Root tips.
Cytokinin structure

Precursor - 5' AMP (isopentenyl group)

Role of Cytokinin in Plants:

⚜ Delaying senescence ( Richmond long effect)
⚜ Enlargement of cells.
⚜ Stimulate cell division and lateral bud development.
⚜ A high cytokinin-auxin ratio (i.e, low amounts of auxin, especially IAA) promotes lateral bud development.
⚜ Diffrentiation of cells (interact with auxins)
⚜ Induces the flowering in short day plants.
⚜ Cytokinins producing large amounts of callus tissue.

Control of morphogenesis:

◼ Callus + low (cytokinin:auxin) - Callus grows well, forms roots

◼ Callus + high (cytokinin:auxin) - Callus grows well, forms meristem & shoots.

4. Abscisic acid (ABA):

     ♠ Naturally occuring plant hormone.
     ♠ Stress hormone 
     ♠ Produced in Terminal bud

Precursor - Mevalonic acid

Role of ABA in Plants:

⚜ Bud dormancy.
⚜ Stimulates the closure of stomata.
⚜ Induction and maintainence of dormancy.
⚜ Disease resistance.
⚜ Protecting cells from dehydration.

5. Ethylene:

     ♠ Gaseous hormone.
     ♠ Ripening hormone.
     ♠ Ethylene is produced at a faster rate in          rapidly growing and dividing cells,                  especially in darkness.
     ♠ It is produced highly in mature fruits              resulting to its ripening.
Ethylene structure

Precursor - Methionine

Role of Ethylene in Plants:

⚜ Stimulates leaf and fruit abscission.
⚜ Induction of flower e.g. Pineapple.
⚜ Stimulates shoot and root growth and diffrentiation (triple response)
⚜ Initiation of fruit ripening e.g. Citrus, Banana and Tomato.
⚜ Stimulates flower and leaf senescence.

Thank you.
Green Doctor™

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