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Sunday, 26 April 2020

Role of PGR's in Horticultural Crops

Plant Hormones and Their Role in Horticultural Crops.

Introduction:
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.

Classes:

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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