Plant Growth and Development

Phytohormones


Phytohormones

  • Phytohormones (or) plant growth regulators are of 2 types (i) Plant growth promoters, (ii) Plant growth inhibitors
  • Auxins, gibberellins and cytokinins are promoters, Ethylene and Abscisic acid (ABC) are plant growth inhibitors.
  • Auxins are weakly acidic hormones that are capable of promoting cell elongation (10ppm - stem ; 0.0001 ppm - root).
  • At higher concentrations, auxin inhibits growth.
  • Auxin a, b and hetero auxin are the three chemicals isolated from human urine by kogl and smith [especially when a human is affected by pellagra].
  • IAA (Indole - 3 - acetic acid) is natural universal auxin synthesized in shoot apices. leaf primordia and developing seeds from an amino acid tryptophan (Pre-cursor).
  • 2:4 - dichlorophenoxy acetic acid, 2:4:5-Trichlorophenoxy acetic acid, IBA (Indole - 3 - Butyric acid), NAA (Naphthalene acetic acid) are synthetic auxins. [IBA is natural as well as synthetic auxin].
  • Synthetic auxins show apolar transport but natural auxins undergo polar movement ie. It is basipetal in stem; Acropetal in root.
  • TIBA [2, 3, 5 - Tri iodo - benzoic acid], 4 - chlorophenoxy isobutyric acid are anti - auxins.
  • From Avena curvature test, it is observed that 10° curvature is produced by the auxin concentration of 150 μg/litre at 25°C and 90% humidity.

Important functions :

  • Respiaration & metabolism
    Inhibition of abscission
    Cell enlargement and division
    seedless fruits production
    Canbial activity
    defoliation by 2 : 4 - D
    root formation
    Induction of fruiting by 2 : 4 -D & NAA
    Xylen differentiation
    2 : 4 -D; 2 : 4 : 5 T as weedicides.
  • Gibberellins having gibbane ring structures, whose effect had been known in Japan where certain rice plants were found to suffer from foolish seedling (bakanae) disease and such affected plants were found to be longer by 50% than the healthy plants and were without seeds.
  • The disease is caused by an ascomycete, named as Gibberella fujikori.
  • Gibberellin induces the enzyme production such as amylase, protease, lipases and ribonucleases
  • Two common methods of gibberellin bioassay are - (i) shoot elongation in some dwarf mutants and (ii) Barley endosperm test.
  • Bolting , Dormany break, seed germination, Fruit development, Flowering, vernalization, sex expression are important functions of Gibberellins.
  • It can be also used in Parthenocarpy, delayed ripening, Early maturation of seeds.
  • Cytokinins are basic in nature that promote cytokinesis either alone or in combination with auxin.
  • It was discovered as kinetin from the DNA of herrings sperm, which does not occur naturally in plants.
  • Roots seem to be the major source of cytokinin synthesis which pass upwardly through stem's xylem.
  • Promotion of cell division in tabacco pith culture, expansion of excised radish cotyledons, delay in senescence of leaves are the experiments to check the power of cytokinin.
  • Cell division, elongation, Morphogenesis, apical dominance, seed dormancy, Resistance to high temperature, phloem transport, accumulation of salts, Flowering are certain functions of cytokinins.
  • Cytokinin is used in tissue culture experiments, increment in shely life, creating resistance to pathogens and to overcome sensecence.
  • Ethylene (CH2 = CH2) is a gaseous hormone that stimulates transverse or isodiametric growth but retards the longitudinal growth, which is produced from methionine in plants.
  • Maximum production of ethylene occurs during senescence and Climacteric ripening of fruits and also due to excess auxins.
  • The main functions of Ethylene are listed below :
    (i) decreasing the sensitivity to gravity
    (ii) Hasters the senescence
    (iii) Dormancy breakage
    (iv) Growth of rice seedling
    (v) Root initiation
    (vi) Flowering and fruit ripening.
  • Thinning of flowers and young fruits, fruit ripening, inducing feminising effect, sprouting of storage organs are the uses of Ehtylene.
  • ABA (Abscisic acid) is also called stress hormone as its production is stimulated by drought , water logging and other adverse conditions.
  • ABA is mildly acidic that acts as a growth inhibitor by counteracting other hormones, where in most causes, ABA acts as an antagonist to GAS.
  • Liverworts and thallophytes do not possess ABA, instead they have another inhibitor called lunularic acid.
  • ABA functions :
    (i) Bud and seed dormancy
    (ii) stoppage of cambial activity
    (iii) Prevention of transpiration
    (iv) Flowering
    (v) Resistance to cold
  • ABA uses :
    (i) Flowering and rooting
    (ii) Prolonging dormancy

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