Wednesday, September 9, 2020

Plant Growth and Development - Notes | Class 11 | Part 3: Plant Hormones

15. PLANT GROWTH AND DEVELOPMENT

PLANT GROWTH REGULATORS (PLANT HORMONES OR PHYTOHORMONES)

Plant growth regulators (PGRs) are small, simple molecules that regulate plant growth.

Based on the functions, PGRs are 2 groups:
  • Plant growth promoters: For growth promoting activities like cell division & enlargement, tropic growth, pattern formation, flowering, fruiting & seed formation. E.g. auxins, gibberellins and cytokinins.
  • Plant growth inhibiters: For growth inhibiting activities like dormancy & abscission. Respond to wounds & stresses of biotic and abiotic origin. E.g. abscisic acid & ethylene. (Ethylene fits either of the groups, but it is largely a growth inhibitor).

1. Auxins


Charles Darwin & his son Francis Darwin observed that the coleoptiles of canary grass responded to unilateral illumination by growing towards the light source (phototropism). It was concluded that the tip of coleoptile caused the bending of the entire coleoptile.


F.W. Went isolated Auxin (Greek ‘auxein’: to grow) from tips of coleoptiles of oat seedlings.

Auxin was first isolated from human urine.

Auxins are produced by the growing apices of stems & roots, from where they migrate to regions of their action.

Types of Auxins:
  • Natural: E.g. Indole-3-acetic acid (IAA) and indole butyric acid (IBA). They are isolated from plants.
  • Synthetic: E.g. NAA (naphthalene acetic acid) and 2, 4-D (2, 4-dichlorophenoxyacetic).
Functions of auxins:
  • Initiate rooting in stem cuttings for plant propagation.
  • Promote flowering. E.g. in pineapples.
  • Prevent fruit and leaf drop at early stages.
  • Promote the abscission of older leaves and fruits.
  • Induce parthenocarpy. E.g., in tomatoes.
  • Used as herbicides. E.g. 2, 4-D is used to kill dicot weeds. It does not affect mature monocot plants. It is used to prepare weed-free lawns.
  • Controls xylem differentiation and helps in cell division.
In higher plants, the growing apical bud inhibits the growth of lateral (axillary) buds. It is known as apical dominance. Removal of shoot tips (decapitation) results in the growth of lateral buds. It is applied in tea plantations & hedge-making.

2. Gibberellins


These are acidic PGR.

E. Kurosawa treated the sterile filtrates of Gibberalla fujikuroi (a fungus that causes ‘bakane’ disease or foolish seedling in rice) to healthy rice seedlings. As a result, it showed the symptoms of ‘bakane’ disease. Later, the active substances were identified as gibberellic acid.

There are more than 100 gibberellins (GA1, GA2, GA3 and so on) in fungi and higher plants.

Gibberellic acid (GA3 or Terpenes) is one of the first discovered and most intensively studied gibberellins.

Functions:
  • They cause an increase in length of axis. So they are used to increase the length of grapes stalks.
  • To elongate and improve the shape of fruits such as apple.
  • They delay senescence. So the fruits can be left on the tree to extend the market period.
  • GA3 is used to speed up malting process in brewing industry.
  • Sugarcane stores sugar in stems. Spraying sugarcane crop with gibberellins increases the length of the stem. It increases the yield by as much as 20 tonnes per acre.
  • Spraying juvenile conifers with GAs hastens the maturity period. It leads to early seed production.
  • For bolting (internode elongation just prior to flowering) in beet, cabbages and many plants with rosette habit.

3. Cytokinins


F. Skoog and co-workers observed that from the internodal segments of tobacco stems, the callus (a mass of undifferentiated cells) proliferated only if the nutrients medium was supplemented with extracts of vascular tissues, yeast extract, coconut milk or DNA.

Skoog & Miller later identified and crystallized the active substance and termed as kinetin.

Cytokinins were discovered as kinetin (N6-furfurylamino purine - an Adenine derivative) from the autoclaved herring sperm DNA.

Kinetin does not occur naturally in plants.

Zeatin (from corn-kernels and coconut milk) is the natural substances with cytokinin-like activities.

There are some synthetic compounds with cell division promoting activity.

Natural cytokinins are synthesized in regions of rapid cell division (root apices, shoot buds, young fruits etc).

Functions:
  • Play a role in cytokinesis.
  • Help to produce new leaves, chloroplasts in leaves, lateral shoot growth and adventitious shoot formation.
  • Help overcome the apical dominance.
  • Promote nutrient mobilization which helps in the delay of leaf senescence.

4. Ethylene (C2H4)


Cousins confirmed that ripened oranges released a volatile substance that hastened the ripening of stored bananas. Later this substance was identified as ethylene.

Ethylene is a simple gaseous PGR.

It is synthesized in large amounts by tissues undergoing senescence and ripening fruits.

Functions:
  • Influences horizontal growth of seedlings, swelling of the axis and apical hook formation in dicot seedlings.
  • Promotes senescence and abscission of plant organs especially of leaves and flowers.
  • Promotes fruit ripening. It enhances respiration rate during fruit ripening. This is called respiratory climactic.
  • Breaks seed and bud dormancy, initiates germination in peanut seeds, sprouting of potato tubers.
  • Promotes rapid internode/petiole elongation in deep water rice plants. It helps leaves/upper parts of the shoot to remain above water.
  • Promotes root growth and root hair formation. It increases absorption surface.
  • Used to initiate flowering and for synchronising fruit-set in pineapples. It also induces flowering in mango.
  • It is widely used in agriculture.
The most widely used source of ethylene is ethephon. Ethephon in an aqueous solution is readily absorbed and transported within the plant and releases ethylene slowly. Ethephon hastens fruit ripening in tomatoes & apples and accelerates abscission in flowers and fruits (thinning of cotton, cherry, walnut). It promotes female flowers in cucumbers thereby increasing the yield.

5. Abscisic acid (ABA)


During mid-1960s, it was reported 3 kinds of inhibitors: inhibitor-B, abscisin II & dormin. They were chemically identical and now known as abscisic acid.

ABA is the derivatives of carotenoids.

It regulates abscission and dormancy.

Functions:
  • Inhibitor of plant growth and metabolism.
  • Inhibits seed germination.
  • Stimulates the closure of stomata in the epidermis.
  • Increases the tolerance of plants to various kinds of stresses. Therefore, it is also called the stress hormone.
  • For seed development, maturation & dormancy (it helps to withstand desiccation and other unfavourable factors).
Interactions of PGRs 

PGRs play individualistic or synergistic role. Such roles may be complimentary or antagonistic.

PGRs interact to affect dormancy in seeds/ buds, abscission, flowering, senescence, vernalisation, apical dominance, seed germination, plant movements etc.

In most situations, ABA acts as an antagonist to GAs.

Factors influencing the action of PGR
  • Intrinsic factor: Genomic control.
  • Extrinsic factors: Light and Temperature.

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