Physiological effects of plant growth regulators
- IAA and indole butyric acid (IBA) have been isolated from plants and NAA (naphthalene acetic acid) and 2, 4-D (2, 4-dichlorophenoxyacetic) are synthetic auxins.
- They help to initiate rooting in stem cuttings, promote flowering, prevent fruit and leaf drop at early stages, induce parthenocarpy (example- tomato).
- In most higher plants, the growing apical bud inhibits the growth of the lateral (axillary) buds, a phenomenon called apical dominance.
- Auxins are widely used as herbicides.
Example- 2, 4-D is widely used to kill dicotyledonous weeds.
- Gibberellins are denoted as GA1, GA2, GA3.
- All gibberellins are acidic.
- They help to increase in length of axis, elongate and improve shape of fruits, delay senescence,
- Spraying juvenile conifers with GAs hastens the maturity period, thus leading to early seed production.
- Gibberellins also promotes bolting, defined as internode elongation just prior to flowering, in beet, cabbages.
- Cytokinins were discovered as kinetin from the autoclaved herring sperm DNA.
- Natural cytokinins are synthesised in regions where rapid cell division occurs, for example, root apices.
- Cytokinin helps to produce new leaves, chloroplasts in leaves, lateral shoot growth and adventitious shoot formation.
- Cytokinins help overcome the apical dominance and promote nutrient mobilisation which helps in the delay of leaf senescence.
- Ethylene is a simple gaseous plant growth regulator synthesized by tissues undergoing senescence and ripening fruits.
- Influences of ethylene on plants include horizontal growth of seedlings, swelling of the axis and apical hook formation in dicot seedlings.
- Ethylene promotes senescence and abscission of plant organs, fruit ripening, enhances the respiration rate during ripening of the fruits
- The rise in rate of respiration is called respiratory climactic.
- Ethylene breaks seed and bud dormancy, initiates germination in peanut seeds, sprouting of potato tubers, promotes rapid internode/petiole elongation in deep water rice plants.
- Ethylene also promotes root growth and root hair formation, thus helping the plants to increase their absorption surface.
- The most widely used compound as source of ethylene is ethephon, which hastens fruit ripening and accelerates abscission in flowers and fruits and promotes female flowers in cucumbers.
- Abscisic acid (ABA) was discovered for its role in regulating abscission and dormancy.
- ABA inhibits seed germination, stimulates the closure of stomata, promote seed development, maturation and dormancy.
- ABA increases the tolerance of plants to various kinds of stresses, that is why it is also called the stress hormone.
- ABA acts as an antagonist to gibberellins.
Fig. Apical dominance