Plant Growth Regulators Overview

Study Notes

Plant Growth Regulators (PGRs)

Auxin: Promotes cell elongation, root initiation in stem cuttings, and apical dominance.
Gibberellins:
- Stimulate stem growth through cell elongation and division.
- Overcome seed and bud dormancy.
- Cause internodes to lengthen in response to light intensity.
- Commercially used for seed germination and seedling growth.
- Stimulate flower development.
- Increase fruit size in seedless grapes (parthenocarpy).
- Delay senescence by increasing photosynthesis and protein synthesis, reducing leaf abscission.
- Increase cambial growth and differentiation.
- Break dormancy and promote leaf expansion.
Cytokinins:
- Promote cell division (cytokinesis).
- Contribute to cell elongation in leaves.
- Stimulate cell differentiation in combination with auxins.
Ethylene:
- Inhibits root and shoot initiation by blocking auxin transport from apical meristems.
- Stimulates leaf curling (epinasty).
- Stimulates fruit ripening (known as the "ripening hormone").
- Promotes flowering in mango, pineapple, and some ornamentals.
- Accelerates leaf abscission due to the formation of an abscission layer.
- Deterines sex expression in monoecious plants (plants with both male and female flowers).
Abscisic Acid (ABA):
- Closure of Stomata: During drought, ABA causes stomata to close, conserving water.
- Delays Seed Dormancy: ABA delays seed germination, controls bud dormancy, and counteracts the effects of other hormones.
- Inhibits IAA-stimulated cell growth.
- Inhibits gibberellin-induced amylase production, further delaying seed germination.
- Induces chlorosis and senescence, unlike cytokinins which promote leaf rejuvenation and delay senescence.

Fungicides

Based on Time of Application: - -
Based on Movement: - -
Based on Use:
- Foliar-Applied Fungicides: Glyodin, Fenopanil, Prochloraz, Imazalil, Carbendazim, Benomyl, Cypendazole, Thiophanate-methyl, Fluotrimazole, Triadimenol, Bibertanol, Diclobutrazol, Triazbutil control a variety of fungal pathogens.
- Seed Treatment Fungicides: - -

Herbicides

Based on Mode of Action:
- Seedling Growth Inhibitor: -

Insecticides

Chitin Synthesis Inhibitors: - Inhibit reproduction and development of chitin-synthesizing organisms.
Avermectins: - -
Spinosyns: Derived from Saccharopolyspora spinosa, spinosyns are a group of compounds including spinosyn A and spinosyn D. Spinosyn A is effective against caterpillars, leaf miners, thrips, and foliage-feeding beetles.
Botanical Insecticides: Aromatic plants with pesticidal properties
Microbial Insecticides: Different species and strains of Bacillus bacteria target different insect pests. Bacillus thuringiensis var. kurstaki (Dipel, Javelin) effectively controls caterpillars of moths and butterflies.