Growth Regulators Original by Libby Astrachan Modified by Georgia Agriculture Education Curriculum Office- July 2004

Plant Hormones Natural (made by plants) – also called hormones Synthetic (man made) Also called PGRs (plant growth regulators) Purposes: start growth, stop growth, modify growth & development

5 Known Plant Hormones: Auxins (ox ins) Gibberellins (jib ber ill ins) Cytokinins (site oh kine ins) Ethylene (eth el een) Abscisic acid (ab sis ick) Hormones may act individually or together

Auxins Stem elongation Produced in tips of stems (“B” in photo) Migrate from cell to cell in stems

Phototropism – ability to bend towards light Auxins - responsible for plants bending towards light. Auxins - move down shaded side of the stem and cause cells to elongate

Gravitropism (geotropism) – plant response to gravity Auxins – responsible for plant response to gravity Auxins – move to lowest side and cause stem tissue to elongate – stem curves upwards

Apical dominance Auxins – move down the stem from the terminal bud and inhibit growth of side shoots

Pinching Pinching = removing the terminal bud Pinching - stops flow of auxins down the stem and allows side shoots to develop Produces bushy, well- branched crops

Root development Auxins encourage root development in cuttings Some plants produce plenty of auxins to make rooting cuttings easy Other plants need synthetic auxins such as IBA

Gibberellins Cell elongation and cell division Stimulate development of flowers (as in “gibbing” camelias) Cause internodes to stretch Produced in stem and root apical meristems, seed embryos, young leaves

Internode Elongation Gibberellins cause internodes to stretch in relation to light intensity. High light intensity = no stretch Low light intensity = long internodes. Leaves are raised to capture light

Problems with Internode Elongation Greenhouse problem – plants spaced too closely to one another Plants shade one another – results in stretching, less compact plants, weaker stems, loss in value $$$ B-Nine is a growth regulator that inhibits gibberellin and controls plant height in bedding plants

Cytokinins Cell division (used in tissue culture) Cell differentiation (used in tissue culture for plant organ formation) Formation of callus tissue Delay aging process in plants Produced in roots Transported through xylem Still researched

Cytokinins vs. Auxins In stems – auxins inhibit lateral shoots, cytokinins promote lateral shoots In roots – Auxins promote root branching, cytokinins inhibit root branching Work together to control cell differentiation and cell division

Ethylene Gas Colorless gas Produced in nodes of stems, ripening fruits, dying leaves

Ethylene exposure Thickens stems Breaks down chlorophyll Weakens cell membranes Softens cell walls

The Holly and the Ethylene

Abscisic Acid – The Plant Stress Hormone Growth inhibiting hormone Responsible for seed dormancy Responsible for closing stomata during drought

Synthetic Growth Regulators Rooting Compounds – increase rooting % speed rooting increase number and quantity of roots increase uniformity of roots

Rooting compounds Liquid or mixed with talc

Growth Retardants Widely used in the greenhouse industry Inhibit action of gibberellins on stem elongation

Goal of Synthetic Growth Retardants: Compact plants More attractive Greener Easier to transport Flowering NOT affected

Application Guidelines Read label Wear safety gear Apply correct amount at correct time Plant should be well-established Apply uniformly as foliar spray or drench Split into two half-strength applications