Growth Regulators Original by Libby Astrachan Modified by

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, wellbranched 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