Growth Regulators

Original by Libby AstrachanModified by Georgia Agriculture Education Curriculum Office- July 2004

Modified by Georgia Organics 2008

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 $$$

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 being 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

Growth Regulators• Rooting Compounds –

• increase rooting %• speed rooting• increase number,

quantity of roots

• increase uniformity of roots

Rooting compounds• Rooting Compounds – • Instead of synthetic rooting compounds, try willow water• Strip leaves and terminal stems from present year’s growth of any willow species and soak in water for 24 hours.• Soak cuttings for 24 hours in willow water before sticking in medium• Water with willow water