Plant Growth Regulators

A hormone is a naturally produced chemical synthesized in one part of

the plant and then travels to another part where it effects growth and

development. There are five main groups:

Auxin or Indole Acetic Acid (IAA)

Gibberellin or Gibberellic Acid (GA)

Cytokinin (Ck)

Ethylene (C2H4)

Abscisic Acid (AbA)

Hormones discovered more recently include salicylic acid, jasmonates,

brassinolides, etc.

Plant growth regulators are man-made chemicals applied to plants to

produce a desired effect (some are chemically identical to hormones).

https://en.wikipedia.org/wiki/Plant_hormone

Phototropism: shoots bend towards light due to auxin migrating to darker side of stem.

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Light source

Gibberellic acid and low light levels cause stretching due to the elongation of internode cells. Plant Growth Regulators © KPU.ca/Hort

Effects of gibberellic acid and low

light on bean seedlings and petunia.

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Auxin regulates apical dominance: pinching/ shearing plants will result in more branches. Plant Growth Regulators © KPU.ca/Hort

Senescence (aging) and dormancy: Ethephon causes plants to release ethylene and senesce faster.

Control

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Control

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Senescent parts due to Ethephon spray abscise faster due to more ethylene gas being released.

Leaf abscission: occurs when auxin levels decrease and ethylene levels increase.

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A separation layer

forms in the abscission

zone where the petiole

joins the stem.

Fruit and flower part drop: ethylene released from apple cause separation layer formation.

Control

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Ethephon can also be used to de-green citrus fruits, and to ripen bananas or tomatoes. Plant Growth Regulators © KPU.ca/Hort

Shoot proliferation in tissue culture: high cytokinin and auxin level promotes shoot growth.

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Effect of cytokinin (e.g. Benzyl Adenine) to Auxin (IAA) levels on African violet in culture.

BA >> IAA

BA << IAA

Synthetic auxin (e.g. 0.1-0.8% IBA) can be used to stimulate adventitious rooting on cuttings.

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Under watered plants are stressed and have stomata closed more often. Why?

Under-watered

Under-

watered

Under-

watered

Plant Growth Regulators © KPU.ca/Hort

Under-

watered

Auxins

Cell enlargement, role in apical dominance

Gibberellins

Cell (internode) elongation, seed germination

Cytokinins

Cell division, delays senescence

Abscisic Acid

Promotes dormancy, stomatal regulation

Ethylene

Fruit ripening, senescence, organ abscission

Summary: Main Effects of Plant Hormones (and related PGR’s)