Section 1

Plant Hormones

• Plant hormones are formed in many plant parts and regulate many aspects of growth and development. There are five major groups of plant hormones: auxins, gibberellins, ethylene, cytokinins, and abscisic acid.

• Hormonal responses often have adaptive advantages.

• Synthetic auxins are used for killing weeds, stimulating root formation, and stimulating or preventing fruit drop.

• Gibberellins are used to increase the size of fruit, to stimulate seed germination, and to brew beer.

• Ethylene is used to ripen fruit and promote abscission.

• Cytokinins are used to culture plant tissues in the lab and to promote lateral bud growth of flower crops.

• Abscisic acid promotes dormancy in plant buds, maintains dormancy in seeds, and causes stomata to close.

• Growth retardants are widely used to reduce plant height.

Vocabulary plant hormone (p. 631) growth regulator (p. 631) auxin (p. 631) indoleacetic acid (p. 631)

naphthalene acetic acid

apical dominance (p. 633) 2,4-D (p. 633)

Agent Orange (p. 633) gibberellin (p. 633) ethylene (p. 634) ethephon (p. 634)

abscission (p. 634) cytokinin (p. 635) abscisic acid (p. 635) growth retardant (p. 635)

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