How auxin, gibberellin, cytokinin differ in function

While all three are "growth" hormones, they have very different primary jobs. The easiest way to think of it is: Auxin promotes vertical growth, Cytokinin promotes lateral growth, and Gibberellin promotes "dramatic" growth.

 Auxin

• Primary Function: Cell elongation and apical dominance.
• What it does: Auxin is produced at the very tip of the main stem (the apical bud). It flows downward and signals the cells below to get longer, making the plant grow taller.
• Apical Dominance: This downward flow of auxin inhibits the growth of side buds (lateral buds). This is why many conifer trees grow in a "Christmas tree" shape—one single, dominant leader. If you cut off the top bud, the auxin stops, and the side branches start to grow out.
• Motto: "Grow up!"

 Cytokinin

• Primary Function: Cell division (cytokinesis) and lateral bud growth.
• What it does: Cytokinins are produced mainly in the roots and travel up. They promote the growth of lateral (side) buds, making the plant "bushier."
• The Auxin:Cytokinin Ratio: These two hormones work in opposition.
  • High Auxin / Low Cytokinin: Plant grows tall and skinny (strong apical dominance).
  • Low Auxin / High Cytokinin: Plant grows short and bushy (weak apical dominance).
• Motto: "Branch out!"

Gibberellin (GA)

• Primary Function:Stem elongation, seed germination, and flowering.
• What it does: While auxin causes some elongation, gibberellin causes dramatic elongation.
• Seed Germination: Gibberellin is the signal that "wakes up" a dormant seed. It tells the embryo to start growing and break through the seed coat.
• Bolting: When a plant like lettuce or cabbage suddenly sends up a tall flower stalk, this is caused by a surge of gibberellin. It's also used commercially to make table grapes larger and stems longer.
• Motto: "Wake up and stretch!"