Plant Hormones

Overview of Plant Hormones

• Plant hormones guide plant growth and development.

Functions of Plant Hormones

• Plant hormones act as chemical messengers:

  • Produced in one part of the organism.

  • Stimulate or suppress activity in other parts.

Types of Plant Hormones

Gibberellins

• Gibberellins promote:

  • Ending of seed dormancy.

  • Stem elongation.

  • Germination and flowering.

  • Cause dramatic increases in plant size.

Ethylene

• Ethylene is important for:

  • Ripening of fruits, often applied to fruits picked before ripening.

  • Fruits are sprayed with ethylene to ensure ripening upon reaching their destination.

Cytokinins

• Cytokinins are involved in:

  • Stimulating cytokinesis (final stage in cell division).

  • Produced in growing roots, seeds, and fruits.

  • Growth of side branches.

Auxins

• Auxins affect:

  • Lengthening of plant cells in the growing tips.

  • Growth of primary stems.

  • Control certain forms of tropism (response to environmental stimuli).

Types of Tropisms Controlled by Plant Hormones

Phototropism

• Phototropism is the growth of plants toward light:

  • Auxins accumulate on the shaded side of the stem, leading to:

    • Lengthening of cells on that side.

    • Bending of the stem toward the light source.

Thigmotropism

• Thigmotropism is a plant's response to touch-like stimuli:

  • Seen in climbing plants and vines that grow in the direction of constant wind.

Gravitropism

• Gravitropism is how plants respond to gravity:

  • Positive gravitropism: downward growth (roots).

  • Negative gravitropism: upward growth (shoots).

Photoperiodism

• Photoperiodism is a response to seasonal changes in day length:

  • Triggers flowering in certain plants.

  • Influences fall coloration and winter dormancy in deciduous trees.

PLANT HORMONES

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