3.9 - Uses of Plant Hormones

Gibberellins and their effects

• Plant hormones are chemicals that control how a plant grows.
• Gibberellins are one type of plant hormone. They stimulate:
  • Seed germination - A seed starting to grow into a plant
  • Stem growth - Through stem elongation, which helps plants grow tall
  • Flowering
• Gibberellins and auxins can work together to have a large effect on growth - for example, helping a plant grow very tall.

Breaking dormancy

• Many seeds will not germinate until they have been through certain conditions, such as a period of cold or of dryness. This state is called dormancy.
• Gibberellins break this dormancy, allowing the seeds to germinate.

Ethene and its effects on plants

• Plant hormones are chemicals that control how a plant grows.
• Ethene is a gas produced by plants, including ageing leaves.
• It has two main effects: causing leaves to fall and ripening fruit.

Leaf fall

• Ethene stimulates the cells that connect a leaf to the rest of the plant to expand.
• This breaks the cell walls and causes the leaf to fall off.
• Auxins have the opposite effect - they inhibit leaf shedding.
• Auxins are made by young leaves, so as a leaf ages it produces less auxin. This loss of auxin lets the leaf fall.

Fruit ripening

Ethene stimulates enzymes inside fruit that cause the fruit to ripen.