B5__Homeostasis-and-Response

Tropisms

Plants need hormones to coordinate and control growth. They are needed for tropisms. This includes phototropism, geotropism, gravitropism.

Phototropism

Plants show positive phototropism as the grow towards the light source.

  1. Plant is exposed to light on one side.
  2. Auxin (hormone) moves to the shaded side.
  3. Auxin stimulates cells to grow more (Cell elongation).
  4. This leads the shoot to bend towards the light.
  5. The plant receives more light allowing a greater rate of photosynthesis.

Gravitropism

Most shoots show negative gravitropism growing away from gravity (growing up away from floor).

  1. Plant is horizontal.
  2. Auxin moves to the lower side.
  3. Auxin stimulates cells to grow more (Cell elongation).
  4. This makes the shoot bend and grow away from the ground.
  5. This is beneficial as light levels are likely to be higher further away from the ground. Most roots show positive gravitropism growing towards the floor.
  6. Root is horizontal.
  7. Auxin moves to the lower side.
  8. Auxin stimulates root cells to grow LESS (Inhibits cell elongation).
  9. This makes the shoot bend and grow away towards the ground.
  10. This is beneficial as water and nutrient levels are likely to be higher closer away from the ground. It also increases the plant’s stability.

Uses of Plant Hormones

Auxin

  1. Weed Killers
    1. Many weeds are broad-leaved.
    2. Weedkillers contain auxin have been synthesised to only affect broad-leaved plants/
    3. The auxin causes too much growth too rapidly resulting in the weed dying.
  2. Rooting Powders
    1. Plants with desirable features are Cloned with cuttings.
    2. Rooting powder contain auxin is applied to the cutting promoting higher growth rates.
  3. Promote Growth in Tissue Culture
    1. Cells from plants are placed in a growth medium.
    2. Growth hormones such as auxin are added.

Ethene

  1. Fruit is picked when it is not ripe.
  2. It is firm during transport so it gets less damaged.
  3. When it is needed to be sold it is exposed to ethene and warmer temperatures.
  4. Ethene is involved in controlling cell division and stimulates enzymes that result in fruit ripening.
  5. This reduces waste.

Gibberellins

  1. Ending Seed Dormancy
    1. In the brewing industry the germination rate of barley seeds is increased to make malt.
  2. Promote Flowering
    1. Instead of requiring certain conditions such as longer days an low temperatures. Applying gibberellins allows it to flower in any conditions and with bigger flowers.
  3. Increased Fruit Size
    1. The seeds in fruit produce gibberellins to increase fruit size.
    2. This means that seedless fruit are generally smaller.
    3. But they can be sprayed with gibberellins to compensate for this.