auxins and apical dominance

• the apical bud

  The shoot tip at the top of a flowering plant
• apical dominance bud


  Auxins stimulate the growth of the apical bud and inhibit the growth of side shoots from lateral buds.
• Apical dominance prevents side shoots from growing - this saves energy and prevents side shoots from the same plant competing with the shoot tip for light. Because energy isn't being used to grow side shoots, apical dominance allows a plant in an area where there are loads of other plants to grow tall very fast, past the smaller plants, to reach the sunlight.
• If you remove the apical bud then the plant won't produce auxins, so the side shoots will start growing by cell division and cell elongation. However, if you replace the tip with a source of auxin, side shoot development is inhibited. This demonstrates that apical dominance is controlled by auxin
• Auxins become less concentrated as they move away from the apical bud to the rest of the plant. If a plant grows very tall, the bottom of the plant will have a low auxin concentration so side shoots will start to grow near the bottom.
• • auxins can prevent leaf and fruit 'drop and promote flowering
  • but in high concentrations can promote fruit drop as if there are too many small fruits the plants can then produce fewer large fruits
• • rooting powder contains auxins
  • can dip the end of a cutting in it then plant to encourage root growth
• • can treat unpollinated flowers with auxin to promote seedless fruits
  • it promotes ovule growth that triggers automatic production of auxin by tissues in developing fruit which helps complete developmental process
• • artificial auxins can be used as herbicides to kill weeds
  • transported in phloem to all parts of the plant
  • can act in the plant for a long time as they aren't a close fit to the enzymes that break them down
  • promote shoot growth so much that the Sten can't support itself so snaps and dies