Tropic Responses

• Auxin:

  • Plant hormones or growth substances
  • Controls tropisms
  • It is produced by cells at the tip of roots and shoots of plants
• Gravitropism: a response in which a plant grows towards (positive) or away (negative) from gravity.
• Auxins' role in gravitropism:

  • Tend to settle at bottom end of the root.
  • Doesn't make cells at tip of root grow longer; prevents cells at bottom tip from growing, making cells at top of root grow faster.
  • When cells of top of the root grow faster, they push root deeper into soil and root gets longer.
  • Root grows in direction of gravitational pull.
• Phototropism: a response in which a plant grows towards (positive) or away (negative) from the direction from which light is coming.
• Auxins’ role in phototropism:

  • If sun shines on right side of a plant’s shoot, auxins will accumulate on dark opposite left side.
  • Auxins accumulating makes cells on left side grow faster than cells on right side.
  • When left side of shoot starts growing faster than right side, shoot will start to bend to right side towards sunlight.
• Hormones can be used as weed killers: spraying with high concentrations of hormone (2,4-D) upsets normal growth patterns. It affects different species differently so might only kill one species not the other (this is good).