Monday, 10 June 2013

Why plants respond to the environment?

Plants responds to a stimuli

  • When a plant responds to a directional stimulus by a growth movement it is known as tropism
  • If growth is towards the stimulus it is known as positive tropism
  • if growth is away from the stimulus it is described negative tropism
Tropisms
  • Light
    • response: phototropism: eg. shoots grow towards a light source (positively phototropic)
  • Gravity
    • Geotropism: e.g. shoots grow away from gravity (negatively geotropic) but roots grow in the direction of gravity (positively geotropic)
  • Chemicals
    • Chemotropisim: e.g. pollen tubes grow down style attracted by chemicals
  • Water
    • hydrotropism: e.g. roots grow towards water (positive hydrotropic)
Hormones
  • Auxins e.g. IAA
    • promote cell elongation
    • inhibit growth of side shoots
    • inhibit lead abscission (leaf fall)
  • Cytokinins
    • promotes cell division
  • Gibberellins
    • promotes seed germination and growth of stems
  • Abscissic acid
    • inhibits seed germinationa nd growth
    • causes stomatal closure when plant is stressed due to low water availibility
  • ethene
    • promotes fruit ripening



Growing regions
  • Growth only happens in particular places in plants called meristems
    • where there a groups of immature cells capable of dividing
  • apical meristems - located at the tip/apex of roots and shoots, lengthening growth
  • lateral bud meristems - found in the buds, could give rise to side shoots
  • lateral meristems - found in a cylinder near the outside of roots and shoots widening growth

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