• Tropism

    • A tropism is the response of a plant to a directional stimulus (a stimulus coming from a particular direction)
    • Plants respond to stimuli by regulating their growth
    • A positive tropism is a growth towards a stimulus
    • Negative tropism is a growth away from a stimulus

    Phototropism

    • Phototropism is the growth of a plant in response to light
    • Shoots are positively phototropic and grow towards the light
    • Roots are negatively phototropic and grow away from light

    Gravitropism

    • Gravitropism is the growth of a plant in response to gravity
    • Shoots are negatively gravitropic and grow upwards
    • Roots are positively gravitropic and grown downwards

    Auxin

    • Plants respond to directional stimulus using hormonal growth factors which speed/slow growth
    • Plant growth factors are produced only in the tips of the roots and shoots where they can be moved to other parts of the plant
    • Auxin is produced in the tips of shoots and diffuses backwards to stimulate the cells just behind the tips to elongate. The cell walls because loose and stretchy so cells become longer
    • If the tips shoots are removed, no auxin will be produced and therefore shoot will stop growing

    Indoleacetic Acid (IAA)

    • Important auxin produced in tips of shoots and roots
    • Moves around plant to control tropisms
      • Moves via diffusion and active transport over short distances
      • Moves via phloem for long distances
    • Results in different parts having different concentrations of IAA
    • It is the different concentrations which result in uneven growth

    IAA on Phototropism

    • IAA moves to more shaded parts of the shoots and roots

    Shoots:

    • IAA concentration increases on the shaded side
    • Cells elongate and the shoot bends towards the light

    Roots:

    • IAA concentration increases on shaded side
    • Growth is inhibited so the root bends away from the light

    IAA on Gravitropism

    • IAA moves to the underside of shoots and roots

    Shoots:

    • IAA concentration increases on lower side
    • Cells elongate so the shoot grows upwards

    Roots:

    • IAA concentration increases on the lower side
    • Growth is inhibited so the root grows downward