Tropisms in Plants (WJEC GCSE Biology)

• Plants can respond to changes in the environment (stimuli) for survival, e.g. light, water, gravity
• Their responses are usually much slower than animals
• Plants grow in response to light (phototropism) and gravity (gravitropism or geotropism)
  • The shoots must grow upwards, away from gravity and towards light so that leaves can absorb sunlight – shoots show a positive phototropic response 
  • Roots need to grow downwards into the soil, away from light and towards gravity,  to anchor the plant and absorb water and minerals from the soil particles - roots show a positive gravitropic response 
• Plants respond to stimuli by producing a growth hormone called auxin which controls the direction of growth of roots or stems
• Therefore we say plants control their growth chemically
• Auxin is mostly made in the tips of the growing stems and roots and can diffuse to other parts of the stems or roots; spreading from a high concentration in the shoot tips down the shoot to an area of lower concentration

Phototropism

• In the shoots, auxins promote cell elongation (growth); more auxin = more cell elongation = more growth
• If light shines on a growing shoot from all directions, auxin is distributed evenly throughout the shoot and the cells in the shoot grow at the same rate - this is what normally happens with plants growing outside
• When light shines on the shoot predominantly from one side, the auxin produced in the shoot tip concentrates on the shaded side of the shoot, making the cells on that side elongate and grow faster than the cells on the sunny side
• This unequal growth on either side of the shoot causes the shoot to bend and grow in the direction of the light

Phototropism diagram

Auxin accumulates on the shaded side of a plant causing elongation of cells and growth toward the light source, this is phototropism

Gravitropism

• In the roots auxins inhibit cell elongation (growth); less auxin = less cell elongation = less growth
• Roots need to grow downwards into the soil, away from light and towards gravity, in order to anchor the plant and absorb water and minerals from the soil particles
• In the roots, the lower side grows slower than the upper side (as auxin inhibits cell elongation and growth in roots), so the root grows downwards
• Roots therefore show a positive gravitropic response

Gravitropism diagram

Auxin accumulates on the lower side of a plant inhibiting elongation of cells and growth is then toward the direction of gravity, this is gravitropism