Response to Stimuli: Plants (Edexcel IGCSE Biology: Double Science)

• Plants need to be able to grow in response to certain stimuli
• For example, plants need to be able to grow in response to light, to ensure their leaves can absorb light for photosynthesis
• They also need to be able to grow in response to gravity, to ensure that shoots grow upwards and roots grow downwards
• The directional growth responses made by plants in response to light and gravity are known as tropisms
• If the growth is towards the stimulus, the tropism is positive and if the growth is away from the stimulus, the tropism is negative

• A response to light is a phototropism and a response to gravity is a geotropism (or gravitropism)
• As shoots grow upwards, away from gravity and towards light (so that leaves are able to absorb sunlight), shoots show a positive phototropic response and a negative geotropic response
• As roots 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), roots show a negative phototropic response and a positive geotropic response

Geotropism and phototropism table

Plant shoots display a positive phototropic response by growing towards a light source

• Plants produce plant growth regulators (similar to hormones in animals) called auxins to coordinate and control directional growth responses such as phototropisms and geotropism
• Auxin is mostly made in the tips of growing shoots and then diffuses down to the region where cell division occurs (just below the tip)
  • This is an important point - only the region behind the tip of a shoot is able to contribute to growth by cell division and cell elongation

• Auxin stimulates the cells in this region to elongate (get larger); the more auxin there is, the faster they will elongate and grow
• If light shines all around the tip, auxin is distributed evenly throughout 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 tip concentrates on the shaded side, 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

Positive phototropism in plant shoots is a result of auxin accumulating on the shaded side of a shoot