Responses in Plants

Just like animals, plants also need to respond to their environment to survive. If their shoots didn’t grow upwards, towards the sun and their roots didn’t grow downwards into the earth then they wouldn’t do a very good job at photosynthesis and they’d be in a bit of a pickle..

 
 

Tropisms

Plant shoots grow towards sunlight and away from gravity

Plant shoots grow towards sunlight and away from gravity

Tropisms are responses by an organisms in a particular direction as a result of an external stimulus. Plants show phototropism (growth responses to sunlight) and geotropism (growth responses to the force of gravity). These tropisms are a result of growth factors which accumulate in different parts of the plant.

Phototropism is a directional response to sunlight. Plant shoots show positive phototropism and grow towards the sun. This maximises the amount of light they can absorb for photosynthesis. Roots show negative phototropism and grow away from the sun. This ensures that the roots bury themselves deeper within the soil, so they can absorb more water for photosynthesis.

Geotropism is a directional response to gravity. Plant shoots show negative geotropism and grow away from the force of gravity. This ensures that plants grow upwards and means that more light is absorbed for photosynthesis. Roots show positive geotropism and grow towards the force of gravity. Again, this ensures that roots bury themselves deeper within the soil, so they can anchor the plant and absorb more water for photosynthesis.

Growth factors

Plants produce different growth factors which enable them to respond to their environment. Growth factors are produced in regions of the plant that are actively dividing (i.e. the shoots and leaves) and diffuse to other parts of the plant where they are needed. Auxins promote cell elongation in shoots but have the opposite effect in roots (auxins accumulate within roots in high concentrations which inhibits growth).

One of the most important groups of growth factors are auxins such as indoleacetic acid, which is involved in both phototropism and geotropism. There are other growth factors which also play important roles in plant growth:

  • Gibberellins stimulate seed germination and flowering.

  • Abscisic acid (ABA) helps plants respond to environmental stress and is involved in stomatal closure.

  • Cytokinins stimulate cell division and cell differentiation.

  • Ethene stimulates flowering and fruit ripening.

Indoleacetic acid (IAA)

Indoleacetic acid (IAA) is a type of auxin which allows plants to response to light (phototropism) and gravity (geotropism). It works by entering the nucleus of plant cells and binds to the promoter regions of DNA. It then acts as a transcription factor, activating or inhibiting the transcription of genes which code for proteins involved in cell elongation and growth. IAA is transported around the plant in the phloem then moves shorter distances within the plant (from cell to cell) by diffusion and active transport.

In shoots, IAA accumulates on the shaded part of the shoot where it activates genes involved in cell elongation. The activated genes are transcribed into proteins which make the cell walls looser and more stretchy, causing the cells to become longer. The cells on the shaded side of the stem are longer than those on the sunny side, causing the shoot to bend towards the sun.

 
 

In roots, IAA also accumulates on the more shaded side but this time inhibits the growth of cells. This means more cells are produced on the non-shaded side of the root, causing the root to bend away from the sun.

IAA also regulates geotropism by accumulating on the underside of shoots and roots. In shoots, IAA causes cell elongation on the underside of the shoot, causing it to bend upwards, away from the force of gravity. In roots, IAA inhibits cell growth, causing roots to grow downwards, towards the force of gravity.

Phytochromes

Plants don’t just respond to sunlight and gravity - they can also tell whether it is daytime or nighttime and even what season it is using special photoreceptors (receptors which detect light) called phytochromes. Phytochromes are found in the leaves, seeds, roots and stem of plants and exist in one of two states. The PR state absorbs light with a wavelength of 660 nm (the red part of the visible spectrum) and the PFR state absorbs light with a wavelength of 730 nm (the far-red part of the visible spectrum).

When PR absorbs red light, it is quickly converted into the PFR state and when PFR absorbs far-red light, it is quickly converted into the PR state. PFR also turns into PR when there is no light (i.e. during the night).

 
 

Because daylight contains more red light than far-red light, more PR is converted into PFR than the other way around. This means that there is more PFR in the daytime and more PR at night. The proportion of PR to PFR allows the plant to 'know' whether it is daytime, nighttime, summer or winter. For example, high levels of PFR can stimulate flowering. PFR accumulates during the summer, when there is short nights so less conversion of PFR into PR. High PFR stimulates the transcription of genes involved in flowering, which means that flowering only occurs in the summer.

Did you know…

Plant roots grow towards water - but scientists have found that they also grow just towards audio recordings of water. Its thought that the tiny hairs on plant roots act like the tiny hairs that we have in our ears, allowing them to detect the vibrations which indicate the presence of water.