Plant Hormones

To survive, plants need to be able to respond to their environment. All plants need water and light for photosynthesis, so they have developed responses called tropisms.

Tropisms ensure that plants grow in response to stimuli, directing them towards sources of light and water.

There are two types of tropism:

  • Positive tropisms – The plant grows towards a stimulus
  • Negative tropisms – The plant grows away from a stimulus

As a result of tropisms, the stems of plants grow upwards towards the sun (for light) and the roots grow downwards to absorb water in the soil.

  • Phototropism is the response of plants to light
  • Gravitropism (or geotropism) is the response of plants to gravity

Both phototropism and gravitropism play an important role in helping plants adapt to their environments.

Auxins

Plants produce hormones, such as auxins, to coordinate and control growth

Auxins are produced in the shoot tips and root tips of plants by actively dividing cells. They regulate plant growth by stimulating cell elongation in the plant shoot. Shoots and roots have different responses to higher concentrations of auxins:

  • In the shoot tips, high concentrations of auxins promote cell elongation, which contributes to the shoot growing upwards.
  • In the root tips, high concentrations of auxins inhibit cell elongation

Phototropism

Phototropism is the growth of a plant in response to light.

A graphic illustration of a potted plant with green leaves and yellow flowers leaning towards a radiant sun on the top right. The plant exhibits phototropism, as indicated by a label at the bottom that reads, "Bending of the plant towards light." The background is predominantly white, and the pot is brown.

Shoots and roots in plants have opposite responses to light.

  • When plant shoots display positive phototropism, they grow towards the light, maximising the absorption required for photosynthesis.
  • Plant roots display negative phototropism in response to light, causing them to grow deeper into the soil. This action not only maximises water uptake for photosynthesis but also anchors the plant securely into the ground.

The auxins produced in the tip are more concentrated on the shaded side of shoot tips (away from light), which causes the cells on that side to elongate. The elongated cells make the shaded side heavier, which causes it to bend towards the light.

Illustrative diagram showing the effect of sunlight on plant growth. Three sun icons at the top emit orange arrows pointing downwards. Beneath, there are three stages of a green plant stem. In the first stage, sunlight directs "Auxin molecules" towards the shaded side, as indicated by labels and arrows. In the second stage, these molecules are spread across the shaded side. In the final stage, the stem bends towards the sunlight due to "Cell elongation", demonstrated by a curved stem and a large blue arrow indicating the direction of growth. The stem is dotted with small yellow spots, representing cells or molecules. The background is white.

Gravitropism

Gravitropism directs the growth of a plant in response to gravity. Shoots and roots have opposite responses to gravity.

  • Shoots display negative gravitropism by growing against the force of gravity (upwards). This ensures that they grow towards the light.
  • Roots display positive gravitropism, growing in the direction of gravity (downwards) to ensure they penetrate deeper into the soil.
Illustrative diagram contrasting two plant responses to gravity. On the left, a young plant stands upright in a brown pot, with its roots growing straight downwards, representing a standard growth pattern. On the right, the plant displays "Negative gravitropism", growing upwards even though its pot is tilted sideways. The roots, exhibiting "Positive gravitropism", grow in the direction of gravitational pull, heading downwards into the soil of the tilted pot. Both plants have broad green leaves and slender stems. The background is white.

When a plant’s root grows, not all parts of it grow at the same rate. This varying rate of growth is called “differential growth,” which is influenced by auxins.

If a root is growing horizontally (sideways), more auxins will accumulate on the side of the root facing downwards. This high concentration inhibits growth on that side. In contrast, the upper side of the root, with less auxin, experiences more cell elongation.

As a result, the root will grow more on the side facing upwards, causing the root tip to bend downwards. This is an example of positive gravitropism.

You’ve used 0 of your 10 free revision notes for the month

Sign up to get unlimited access to revision notes, quizzes, audio lessons and more

Sign up