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Plant structures and their functions, subtopic 4 of 4Spec 6.15B–6.16BTriple only

Plant hormones

How auxins control phototropism and gravitropism, and the commercial uses of auxins, gibberellins and ethene.

2 sections, with a quick check at the end.

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Plant hormones, phototropism and gravitropismSpec 6.15B

Plant hormones are chemicals that control and coordinate plant growth and development. They are made in one part of the plant and move to other parts, where they change how cells grow.

A tropism is a growth response to a stimulus. A growth towards the stimulus is positive. A growth away from the stimulus is negative. Auxins are the plant hormones involved in two tropisms:

  • Phototropism: growth in response to light.
  • Gravitropism: growth in response to gravity.

Auxins in shoots

  1. Auxin is made in the tip of the shoot.
  2. When light shines from one side, auxin moves to the shaded side of the shoot.
  3. In shoots, auxin makes cells elongate (get longer), so the cells on the shaded side grow faster.
  4. The shoot bends towards the light. Shoots are positively phototropic.

If a shoot is lying on its side, auxin collects on the lower side. The cells there elongate more, so the shoot bends upwards, away from gravity. Shoots are negatively gravitropic.

Auxins in roots

Roots respond differently to auxin. In a root that is lying on its side, auxin collects on the lower side, where it slows the growth of the cells. The cells on the upper side grow faster, so the root bends downwards, in the direction of gravity. Roots are positively gravitropic. Roots also grow away from light, so they are negatively phototropic.

Tropisms of shoots and roots
Response to lightResponse to gravity
ShootPositive phototropism (grows towards light)Negative gravitropism (grows upwards)
RootNegative phototropism (grows away from light)Positive gravitropism (grows downwards)
Top: a shoot lit from the right with auxin made in the tip moving to the shaded left side, then the shoot bent towards the light with longer cells on the shaded side. Bottom: a horizontal root with auxin collected on the lower side, then the root bent downwards because the upper side grows faster.Tap to enlarge
Shoots: auxin makes shaded-side cells elongate, so the shoot bends towards light. Roots: auxin on the lower side slows growth, so the root bends down.

Commercial uses of plant hormonesSpec 6.16B

Growers and farmers use plant hormones, or chemicals that act like them, to control how plants grow.

Commercial uses of auxins, gibberellins and ethene
HormoneUseEffect
AuxinsWeedkillersSelective weedkillers make broad-leaved weeds grow too fast and uncontrollably, which kills them. Narrow-leaved crops such as cereals are not affected.
AuxinsRooting powdersCuttings dipped in the powder grow roots quickly, so new plants can be produced
GibberellinsGerminationUsed to end seed dormancy and make seeds germinate
GibberellinsFruit and flower formationUsed to make flowers and fruits develop
GibberellinsSeedless fruitFruits form without being fertilised, so they contain no seeds
EtheneFruit ripeningUnripe fruit is picked, transported, and then ripened using ethene when it is needed

Quick check

  1. What is a tropism?

    Show answer

    A growth response of a plant to a stimulus.

  2. Which hormone is involved in phototropism and gravitropism?

    Show answer

    Auxin.

  3. Why does a shoot bend towards light?

    Show answer

    Auxin moves to the shaded side and makes those cells elongate more, so that side grows faster.

  4. Are roots positively or negatively gravitropic?

    Show answer

    Positively gravitropic: they grow downwards, in the direction of gravity.