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Plant hormones, phototropism and gravitropism

Homeostasis and response · Plant hormones · note 1 of 2

Spec 4.5.4.1Triple onlyDownload these notes (PDF)4 pages

Plant hormones, phototropism and gravitropismSpec 4.5.4.1

Plants produce hormones to coordinate and control growth and responses to light (phototropism) and gravity (gravitropism or geotropism).

Unequal distributions of auxin cause unequal growth rates in plant roots and shoots.

Auxin and plant responses
ResponseWhere auxin builds upEffect on growthResult
Shoot growing towards lightOn the shaded side of the shootCells on the shaded side elongate moreThe shoot bends towards the light
Root growing downwardsOn the lower side of the rootGrowth of cells on the lower side is slowed downThe root bends downwards, in the direction of gravity
Shoot growing upwardsOn the lower side of the shootCells on the lower side elongate moreThe shoot bends upwards, away from gravity
Three seedlings: a shoot lit from one side with more auxin on the shaded side bending towards the light; a horizontal root with more auxin on the lower side bending downwards; a horizontal shoot with more auxin on the lower side bending upwards.Tap to enlarge
Unequal distribution of auxin causes unequal growth, so shoots bend towards light and away from gravity, and roots bend towards gravity.
Higher tier

Gibberellins and ethene

Higher tier
  • Gibberellins are important in initiating seed germination.
  • Ethene controls cell division and ripening of fruits.
Higher tier

You do not need to know how gibberellins and ethene work.

  1. Germinate seeds (for example cress) on damp cotton wool or filter paper in a dish.
  2. For light: place identical seedlings in a box with a slit that lets light in from one side only. Keep a control set in even light all round.
  3. For gravity: fix seedlings in a dish, turn the dish on its side so the roots point sideways and keep it in the dark. Keep a control set upright.
  4. Measure the length of the shoot (and root) with a ruler at regular intervals, and make a labelled drawing of each seedling to show the direction of growth.
  5. Compare the treated seedlings with the control seedlings.
  • Independent variable: the direction of the light, or the position of the seedling (gravity).
  • Dependent variable: the direction and length of growth.
  • Control variables: the type and age of seedling, temperature and the amount of water. Keep the gravity test in the dark so light does not affect the result.
  • Safety: wash your hands after handling seedlings and compost, and do not eat any seeds or seedlings.
  • Typical results: shoots bend towards the light; roots bend down; shoots grow upwards even in the dark.