plant hormones

• Photosynthesis is important for an ecosystem because plants produce sugars through photosynthesis which provides food for the rest of the organisms in the ecosystem – they are producers
• There are two main types of tropisms: Positive tropisms – the plant grows towards the stimulus. Negative tropisms – the plant grows away from the stimulus
• Shoots are positively phototropic as they grow towards the light to maximise sunlight for photosynthesis
• Tropisms are controlled by hormones called auxins which are produced in the tips of shoots and roots. These hormones accumulate in certain parts of a plant and change how it grows
• Auxins - phototropism

  In a shoot auxins will accumulate on the shaded side. The auxins will trigger cell elongation. This cell elongation will lead to the plant bending away from the shade towards the light
• By cutting off the tip of the plant, Boysen-Jensen proved that the tip was producing the chemical causing phototropism. By covering the tip in a transparent and opaque cap, he proved that sunlight was necessary to cause phototropism
• Boysen-Jensen separated the tips of two shoots with glass and gelatine to prove that auxins were causing phototropism
• By covering the tip in a transparent and opaque cap, it was proved that sunlight was necessary to cause phototropism
• Boysen-Jensen knew that a substance (auxins) was causing phototropism but he did not know it was a hormone
• Boysen-Jensen's experiment

  He separated the tips of two shoots with glass (impermeable to hormones) and gelatine (permeable to hormones). Only the shoot with gelatin showed phototropism, proving that auxins travelled by diffusion and that it was a hormone (not a nervous signal)
• Permeable


  Allowing substances (usually liquids or gases) to pass through it
• Auxins in roots

  Auxins build up on the underside of the root, prevent the cells from growing as much as normal, causing the root to bend and grow down in the direction of gravity
• Roots are positively geotropic
• Gravity causes auxins to accumulate on the underside of the root, prevent cells from growing, and cause the root to bend down
• Shoots growing towards the light enables a plant to photosynthesize more which helps it to grow and survive
• Roots growing down into the soil help a plant access water and minerals deeper below the soil. Deeper roots also help anchor a plant and prevent it from being uprooted
• Auxins can be used to encourage growth of plant seedlings grown in labs. They are also used in rooting powders which help cuttings from adult plants develop their own roots
• In very high concentrations, auxins can disrupt metabolism and kill plants. Some plants (usually broad-leaved plants) are more sensitive to auxins than narrow-leaved plants. Using high concentrations as a selective weed killer can be useful for a farmer who grows wheat as it will only kill the weeds (broad leaves) not the wheat (narrow leaves)
• Gibberellins are important in initiating seed germination
• Weed killer can be useful for a farmer who grows wheat as it will only kill the weeds (broad leaves) not the wheat (narrow leaves)
• GIBBERELLINS

  • End seed dormancy
  • Speed up germination of barley seeds (brewing)
  • Promote flowering throughout the year
  • Increase fruit size and make seedless fruit (parthenocarpy)
  • Important in initiating seed germination
• Ethene (ethylene)


  • Used in the food industry to control ripening of fruit during storage and transport
  • Controls cell division and ripening of fruits
• Auxins are plant hormones that control tropisms
• Shoots are positively phototropic as they grow towards sources of light
• Roots are positively geotropic as they grow down with the pull of gravity
• In shoots, light shining on one side causes auxins to move to the shaded side where they trigger cell elongation
• Gravity causes auxins to accumulate at the underside of roots where they inhibit growth
• Phototropism allows plants to access more light for photosynthesis, while geotropism allows roots to burrow deeper for water, minerals, and better anchorage
• Boysen-Jensen proved that a substance produced in the shoot tips causes phototropism by removing the tips of shoots
• Boysen-Jensen proved that auxin was a hormone, not a nervous message, by placing permeable and impermeable barriers between a shoot and its tip
• What are tropisms in plants?

  Directional growth responses to stimuli
• Why do plants need to respond to light?

  To ensure leaves absorb light for photosynthesis
• How do plants respond to gravity?

  Shoots grow upwards, roots grow downwards
• What is a phototropism?

  A growth response to light
• What is a geotropism?

  A growth response to gravity
• What indicates a positive tropism?

  Growth towards the stimulus
• What indicates a negative tropism?

  Growth away from the stimulus
• How do shoots exhibit phototropic and geotropic responses?

  Positive phototropic and negative geotropic responses
• How do roots exhibit phototropic and geotropic responses?

  Negative phototropic and positive geotropic responses
• What are the effects of auxins in plants?

  • Promote cell elongation in shoots
  • Inhibit cell elongation in roots