OCR A Level Biology

Revision Notes

5.5.6 Practical: Effect of Plant Hormones on Growth

Practical: Effect of Plant Hormones on Growth

  • Indole-3-butyric acid (IBA) is an auxin that is often used as a rooting powder
  • It is possible to investigate the effect of different concentrations of this plant hormone on root growth

Apparatus

  • Indole-3-butyric acid (IBA)
  • Alcohol
  • Measuring cylinder
  • Beakers
  • Plant e.g. Busy Lizzie (Impatiens walleriana)
  • Soil or plant compost
  • Scalpel
  • Digital balance

Method

  • First, a 5% solution of IBA should be made
    • As IBA is insoluble in water, this solution should be made in alcohol
    • Label this as ‘Solution A
  • Serial dilutions of the 5% IBA solution should then be made in the following way:
    • Solution B (0.5%) – add 1cm³ of Solution A to 9cm³ of alcohol
    • Solution C (0.05%) – add 1cm³ of Solution B to 9cm³ of alcohol
    • Solution D (0.005%) – add 1cm³ of Solution C to 9cm³ of alcohol
    • Solution E (0.0005%) – add 1cm³ of Solution D to 9cm³ of alcohol
  • 60 stem cuttings should then be taken from the same plant e.g. Busy Lizzie (Impatiens walleriana)
  • Dip the ends of 10 cuttings into Solution A and place the cuttings into soil or plant compost (dipped-end down)
    • Repeat this process with Solutions B, C, D and E
  • Use the final 10 cuttings as a control group by dipping their ends in the alcohol used to make the IBA solutions and then placing them into the soil or plant compost (dipped-end down)
    • This is to ensure that an effect on root growth was due to the hormone, not the alcohol it was dissolved in
  • After a set time period (e.g. 1 week, during which the cuttings should be regularly watered) carefully remove all the cuttings
  • For each set of 10 cuttings, carefully remove all the roots and weigh them

Results

  • Plot the results on a graph, with the concentration of auxin on the x-axis and the mass of roots grown on the y-axis
  • This should show clearly the effect of different concentrations of this plant hormone on root growth

Limitations

  • Certain factors should be controlled throughout the experiment, including:
    • The volume of water provided to each set of cuttings during the growth period
    • Temperature
    • Humidity
    • Light intensity
  • Although it may be difficult, it is very important to control these factors for each set of cuttings so that the concentration of plant hormone is the only variable that is having an effect on root growth
  • Although it may not be possible to obtain all the cuttings from the same plant, the cuttings must be obtained from the same species of plant and if possible, from genetically identical plants
    • This is to ensure that all of the cuttings respond to the hormone in the same way, as cuttings from genetically dissimilar plants may respond to the hormone in different ways
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