All subjects · Biology · WBI15 (Unit 5)

Core practical 18: gibberellin and amylase in cereal grains

Core practical 18 shows that gibberellin triggers amylase production in germinating cereal grains, using starch agar and iodine.

Method

1. Make a range of gibberellic acid (GA) concentrations, for example 0.0 to 1.0 g dm-3, including distilled water as a control.

2. Sterilise cereal grains (such as barley) in dilute sodium hypochlorite and rinse with sterile water.

3. Cut each grain in half and discard the half containing the embryo, so the only gibberellin is the GA you add.

4. Soak the endosperm halves in each GA solution for 24 to 48 hours.

5. Place the halves cut-face down on starch agar plates, one plate per concentration, and incubate for 24 to 48 hours.

6. Flood the plates with iodine solution. Starch stains blue-black; where amylase has digested the starch, a clear zone (halo) appears around each grain.

Results and analysis

Measure the diameter of each clear zone, calculate the mean, and work out the area: area = πr².

Expected result: higher GA concentration gives larger clear zones, because more amylase is made and secreted. Grains in water alone give little or no clear zone, because the embryo, the natural source of GA, has been removed.

Evaluation

Use aseptic technique so microorganisms on the grains do not digest starch and produce false clear zones. Repeat with several grains per concentration and use means. A whole grain (with embryo) on a plate can be used as a comparison.

Worked example

Three grains soaked in 0.5 g dm-3 GA give clear zones of 10, 12 and 14 mm diameter. Calculate the area of the mean zone.

  1. Mean diameter = (10 + 12 + 14) / 3 = 12 mm
  2. r = 6 mm
  3. Area = π x 6² = 113 mm²

Answer: 113 mm²

Key idea: Embryo-free half grains are soaked in GA, placed on starch agar, then flooded with iodine: clear halos show amylase activity. Higher GA gives larger halos. Area = πr² of the mean diameter. Aseptic technique avoids false results.

Check yourself

  1. A clear zone has a diameter of 18 mm. Calculate its area in mm² to 3 significant figures.
    Show answer

    254 mm². r = 9 mm; π x 81 = 254 mm².

  2. Clear zones of 8, 9 and 13 mm diameter are measured. Calculate the mean diameter in mm.
    Show answer

    10 mm. 30 / 3 = 10 mm.

  3. Why is the embryo half of each grain discarded?
    • The embryo contains starch
    • The embryo is poisonous to bacteria
    • The embryo makes its own gibberellin, which would affect the results
    • The embryo makes iodine
    Show answer

    The embryo makes its own gibberellin, which would affect the results. Removing it means the only GA present is the GA applied.

  4. What does a clear zone around a grain show after adding iodine?
    • Starch is present in large amounts
    • Starch has been digested by amylase from the grain
    • The grain has no aleurone layer
    • Iodine has been absorbed by the grain
    Show answer

    Starch has been digested by amylase from the grain. No starch means no blue-black colour.

  5. What result is expected as GA concentration increases?
    • Smaller clear zones
    • No clear zones at any concentration
    • The agar turns red
    • Larger clear zones
    Show answer

    Larger clear zones. More GA means more amylase.

  6. Why are the grains sterilised with sodium hypochlorite?
    • To provide gibberellin
    • To kill microorganisms that could digest starch and give false clear zones
    • To stain the starch
    • To remove the aleurone layer
    Show answer

    To kill microorganisms that could digest starch and give false clear zones. Bacteria and fungi can also make amylase.

  7. Which is a control variable in this practical?
    • Concentration of gibberellic acid
    • Area of the clear zone
    • Amount of amylase produced
    • Incubation time and temperature
    Show answer

    Incubation time and temperature. GA concentration is the independent variable.

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Last updated 7 November 2026. Spotted a mistake? Tell us.