OCR · A-Level · Biology

    Course version: H420 · Version from Sept 2025

    (d) the experimental evidence for the role of gibberellin in the control…

    Full specification wording

    Plant and animal responses: (d) the experimental evidence for the role of gibberellin in the control of stem elongation and seed germination

    Gibberellins promote stem elongation and seed germination, and the evidence comes from mutants and application experiments.

    Checked against the OCR H420 specification (opens in a new tab) on .

    • Point 4 of 12
    • 4 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    Gibberellins promote stem elongation and seed germination, and the evidence comes from mutants and application experiments. Dwarf varieties (for example short-stemmed mutants) have low gibberellin or reduced sensitivity; applying gibberellin to dwarf plants restores normal elongation, showing the hormone drives stem growth. For germination, gibberellin-deficient seeds fail to germinate, and adding gibberellin triggers germination, including in some light-requiring seeds where it substitutes for the light signal. The evidence combines naturally occurring mutants (loss of function) with replacement by applied hormone, so a strong answer separates the observation from the causal test and notes that sensitivity, not only hormone level, can matter.

    What earns the marks

    • Dwarf (short-stemmed) mutant plants show reduced stem elongation, linking gibberellin to stem growth.
    • Applying gibberellin to dwarf plants restores elongation, showing the hormone causes the growth rather than merely correlating with it.
    • Gibberellin-deficient seeds fail to germinate, and adding gibberellin triggers germination, showing its role in seed germination.
    • In some light-requiring seeds, applied gibberellin can substitute for the light signal that normally promotes germination.
    • Mutant and replacement experiments together provide causal evidence, but reduced sensitivity (not only low hormone) can also produce dwarfism.
    • Conclusions are limited to the species and conditions tested and do not by themselves reveal the full molecular mechanism.

    Common mistakes and how to put them right

    • Claiming dwarf plants always lack gibberellin.

      Put it right: Dwarfism can also result from reduced sensitivity to gibberellin, so level alone is not the whole story.

    • Treating correlation as proof.

      Put it right: The causal evidence comes from applying gibberellin and seeing elongation or germination restored.

    • Confusing gibberellin with auxin roles.

      Put it right: Gibberellin promotes stem elongation and germination, whereas auxin is central to apical dominance.

    Exam tips

    • Pair each observation with its manipulation: dwarf mutant → apply gibberellin → elongation restored.
    • For germination, state that deficient seeds fail and applied gibberellin triggers germination.
    • Mention that sensitivity as well as hormone concentration can explain mutant phenotypes.

    Try the lesson

    The start of the Plant and animal responses: (d) the experimental evidence for the role of gibberellin in the control of stem elongation and seed germination lesson, made from checked teaching notes for this specification point.

    Have a go first

    Which observation would provide evidence that gibberellin is involved in seed germination?

    Listen

    Gibberellins promote stem elongation and seed germination, and the evidence comes from mutants and application experiments. Dwarf varieties (for example short-stemmed mutants) have low gibberellin or reduced sensitivity; applying gibberellin to dwarf plants restores normal elongation, showing the hormone drives stem growth. For germination, gibberellin-deficient seeds fail to germinate, and adding gibberellin triggers germination, including in some light-requiring seeds where it substitutes for the light signal. The evidence combines naturally occurring mutants (loss of function) with replacement by applied hormone, so a strong answer separates the observation from the causal test and notes that sensitivity, not only hormone level, can matter. A strong answer earns marks for these points. Dwarf (short-stemmed) mutant plants show reduced stem elongation, linking gibberellin to stem growth. Applying gibberellin to dwarf plants restores elongation, showing the hormone causes the growth rather than merely correlating with it. Gibberellin-deficient seeds fail to germinate, and adding gibberellin triggers germination, showing its role in seed germination. In some light-requiring seeds, applied gibberellin can substitute for the light signal that normally promotes germination. A common slip: Claiming dwarf plants always lack gibberellin. Instead, dwarfism can also result from reduced sensitivity to gibberellin, so level alone is not the whole story. Exam tip: Pair each observation with its manipulation: dwarf mutant → apply gibberellin → elongation restored.

    See it

    A top-down flowchart. On the left, a dwarf mutant with reduced stem elongation leads to applying gibberellin to the dwarf plant, which leads to elongation being restored. In the middle, gibberellin-deficient seeds failing to germinate lead to adding gibberellin, which leads to germination being triggered. On the right, light-requiring seeds lead to applied gibberellin substituting for the light signal. All three restored or triggered outcomes point to a node reading causal evidence for gibberellin. A separate node noting that reduced sensitivity can also cause dwarfism also points to that causal evidence node.
    Shows how mutant observations paired with gibberellin application provide causal evidence for stem elongation and seed germination.

    Key words

    Say what each word means before you turn the card.

    Gibberellin and stem elongation

    Try answering aloud or in your head before turning the card.

    Gibberellin and seed germination

    Try answering aloud or in your head before turning the card.

    Dwarf mutants and hormone-replacement experiments

    Try answering aloud or in your head before turning the card.

    Hormone sensitivity versus hormone concentration

    Try answering aloud or in your head before turning the card.

    See every definition
    Gibberellin and stem elongation
    Dwarf short-stemmed mutants show reduced stem elongation, and applying gibberellin to dwarf plants restores elongation, showing the hormone causes the growth.
    Gibberellin and seed germination
    Gibberellin-deficient seeds fail to germinate, and adding gibberellin triggers germination, including in some light-requiring seeds where it substitutes for the light signal.
    Dwarf mutants and hormone-replacement experiments
    Naturally occurring loss-of-function mutants combined with replacement by applied hormone provide causal evidence, separating the observation from the causal test.
    Hormone sensitivity versus hormone concentration
    Dwarfism can result from reduced sensitivity to gibberellin as well as low hormone, so level alone is not the whole story.

    Label the diagram

    Some boxes have lost their names. Pick the right name for each number.

    The diagram again, with 3 boxes numbered instead of named. A top-down flowchart. On the left, a dwarf mutant with reduced stem elongation leads to applying gibberellin to the dwarf plant, which leads to elongation being restored. In the middle, gibberellin-deficient seeds failing to germinate lead to adding gibberellin, which leads to germination being triggered. On the right, light-requiring seeds lead to applied gibberellin substituting for the light signal. All three restored or triggered outcomes point to a node reading causal evidence for gibberellin. A separate node noting that reduced sensitivity can also cause dwarfism also points to that causal evidence node.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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