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Difficulty: MediumSuggesting Experiments to Resolve Viewpoints

Three students discuss the mechanism by which a newly discovered plant hormone, *abscisigen*, inhibits seed germination.

* Student 1: Abscisigen directly blocks the synthesis of gibberellins (growth-promoting hormones) in the seed embryo.
* Student 2: Abscisigen prevents water uptake by increasing the solute concentration inside the seed coat, making it hypertonic relative to the surrounding environment.
* Student 3: Abscisigen physically hardens the seed coat by promoting lignin deposition, preventing the embryo's radicle (root) from breaking through.

Match each student's hypothesis with the experimental outcome that would directly invalidate (disprove) that hypothesis.

  • Student 1's hypothesis (abscisigen blocks gibberellin synthesis)Seeds treated with abscisigen fail to germinate even when supplied with an abundant external source of active gibberellins.
  • Student 2's hypothesis (abscisigen prevents water uptake)Treated seeds show a rapid, significant increase in mass and internal water volume when placed in a water bath.
  • Student 3's hypothesis (abscisigen hardens the seed coat)Treated seeds whose outer seed coats are mechanically removed still fail to germinate.

Answer

Student 1's hypothesis is invalidated by the observation that treated seeds fail to germinate even when external gibberellins are supplied. Student 2's hypothesis is invalidated by the observation that treated seeds still absorb water and gain mass. Student 3's hypothesis is invalidated by the observation that treated seeds without seed coats still fail to germinate.
Each hypothesis is invalidated by an experimental outcome that directly violates its prediction: Student 1's prediction of a synthesis-only block is disproved by the failure of external gibberellins to rescue germination; Student 2's prediction of blocked water uptake is disproved by observed water entry and mass increase; and Student 3's prediction of mechanical seed coat restriction is disproved by germination failure in seeds lacking seed coats.

Step-by-Step Solution

1
Analyze Student 1's hypothesis, which proposes that germination is blocked because the hormone prevents the synthesis of gibberellins. Identify an experimental result that bypasses synthesis but maintains the block.
Providing external gibberellins bypasses the synthesis block. If seeds still do not germinate, then the synthesis block is not the primary cause of inhibition, invalidating Student 1.
To test a synthesis block hypothesis, one must supply the synthesis product directly to see if the block is bypassed.
2
Analyze Student 2's hypothesis, which states that the hormone prevents water uptake by creating hypertonic conditions inside the seed coat.
If treated seeds are placed in water and show a significant increase in internal water volume and mass, then water is entering the seeds.
An increase in water volume directly contradicts the claim that water uptake is prevented.
3
Analyze Student 3's hypothesis, which attributes germination failure to the mechanical restriction of a hardened seed coat.
If the seed coat is removed and the embryo still fails to germinate, the mechanical constraint of the seed coat is not the active inhibitor.
Removing the proposed barrier should allow germination to proceed if that barrier were the sole cause of the inhibition.

Key Concept

Identifying experimental conditions or observational results that isolate a specific biological mechanism to test and potentially invalidate a hypothesis.
Estimated Time:1m 30s
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Suggesting Experiments to Resolve Viewpoints

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