[Campbell Biology P.906] The "WHAT IF?" section discusses placing seeds in white light inste... | Practice Question

The "WHAT IF?" section discusses placing seeds in white light instead of darkness after red and far-red light treatments. Given that white light contains both red and far-red light, and Pfr conversion is faster than Pr conversion in sunlight, how would the results likely differ from those described in the experiment?

  • A: All seeds would show high germination rates regardless of the final treatment before white light, as white light would quickly convert all phytochrome to Pfr.
  • B: Seeds last exposed to far-red light would still not germinate, as the white light would maintain the Pfr form.
  • C: The results would be identical to the experiment, as the dark period is essential for phytochrome to stabilize.
  • D: Seeds last exposed to red light would show inhibited germination due to the faster conversion to Pr in white light.

Explanation

The text states that "Sunlight contains both red light and far-red light, but the conversion to Pfr is faster than the conversion to Pr. Therefore, the ratio of Pfr to Pr increases in the sunlight. When seeds are exposed to adequate sunlight, the production and accumulation of Pfr trigger their germination." If seeds were placed in white light which mimics sunlight after treatments, the phytochrome would quickly shift to a Pfr-dominant state, leading to germination regardless of the preceding light sequence.