[Campbell Biology P.905] A batch of lettuce seeds that have been water-swollen are first exp... | Practice Question

A batch of lettuce seeds that have been water-swollen are first exposed to a brief flash of red light 660 nm , then immediately followed by a brief flash of far-red light 730 nm . According to the text, what would be the expected outcome regarding their germination, and why?

  • A: The seeds would germinate because red light stimulates germination, and its effect is dominant.
  • B: The seeds would not germinate because far-red light inhibits germination, and the last flash determines the outcome.
  • C: The seeds would germinate, but at a reduced rate, as the far-red light partially negates the red light's effect.
  • D: The outcome would be unpredictable, as the sequence of light flashes creates a complex response.

Explanation

The text explains the concept of phytochrome reversibility, stating: 'The last flash of light determines the seeds' response: The effects of red and far-red light are reversible.' Red light converts P r to P fr stimulating germination , while far-red light converts P fr back to P r inhibiting germination . Since the last flash was far-red light, it would convert the P fr created by the red light back to P r, thereby inhibiting germination.