Grand Rapids variety of lettuce produces seeds that will not germinate unless they are exposed to light. Phytochrome is also involved in that exposure to red light stimulates germination and far-red light inhibits germination. However, the effect of
red light can be reversed by an immediate exposure to far-red light--and vice versa. You design an experiment in which you expose seeds of Grand Rapids lettuce first to red light, then immediately to far-red light, and then immediately to red light again. Would these seeds germinate? Use your knowledge of the phytochrome molecule to explain your reasoning. What if the seeds were exposed first to red light, then immediately to far-red light, then immediately to red light again, and then immediately to far-red light again?Grand Rapids variety of lettuce produces seeds that will not germinate unless they are exposed to light. Phytochrome is also involved in that exposure to red light stimulates germination and far-red light inhibits germination. However, the effect of red light can be reversed by an immediate exposure to far-red light--and vice versa. You design an experiment in which you expose seeds of Grand Rapids lettuce first to red light, then immediately to far-red light, and then immediately to red light again. Would these seeds germinate? Use your knowledge of the phytochrome molecule to explain your reasoning. What if the seeds were exposed first to red light, then immediately to far-red light, then immediately to red light again, and then immediately to far-red light again? What will be an ideal response?
ANSWER: Concepts to Consider: Phytochrome exists in two interconvertible forms: Pr and Pfr.
Exposure to red light converts Pr to Pfr, which is the active form and thus stimulates
germination. However, if the exposure to red light is followed immediately by an exposure to
far-red light, Pfr will be converted back to Pr before germination is stimulated, and the seeds
will not germinate. Using this reasoning, the result will depend on which wavelength the
seeds receive last.
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