International A Level Biology / Coordination in animals and plants
Phytochrome, transcription and plant responses
Edexcel International A Level Biology (YBI11), chapter 24: Coordination in animals and plants, specification point 8.11. Part of the full course.
Read the revision notes · International A Level Biology past papers
Phytochrome links light to changes in gene transcription, causing seedlings to develop differently and helping some seeds germinate. Red and far-red light can switch phytochrome between forms, so the final flash can determine the response.