Plant responses to environmental cuesEdexcel A-Level Biology A: Flashcards

Exam questions on Plant responses to environmental cues

1. Lettuce seeds were kept in the dark and given brief flashes of red light (R) and far-red light (FR) in different sequences. Seeds given R alone, or R then FR then R, mostly germinated (92% and 90%). Seeds given R then FR germinated poorly (8%). The germination of the seeds depended on the last flash that they received.
   Explain why seeds given red light then far-red light germinated poorly, but seeds given red, far-red and then red light germinated well.2 marks
2. Young oat seedlings were illuminated from the right. Normal seedlings bent towards the light. Seedlings whose tips had been removed did not bend. Seedlings with an opaque cap over the tip did not bend, but seedlings with a cap over the lower part of the shoot did bend towards the light.
   Explain why the seedlings with the tips removed did not bend.2 marks
3. Seedlings of a plant species were grown for ten days in white light with a normal ratio of red to far-red light, and in white light with extra far-red added to give a low red to far-red ratio, as occurs under the leaves of neighbouring plants. The mean stem length of seedlings in normal light was 4 cm. The mean stem length with extra far-red was 11 cm.
   Explain the difference in mean stem length between the two groups of seedlings.3 marks