Plant responses and plant hormonesEdexcel International A Level Biology: Mind map

Exam questions on Plant responses and plant hormones

1. Oat seedlings are grown in complete darkness so that their coleoptiles (the protective sheaths around the shoots) grow straight upwards. A student then lights a group of seedlings from one side only with a dim lamp. After several hours the coleoptiles have curved towards the lamp. The student knows that the response depends on the plant hormone indoleacetic acid (IAA), which is made in the coleoptile tip.
   Explain why the coleoptiles curve towards the lamp.2 marks
2. Seeds of a light-sensitive lettuce variety are soaked in the dark and then given brief flashes of red light (660 nm) and far-red light (730 nm) in different orders, before being returned to the dark. Germination is scored after three days. Batches whose final flash was red germinated; batches whose final flash was far-red did not.
   Explain how the formation of Pfr leads to the germination of the lettuce seeds.2 marks
3. In Core Practical 18, a student cuts barley grains in half across the middle. Half grains with the embryo and half grains without the embryo are placed cut side down on starch agar plates and incubated for 48 hours at 25 °C. The plates are then flooded with iodine solution and the diameter of the clear zone around each half grain is measured. Mean diameters of clear zone: embryo half grains on water 14 mm; embryo-less half grains on water 0 mm; embryo-less half grains on gibberellin solution 12 mm; embryo-less half grains on gibberellin solution plus actinomycin D (an inhibitor of transcription) 0 mm.
   Explain why a clear zone forms around the half grains with an embryo but not around the embryo-less half grains on water.3 marks