Question

Active form of phytochrome for seed germination is

• A. P${}_r$-form
• B. P${}_{fr}$-form
• C. Both of A and B
• D. None of the above

Answer 1

The correct option is B P${}_{fr}$-form

The main photoreceptor for photoperiodism and many other light-initiated plant responses is phytochrome, a family of about five blue-green pigment proteins, each of which is coded for by a different gene. Phytochrome is involved in the light requirement that some seeds have for germination. Seeds with a light requirement must be exposed to light containing red wavelengths. Exposure to red light converts $P_r$ to $P_{fr}$ and germination occurs. When $P_{fr}$ absorbs far-red light it is converted back to $P_r$. Hence, red light makes $P_{fr}$, far-red light makes $P_r$. In plants at least $P_{fr}$ is the physiologically active or "signalling" state.