119. Phytochromes A and B maximally absorb light at wavelength range:
(1) 150-300 nm
(2) 300-450 nm
(3) 450-550 nm
(4) 600-750 nm

Detailed Explanation:

Question:
Phytochromes A and B maximally absorb light at which wavelength range?

✅ Correct Answer: (4) 600–750 nm

What Are Phytochromes?

Phytochromes are a class of photoreceptor proteins in plants that regulate growth and development in response to red and far-red light. The two primary forms found in higher plants are:

• Phytochrome A (phyA): Sensitive to very low fluence light and mediates far-red light responses.

• Phytochrome B (phyB): More stable and primarily responsible for red light responses.

Absorption Characteristics of Phytochromes:

Phytochromes exist in two interconvertible forms:

• Pr form: Absorbs red light (~660 nm).

• Pfr form: Absorbs far-red light (~730 nm).

Both phytochrome A and B function within the 600–750 nm wavelength range, making option (4) the correct choice.

Why This Wavelength Range Matters:

• This range corresponds to red and far-red light, which are abundant in natural sunlight.

• Plants detect shade and light quality through phytochrome signaling.

• Activation and conversion between Pr and Pfr forms influence germination, stem elongation, flowering, and circadian rhythms.

Why Other Options Are Incorrect:

• (1) 150–300 nm: Ultraviolet (UV-C and UV-B) region; not detected by phytochromes.

• (2) 300–450 nm: UV-A to blue light range; mainly sensed by cryptochromes and phototropins.

• (3) 450–550 nm: Blue to green light range; still outside the optimal red/far-red detection range for phytochromes.

Conclusion:

Phytochrome A and B are vital for plant photoreception and regulate key developmental processes in response to light, especially in the 600–750 nm red/far-red spectrum.

✅ Correct Answer: (4) 600–750 nm