Question

Phytochrome becomes active in
(a) Green light
(b) Blue light
(c) Red light
(d) None of the above

Answer 1

Phytochrome originally described the proteinous pigment that controls photoperiod detection and floral induction of specific short-day plants (such as soybean) and the term means “plant colour”.

Complete answer:
In plants, phytochrome becomes biologically active in red light and it absorbs visible light max in the red region at 660nm. Phytochromes are photoreceptors, i.e. sense light attached to the small protein of about 1,24,000 Daltons in plants, bacteria, and fungi. They regulate the germination of seeds (photoblasty), stem growth, the synthesis of chlorophyll, root growth, pigment synthesis, the size, shape and number, and movement of leaves and flowering. The physiologically active form or ‘signaling’ state of the protein is $P_{fr}$.
-Phytochrome is a blue or bluish-green pigment and is located in most of the organs of seed plants and free-sporing plants, also found in green algae.
-The far-red light converts the $P_{fr}$ to the inactive $P_{r}$ form. In other words red light makes $P_{fr}$, far-red light makes $P_{r}$.
-If $P_{730}$ is exposed to far-red light, it changes quickly back to $P_{660}$​ and if phytochrome $P_{660}$​ is exposed to red-light, it changes to $P_{730}$​ so we say both are in interconvertible form.
-Phytochromes also can make the plant grow towards the light. This is called phototropism.

So, the correct answer is ‘red-light’.

Note:
-If a plant is subjected to a continuous red light then the phytochrome is destroyed.
-The Dalton (symbol Da) is often used as a unit of molecular weight or molecular mass, with the definition 1 Da = 1 g/mol.