Question

What will be the effect on phytochrome in a plant subjected to a continuous red light?
(a) Phytochrome synthesis will increase
(b) The level of Phytochrome will decrease
(c) Phytochrome will be destroyed
(d) First (b) and (a)

Answer 1

Hint: Phytochrome is a pigment which induces morphological changes in the plant upon exposure to light. It does so by the interconversion of its two reversible forms upon absorption of a particular wavelength of light.

Complete answer:
Phytochrome is a pigment whose structure changes upon absorption of light. It absorbs red and far-red light of the absorption spectrum and causes photomorphogenesis. It is a set of morphological changes that can be observed in plants upon exposure to light of certain wavelengths. These changes involve flowering, chloroplast development (but not chlorophyll synthesis), leaf senescence followed by its abscission, seed germination, and also stem elongation to name a few. The phytochrome pigment exists in two reversible forms in nature, whose reversibility depends on the wavelength of light available and absorbed by the plants. Continuous exposure of red light will lead to the conversion of phytochrome and thus its levels are decreased. The decrease will be maintained by its synthesis initiation.
So, the correct answer is ‘First (b) and (a).’

Additional Information:
 In phytochromes, a pigment molecule called chromophore is covalently attached to two polypeptides of 125 kDa each. The two reversible forms of phytochrome are Pr and Pfr. Pr form absorbs red light in the wavelength of 666nm and the other one, Pfr form absorbs far-red light in the wavelength of 730 nm. The Pr form absorbs red light and it is converted to Pfr form. The Pfr form absorbs far-red light and is converted to Pr form. It can also spontaneously revert to the Pr form in the dark over time.
The photoperiodic response is mediated by phytochromes. Depending on this photoperiodic response, there can be short-day plants and long-day plants.

Note:
Short day plants are also called long night plants because of their requirement of a continuous uninterrupted critical dark period. If it is interrupted by even a flash of light or red light of 660nm in this continuous darkness, then flowering is inhibited in such plants. On the other hand, long day plants demand a shorter period of darkness which is lower than a critical dark period. Even if the dark period is exposed to any light, it does not affect the flowering. Rather, it promotes flowering.