Phytochrome: Definition, Mechanism, Role, Functions MCQ - Practice Questions with Answers

• It is observed that that a brief exposure with red light during critical dark period inhibits flowering in a short day plant and this inhibitory effect can be reversed by a subsequent exposure to far-red light.
• Similarly, prolongation of the critical light period or the interruption of the dark period stimulates flowering in long-day plants.
• This inhibition of flowering in short day plant and stimulation of flowering in long day plants involves the operation of a proteinaceous pigment called phytochrome. 
• Phytochromes are the chemicals which perceive the photoperiodic stimulus.
• The wavelength of light is perceived by the leaves. 
• A defoliated plant does not flower.
• A single leaf is sufficient to stimulate the flowering.
• Phytochrome is present in roots, coleoptiles, stems, hypocotyls, cotyledons, petioles, leaf blades, vegetative buds, flower tissues, seeds and developing fruits of higher plants. 

Forms of Phytochromes:

• The pigment, phytochrome exists in two different forms:
• red light absorbing form which is designated as Pr. it is the inactive form.
• far red light absorbing form which is designated as Pfr. it is the active form.
• These two forms of the pigment are photo chemically inter convertible.
• When Pr form of the pigment absorbs red light (660-665 nm), it is converted into Pfr form. 
• When Pfr form of the pigment absorbs far red light (730-735 nm), it is converted into Pr form. 
• The Pfr form of pigment gradually changes into Pr form in dark.

Effect of Phytochromes

• It is considered that during the day time, the Pfr form of the pigment is accumulated in the plants which are inhibitory to flowering in short day plants but is stimulatory in long day plants. 
• During critical dark period in short day plants, this form gradually changes into Pr form resulting in flowering. 
• A brief exposure with red light will convert this form again into Pfr form thus inhibiting flowering in short day plant.

Difference between Pr and Pfr forms:

Some Phytochrome Mediated Photo responses in Plants are as follows:

1. Photoperiodism
2. Seed germination
3. Elongation of leaf, petiole, stem
4. Hypocotyl hook unfolding
5. Unfolding of the grass leaf
6. Sex expression
7. Bud dormancy
8. Plastid morphology
9. Plastid orientation
10. Rhizome formation
11. Bulb formation
12. Leaf abscission
13. Epinasty
14. Succulency
15. Enlargement of cotyledons
16. Hair formation along cotyledons
17. Formation of leaf primordia