🌿 Photoperiodism

This lesson provides exam-focused context on key concepts in crop physiology and connects the section topics for quick revision.

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Differences between short day and long day plants

Col1	Short day plant	Long day plant
1	Plants flower when photoperiod is less than the critical day length	Plants flower when photoperiod is more than the critical day length
2	Interruption during light period with darkness does not inhibit flowering	Interruption during light period with darkness inhibit flowering
3	Flowering is inhibited if the long dark period is interrupted by a flash of light	Flowering occurs if the long dark period is interrupted by a flash of light
4	Long continuous and uninterrupted dark period is critical for flowering	Dark period is not critical for flowering
5	Flowering does not occur under alternating cycles of short day and short light period.	Flowering occurs under alternating cycles of short day followed by still shorter dark periods

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Phytochrome

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 with 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 . It is

present in the plasma membrane of cells and it has two components, chromophore and

protein. Phytochrome is present in roots, coleoptiles, stems, hypocotyls, cotyledons, petioles,

leaf blades, vegetative buds, flower tissues, seeds and developing fruits of higher plants.

The pigment, phytochrome exists in two different forms i.e., red light absorbing form

which is designated as Pr and far red light absorbing form which is designated as Pfr . 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.

It is considered that during 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.

Reversal of the inhibitory effect of red light during critical dark period in SDP by

subsequent far-red light exposure is because, the Pfr form after absorbing far-red light (730

354 nm) will again be converted back into Pr form.

Prolongation of critical light period or the interruption of the dark period by red- light

in long day plants will result in further accumulation of the Pfr form of the pigment, thus

stimulating flowering in long-day plants.

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Differences between Pr and Pfr forms of phytochrome

Col1	Pr form	Pfr form
1	It is blue green in colour	It is light green in colour
2	It is an inactive form of phytochrome and it does not show phytochrome mediated responses	It is an active form of phytochrome and hence shows phytochrome mediated responses
3	It has maximum absorption in red region (about 680nm)	It has maximum absorption in far-red region (about 730nm)
4	It can be converted into Pfr form in red region (660-665nm)	It can be converted into Pr form in far red region (730-735nm)
5	It is found diffused throughout the cytosol	It is found in discrete areas of cytosol
6	The Pr form contains many double bonds in pyrrole rings	The Pfr form contains rearranged double bonds in all pyrrole rings

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Significance of photoperiodism

Photoperiodism is an example for physiological preconditioning . The stimulus is

given at one time and the response is observed after months. Exposure to longer photoperiods

hastens flowering (E.g). In wheat, the earing is hastened. During long light exposure, Pr form

is converted into Pfr form and flowering is initiated. If dark period is greater, Pfr is converted

into Pfr form that inhibits flowering.

The important phytochrome mediated photo responses in plants include

photoperiodism, seed germination, sex expression, bud dormancy, rhizome formation, leaf

abscission, epinasty, flower induction, protein synthesis, pigment synthesis, auxin

catabolism, respiration and stomatal differentiation.

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Summary Cheat Sheet

• Review each concept section above in sequence to connect definitions, processes, and applied crop-physiology outcomes.
• Focus on high-yield terms, pathways, and condition-dependent responses for exam-ready recall.
• Use the listed examples, comparisons, and cycles as rapid-revision anchors before practice questions.

References

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