Mechanism of photoperiodism

a) Phytochrome theory

• Phytochrome is a homodimer: two identical protein molecules each conjugated to a light absorbing molecule (compare rhodopsin).
• Plants make 5 phytochromes: PhyA, PhyB, as well as C, D, and E.
• Plants make such adjustments by utilizing the pigment phytochrome, which exists in two forms: Pr, which absorbs red light, and Pfr, which absorbs far-red light. Each can convert to the other when they absorb light. During the day, the two forms convert back and forth (Pr becomes Pfr, and vice versa), until they reach an equilibrium of 60:40 Pfr: Pr in plant tissues. During the night, Pfr slowly converts to Pr or else disintegrates. Pr is stable in the dark.

• Phytochromes exist in two interconvertible forms

o PR because it absorbs red (R; 660 nm) light;

o PFR because it absorbs far red (FR; 730 nm) light.

• These are the relationships:

o Absorption of red light by PR converts it into PFR.

o Absorption of far red light by PFR converts it into PR.

o In the dark, PFR spontaneously converts back to PR.

• Pr usually stimulate flowering where as Pfr checks it.
• Pr promotes flowering in SDP and Pfr in LDP.
• Pr+Pfr promotes flowering in both SDP and LDP.
• Pr+Pfr+Pr inhibit flowering
• Pr+Pfr+Pr+Pfr promotes flowering.
• Pfr+Pr no flowering in SDP but promotes in LDP.

b) Hormonal theory ( Long, Brain, Naylor)

According to this theory CO2 produce a precursor and leads to gibberlin like hormone during appropriate light condition. During night it gets converted back to the precursor. Pr and Pfr play a role in the formation of gibberlin like hormone for flower induction. CO2 precursor GA like hormone flower induction

Florigen

• Because hormones control so many metabolic activities in plants, flowering has long seemed likely to be under the control of one or more hormones. Early experiments sought to determine which part of a plant is sensitive to the light that initiates flowering.
• The results suggested the presence of a substance that moved from the leaves to the flower buds. Although the substance was not identified then—nor has it been isolated now—it was named florigen.
• Florigen is the hypothetical flowering hormone; it may or may not actually exist. Note that flowering most likely is not controlled by a single hormone, but is the result of a combination of internal and external signals and responses.

Theory of endogenesus rhythm

• Photophilous phase: light is essential. Anabolism is dominant and catabolism is less or weak hence stimulates the initiation of flowering.
• Scotophilous phase: dark is essential where the catabolism is dominant and anabolism is weak or less hence inhibiting flowering.