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 stimulates 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 has no flowering in SDP but promotes in LDP.

b) Hormonal theory ( Long, Brain, Naylor)

According to this theory, CO2 produces a precursor and leads to gibberellin-like hormone during appropriate light conditions. During the night it gets converted back to the precursor. Pr and Pfr play a role in the formation of gibberellin-like hormones for flower induction. CO2 precursor-likee hormone flower induction