47. Light signals for flowering is received by
    (1) Flower bud (2) Apical bud
    (3) Leaves (4) Flower bract

---

Introduction

Plants rely on environmental cues like light duration to time their flowering, a process called photoperiodism. Though flowering occurs at the shoot apex, it is the leaves that detect the light signals, integrating photoperiod information and producing florigen, a mobile flowering signal that travels to the shoot apical meristem to induce flowering.

Leaves Detect Light Signals

• Leaves harbor photoreceptors such as phytochromes and cryptochromes that sense changes in day length, which regulate gene expression controlling flowering time.

• The FLOWERING LOCUS T (FT) gene in leaves encodes florigen, a protein that moves from leaves to the shoot apex triggering floral development.

• Experiments with partially illuminated plants show that exposure of leaves to inductive photoperiod is sufficient to induce flowering even if the shoot apex is shaded, confirming leaves’ role as the primary photoreceptor site.

Photoperiodic Signal Transduction

• Light signals activate photoreceptors in leaves, which regulate CONSTANS (CO) gene expression via the circadian clock.

• CO protein promotes FT transcription in leaves during inductive photoperiods, launching the flowering process through florigen transport.

• Florigen moves via the phloem to the shoot apical meristem, converting it into a floral meristem to start flower formation.

Contrasts with Other Plant Parts

• Flower buds and shoot apical buds do not themselves perceive photoperiodic light signals directly but respond to systemic signals from leaves.

• Flower bracts have no known role in perceiving light signals for flowering induction.

---

Summary Table: Sites of Photoperiodic Signal Reception

---

Conclusion

Leaves are the principal organs detecting light signals for flowering induction in plants by integrating photoperiod and circadian information, producing florigen to stimulate floral development at the shoot apex.

---

FAQ