flowchart LR PR["Pr\n(Red-absorbing form)\nInactive\nλ = 660 nm"] PFR["Pfr\n(Far-red-absorbing form)\nActive — promotes flowering\nλ = 730 nm"]

PR -->|"Red light\n660 nm"| PFR
PFR -->|"Far-red light\n730 nm"| PR
PFR -->|"Darkness / slow\nreversion"| PR
PFR -->|"Biological\nresponse"| R["Seed germination\nFlowering\nDe-etiolation"]

### Phytochrome Interconversion Diagram

            RED LIGHT (660nm)
    ┌─────────────────────────────►
    │                              │
   Pr                             Pfr

(red-absorbing, (far-red absorbing, inactive form, ACTIVE FORM, ~660nm peak) ~730nm peak) │ │ ◄─────────────────────────────┘ FAR-RED LIGHT (730nm) or DARK (slow dark reversion)

### Functional Outcomes of Pfr

| Pfr Level | Effect in LDP | Effect in SDP |
|---|---|---|
| HIGH (long days) | Promotes flowering | Inhibits flowering |
| LOW $\frac{long nights}{dark reversion}$ | Inhibits flowering | Promotes flowering |

### Night-Break Experiment Summary

| Treatment | Phytochrome Status | SDP Result | LDP Result |
|---|---|---|---|
| Long night (no interruption) | Pfr → Pr (dark reversion) | Flowers | No flowers |
| Long night + red flash | Pfr accumulated | No flowers | Flowers |
| Long night + red then far-red | Pfr → Pr (FR reversal) | Flowers | No flowers |

### Key Fact for NEET
Phytochrome mediates:
- Photoperiodism (flowering)
- Seed germination (light requirement in small seeds)
- De-etiolation (Pfr promotes chloroplast development)
- Shade avoidance (R:FR ratio sensing)