Photoperiodism — Complete Key Points
Definition: Response of plants to the relative duration of light and dark periods; specifically controls the timing of flowering.
Critical Signal: The length of the UNINTERRUPTED DARK period (not the light period). SDPs are really "long-night plants."
Three Categories:
| Type | Requirement | Examples |
|---|---|---|
| Short-Day Plants (SDP) | Night > critical night length | Rice, Chrysanthemum, Tobacco, Xanthium, Soybean |
| Long-Day Plants (LDP) | Night < critical night length | Wheat, Radish, Spinach, Henbane (Hyoscyamus) |
| Day-Neutral Plants (DNP) | No photoperiod requirement | Tomato, Cucumber, Sunflower, Cotton |
Phytochrome:
- Photoreceptor protein with two forms:
- Pr (red-absorbing, ~660nm): INACTIVE; accumulates in dark
- Pfr (far-red absorbing, ~730nm): ACTIVE; accumulates in light
- Red light: Pr → Pfr (rapid)
- Far-red / dark: Pfr → Pr (dark reversion, slow)
Phytochrome Roles:
- In LDP: Pfr PROMOTES flowering (accumulated during long days)
- In SDP: Pfr INHIBITS flowering
Night-Break Experiment:
- Brief red light flash in the dark period → Pr → Pfr → SDP does NOT flower
- Far-red flash reverses red light effect → Pfr → Pr → SDP DOES flower
- Proves: Uninterrupted dark period is critical; phytochrome measures darkness duration
Florigen:
- Hypothetical flower-promoting substance (now identified as FT protein)
- Produced in LEAVES (site of photoperiod perception)
- Transported via PHLOEM to SHOOT APICAL MERISTEM
- Triggers meristem transition from vegetative to reproductive
Manipulation:
- SDP: Black cloth to extend dark period (greenhouse chrysanthemums)
- LDP: Artificial lighting to extend day length
- Night-break: Brief light to prevent SDP flowering in long nights