Comparison Note: SDP vs LDP vs DNP

Master Comparison Table

Feature	Short-Day Plants (SDP)	Long-Day Plants (LDP)	Day-Neutral Plants (DNP)
Definition	Flower when night > critical night length	Flower when day > critical day length	Flower regardless of photoperiod
Critical signal	Long uninterrupted DARK period	Short night period	Internal developmental age
Pfr effect	Pfr INHIBITS flowering	Pfr PROMOTES flowering	Pfr irrelevant to flowering
Examples	Rice, Chrysanthemum, Tobacco, Xanthium, Soybean	Wheat, Radish, Spinach, Henbane (Hyoscyamus)	Tomato, Cucumber, Sunflower, Cotton
Night-break effect	Red flash prevents flowering	Red flash promotes flowering	No consistent effect
Season of flowering	Autumn/winter (short days = long nights)	Spring/summer (long days = short nights)	Any season when mature
Manipulation method	Black cloth to extend night	Artificial light to extend day	Not needed
Agricultural example	Manipulate rice flowering seasons	Winter wheat needs natural long days	Tomato in greenhouses year-round

Mnemonic

SDP: "Crops and Rice Stay Short-day" — Chrysanthemum, Rice, Tobacco, Xanthium, Soybean
LDP: "Wheat and Radish Love (Long) Light" — Wheat, Radish, Spinach, Henbane
DNP: "TCS doesn't care" — Tomato, Cucumber, Sunflower

Phytochrome Roles Summary

Light (day) → Pr → Pfr (accumulates)
Dark (night) → Pfr → Pr (dark reversion, slow)

LDP: Pfr → PROMOTES flowering (long days = high Pfr = flower)
SDP: Pfr → INHIBITS flowering (long nights = low Pfr after dark reversion = flower)