Day length sensing
Short-day plants flower when nights exceed critical length (chrysanthemum, soybean). Long-day plants flower when nights are shorter than critical (spinach, Arabidopsis).
Phytochrome mechanism
Night length measurement via phytochrome Pr/Pfr interconversion. Interrupting dark period with flash of light can prevent or trigger flowering depending on species.
Commercial use
Greenhouse growers manipulate photoperiod with blackout curtains and supplemental lighting to schedule flowering for market dates.