Fotoperioden

Fotoperioden, or photoperiod, is the length of the daily light period in a 24-hour cycle and acts as a primary seasonal cue for many organisms. The relative duration of light and dark helps determine when processes such as flowering, reproduction, dormancy, and migration should occur. While photoperiod is defined by the hours of light, its biological impact depends on night length and the organism’s internal circadian clock.

In plants, photoperiodism refers to the ability to respond to day length. Plants sense night length through photoreceptors such as phytochromes and cryptochromes, and integrate signals to regulate flowering and growth. They are categorized as short-day, long-day, or day-neutral depending on whether they flower when nights are long, short, or regardless of day length. The molecular basis includes a florigen signal (FT gene) and regulatory networks that connect light perception to developmental transitions.

In animals, photoperiod affects reproductive timing via melatonin secretion by the pineal gland during darkness. Longer nights extend melatonin signals and can suppress or stimulate the reproductive axis depending on the species. Photoperiod also governs seasonal migration, fur growth, and hibernation in some mammals and birds. In humans, photoperiod and circadian rhythms influence sleep, mood, and seasonal affective disorder.

Applications and relevance: Photoperiod management in agriculture and horticulture uses artificial lighting to advance or delay flowering; in animal husbandry to optimize breeding; and researchers study photoperiod responses to understand climate adaptation and ecological dynamics. The concept has evolved since early 20th-century studies of day length signaling in plants and remains a central topic in chronobiology.