Photoperiodisme

Photoperiodism is the physiological reaction of organisms to the length of day or night. It serves as a time-keeping mechanism that allows many species to anticipate seasonal changes and synchronize development and behavior with the environment. The cue is the photoperiod, the proportion of light and dark within a 24-hour cycle, which is detected by light receptors and integrated with an internal circadian clock.

In plants, photoperiodism governs flowering and dormancy. Plants can be categorized as short-day, long-day, or day-neutral according to the photoperiod required to flower. The process involves photoreceptors such as phytochromes (sensitive to red and far-red light) and cryptochromes, which perceive light signals, and a circadian clock that times night length. A key result is the production of florigen, a flowering signal produced in leaves and transported to the shoot apical meristem to initiate flowering. The concept of a critical photoperiod describes the night length threshold needed for a species to respond. Temperature can modulate the strength and timing of photoperiodic responses.

In animals, photoperiodism influences reproduction, migration, and other seasonal activities through melatonin secretion by the pineal gland, regulated by the light-dark cycle via the hypothalamic clock. Longer or shorter nights alter melatonin duration, signaling winter conditions to seasonal breeders and affecting reproductive cycles, diapause, and migratory behavior. In non-seasonal species, photoperiod can still shape circadian rhythms and metabolic processes.

Historically, the concept was advanced by researchers such as Garner and Allard in the 1920s. Today, photoperiodism informs agriculture and horticulture, enabling manipulation of flowering times, and helps ecologists understand how organisms time their life histories to seasonal cues.