Photoperiodism

Photoperiodism is an organism’s response to the relative lengths of day and night. It regulates seasonal processes such as flowering, migration, and reproduction.

Circadian rhythm: An approximately 24-hour cycle in biological activity, generated by an organism’s internal timekeeping system. Circadian timing helps organisms measure the duration of uninterrupted darkness.
Short-day plant: A plant that flowers when nights exceed a species-specific duration. Its flowering is induced by long, uninterrupted nights.
Day length: The interval between sunrise and sunset at a particular place and date. Its seasonal change supplies the environmental timing cue.
Phototaxis: Directional movement of an organism toward or away from a light source. Phototaxis responds to light direction, not seasonal day–night duration.
Phytochrome: A light-sensitive pigment in plants that shifts between forms in response to red and far-red light. Its light-dependent state helps plants track night length.
Long-day plant: A plant that flowers when nights are shorter than a species-specific duration. Its flowering is promoted by short nights or long days.
Seasonality: Regular changes in environmental conditions and biological activity over the course of a year. Photoperiodism is one way organisms anticipate seasonal change.
Phototropism: Directional growth of a plant in response to the direction of light. It concerns growth direction rather than seasonal timing.
Florigen: A mobile flowering signal produced in leaves and transported to a plant’s growing regions. Photoperiodic leaves can send this signal to initiate flowering.
Day-neutral plant: A plant whose flowering is largely independent of day length. It contrasts with plants whose flowering depends on photoperiod.

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