photoperiodic

Photoperiodic describes physiological responses in organisms that are regulated by the length of day and night. The related concept photoperiodism refers to how organisms detect and respond to photoperiod to time seasonal activities such as flowering, diapause, reproduction, and migration.

In plants, photoperiodic signaling involves light perception by photoreceptors such as phytochromes and cryptochromes, integration with the plant’s circadian clock, and downstream flowering pathways. A key mechanism is the regulation of CONSTANS and FLOWERING LOCUS T, which promotes or suppresses flowering depending on whether the night length fits a species’ critical photoperiod. Long-day plants flower when days are long; short-day plants flower when days are shorter than a threshold. The concept of a critical photoperiod and its seasonal timing emerged from early 20th-century experiments by Garner and Allard.

In animals, photoperiodism often operates through melatonin: nocturnal melatonin secretion reflects night length and influences the neuroendocrine axis that controls reproduction, molt, and migration. The specific responses are species-dependent, with some temperate species breeding when day length increases and others responding to decreasing photoperiods.

Applications and relevance include agriculture and horticulture, where manipulating photoperiod can control flowering and breeding cycles, and ecological management, where artificial lighting and climate change may disrupt natural photoperiodic cues. Ongoing research aims to unravel the molecular clockwork behind photoperiodic responses across plants and animals.