photoperiodism

Biological mechanism that determines the timing of certain activities by responding to changes in day length. The flowering of many plants is initiated in this way. Photoperiodism in plants is regulated by a light-sensitive pigment, phytochrome. The breeding seasons of many temperate-zone animals are also triggered by increasing or declining day length, as part of their biorhythms.

Autumn-flowering plants (for example, chrysanthemum and soybean) and autumn-breeding mammals (such as goats and deer) require days that are shorter than a critical length; spring-flowering and spring-breeding ones (such as radish and lettuce, and birds) are triggered by longer days.