Orientation in time: photoperiodism

Abstract

Obvious features of plant behaviour are its periodicity and synchrony with environmental conditions - different activities are associated with particular times of the day and with certain seasons. What is not so obvious is that much of this periodicity is not imposed directly by the environmental conditions at that time. Plants, like all other eukaryotes, have the ability to anticipate many changes in the environment, inasmuch as certain signals induce sequences of development or patterns of behaviour which prepare the organism for future conditions. This applies both to daily and to seasonal cycles of change: plants seem to have both a clock and a calendar. Light is strongly involved in each of these aspects of time measurement.