Coordination of Plant Metabolism and Development by the Circadian Clock.

Many plant cellular activities occur with a daily rhythmicity. In some cases, the rhythmicity of these cellular activities is maintained in plants growing under constant environmental conditions, such as continuous light (LL) or darkness (DD) and constant temperature. Because rhythms can persist in the absence of externa1 time cues (known as free-running conditions), they must be driven by an internal oscillator. This oscillator, which is known as the circadian clock, generates circadian rhythms. For the circadian clock to regulate rhythms such that they occur at the correct time of day throughout the year, it must be able to perceive the seasonal changes in day length. This adjustment of the clock is known as entrainment, and the environmental cues that are perceived are called Zeitgebers, from the German word meaning "time giver." Thus, the circadian clock can be considered to be an internal processor of temporal inputs from the environment (such as light and temperature). Output from the processor regulates the timing of metabolic and developmental events within the plant.