23 Circadian Rhythms in Arabidopsis thaliana

Abstract

INTRODUCTION
Much of an organism’s physiology, biochemistry, and behavior is temporally organized with respect to the environmental oscillation of day and night. Thus, most organisms express diurnal rhythms. When deprived of environmental time cues, such as light-dark cycles or temperature cycles, many of these rhythms persist, indicating that organisms have the capacity to measure time and to use this time information to temporally regulate aspects of their biology. Circadian rhythms, that subset of endogenous rhythms with periods of approximately 24 hours, have been known for many years, and a great deal of information has been gathered concerning the rhythmic processes of many organisms (Pittendrigh 1981b; Edmunds 1988). Circadian rhythms share certain characteristics. They persist in the absence of environmental time cues. The period of the free-running rhythm is approximately, although not exactly, 24 hours. The deviation of the free-running period from exactly 24 hours has been termed the “strongest single piece of evidence that the overt rhythm is under the control of an endogenous timing mechanism” (Feldman 1982). The period of the rhythm is temperature-compensated; that is, the period remains relatively constant over the range of physiologically relevant temperatures. Note, however, that temperature compensation applies to organisms maintained at one (of several possible) constant temperature and does not imply that temperature changes or cycles are without effect on the clock; indeed, temperature cues and light cues are two of the most potent stimuli that can shift the phase of the clock. The primary effect of the environmental time cues associated...