The Physiology and Molecular Bases of the Plant Circadian Clock

Periodicity in biology comes in many shapes and sizes. Among the very rapid are ultradian rhythms in the courtship song of the Drosophila male, which recur every 50 to 60 s (Edmunds, 1988). Running at a slightly faster pace is the glycolytic oscillator in yeast, which exhibits selfsustained oscillations in NADH fluorescence. These are typically between 2 and 70 min long, though periods of up to 6 h have been obtained by manipulating metabolite concentrations (Edmunds, 1988). Hypocotyl circumnutations with period lengths ranging from 25 min to 8 h are part of the process of stem elongation in Arabidopsis and Sinapis (Engelmann and Johnsson, 1998).