A Re-Examination of the Role of the Nucleus in Generating the Circadian Rhythm in Acetabularia

The role of the nucleus in the generation of the circadian rhythm in Acetabularia has been unclear. Early experiments showed that the plant could exhibit a circadian rhythm in the absence of a nucleus. However, other experiments appeared to show that the nucleus could impart phase information to the rhythm, and so therefore must be a part of the system that generates the rhythm. We have conducted experiments similar to these—in particular, one in which the nuclear end of the plant was entrained on a light-dark cycle that was opposite that of the rest of the plant. The phase of the free running rhythm of this type of plant is not consistent with the conclusion that the nucleus is part of the circadian oscillator. We have also tried entraining opposite ends of plants with no nuclei on opposite light-dark cycles. The ultimate phases of these plants appear to be nearly random. A possible interpretation of these experiments is discussed.