Frequency Resonances of the Circadian Rhythm of CAM Under External Temperature Rhythms of Varied Period Lengths in Continuous Light*

Net CO₂ exchange (J_co2) of leaves of the CAM plant Kalanchoë daigremontiana Hamet et Perrier de la Bǎthie was studied in continuous light under the influence of temperature jumps of different period lengths. Each period consisted of two equal time intervals at a lower and a higher temperature, i.e. ?24 and ?29 °C respectively. At a period length of the imposed external temperature rhythm of 8 h the free running endogenous rhythm of J_co2 with a period length of 21.7 h was expressed. Under external rhythms of 12–28 h period length there was entrainment of J_co2. Under external rhythms of 40 and 56 h the time course of J_co2 became arrhythmic. In power spectra of the J_co2 time series obtained by Fourier analysis, besides the dominant frequency, there were other frequency peaks with lower Fourier amplitudes of J_co2. It was noted that these are natural number multiples of the major frequencies. A comparison shows that these frequencies correspond to the harmonic overtones of the basic oscillation of the free running endogenous rhythm of J_co2 at 24°C, viz. 1/22.5 h^?1. The overall power of the enforced oscillations shows a typical resonance behaviour around the natural, free running frequency. The intriguing observation that external temperature jump rhythms lead to resonance, not only of the basic oscillation but also of its harmonic overtones, is discussed in relation to earlier findings suggesting the occurrence of paths between order and deterministic chaos in the CAM rhythm.