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 Kalanchoe daigremontiana Hamet et Perrier de la Bathie 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 56h the time course of ion: 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.