Responses of a plant circadian rhythm to thermoperiodic perturbations with asymmetric temporal patterns and the rate of temperature change

The circadian CO2 exchange rhythm of the CAM plant Kalanchoe daigremontiana Hamet et Perrier de la Bathie in continuous light has been exposed to various thermoperiodic perturbation patterns. Under a fixed thermocycle period of 16-h the time of temperature exposure has been varied, yielding asymmetric temporal patterns of the perturbation. The data are analyzed considering frequency spectra and a correlation analyses of temperature and gas exchange time series. It is shown that short high-temperature patterns give rise to a significant contribution of the plant's circadian rhythm in the response of the CAM plant to the thermoperiodic perturbation. Comparison with numerical simulations using a theoretical model of CAM dynamics suggests that the experimental data can be understood as emerging from a continuous transition between free running oscillations and full synchronisation to the external driver. Furthermore, evidence for an acute response of the plant to the rate of temperature change is provided. It is proposed that the absolute value and the rate of change of temperature, respectively, should be considered as independent experimental variables.