Temperature profiles for the expression of endogenous rhythmicity and arrhythmicity of CO2 exchange in the CAM plant Kalanchoe daigremontiana can be shifted by slow temperature changes

The crassulacean acid metabolism (CAM) plant Kalanchoe daigremontiana Hamet et Perrier de la Bathie shows an endogenous circadian rhythm of net CO2 exchange (J(CO2)) under constant conditions in continuous light. Previous studies have shown, however, that above a certain threshold temperature J(CO2) changes from rhythmic to arrhythmic behaviour and that this is reversible when the temperature is lowered again. It is now demonstrated here, that this re-initiation of rhythmic J(CO2) from arrhythmicity needs a sufficiently strong temperature signal as defined by its abruptness. Rhythmicity reappears only if the temperature is reduced rather rapidly. If the temperature is reduced slowly then arrhythmicity is retained even at a low temperature level which normally would allow rhythmicity. Under these circumstances, however, a distinct temperature increase followed by an abrupt temperature decrease immediately elicits regular oscillations of J(CO2) at this lower temperature. We suggest that the strong temperature signals function as a definite synchronizer ("zeitgeber") which synchronizes different cells and/or different leaf areas which remain desynchronized after application of only slow temperature changes. This is further supported by Fourier transform analyses, revealing a harmonic structure of the superficially arrhythmic time series of J(CO2) after application of slow temperature reductions. This conclusion adds a spatial dimension to the otherwise purely time-dependent rhythmicity and arrhythmicity of J(CO2) in CAM.