Responses to different external light rhythms by the circadian rhythm of Crassulacean acid metabolism in Kalanchoe daigremontiana

Time series of net CO2 exchange (J(CO2)) and leaf conductivity for water va pour (g(H2O)) were measured and subsequently analysed mathematically in the Crassulacean acid metabolism (CAM) plant Kalanchoe daigremontiana (Hamet e t Perrier de la Bathie) under constant environmental conditions and under i mposed external rhythms of lower and higher light intensity. The time serie s were analysed by Fourier methods and a correlation analysis considering t he first time derivatives of J(CO2),g(H2O) and photosynthetically active ph oton flux density (PPFD). The ratio of internal to external CO2 (c(i)/c(a)) was also considered in the analysis, leading to a discussion of the intera ction of stomata and carbon assimilation under periodic stimulation. It is suggested that for stimulation with frequencies close to the endogeneous ci rcadian period, stomatal conductance and carbon assimilation oscillate sync hronously, guard-cell movements trailing behind changes in internal CO2 wit h a delay of 10-15 min. For stimulation frequencies far shorter than the en dogeneous period, this synchrony can be disturbed due to independent respon ses of stomata and assimilation to light pulses, leading to an arrhythmic g as exchange pattern. These results are discussed in the context of understa nding circadian oscillations as the output of a multioscillator, multisigna lling pathway system on the organismic and metabolic level.