Perturbations of malate accumulation and the endogenous rhythms of gas exchange in the Crassulacean acid metabolism plant Kalanchoë daigremontiana: testing the tonoplast-as-oscillator model.

In continuous light, leaves of the Crassulacean acid metabolism (CAM) plant Kalanchoë daigremontiana Hamet et Perrier exhibit a circadian rhythm of CO2 uptake, stomatal conductance and leaf-internal CO2 pressure. According to a current quantitative model of CAM, the pacemaking mechanism involves periodic turgor-related tension and relaxation of the tonoplast, which determines the direction of the net flux of malate between the vacuole and the cytoplasm. Cytoplasmic malate, in turn, through its inhibitory effect on phospho enolpyruvate carboxylase, controls the rate of CO2 uptake. According to this mechanism, when the accumulation of malate is disrupted by removing CO2 from the ambient air, the induction of a phase delay with respect to an unperturbed control plant is expected. First, using the mathematical model, such phase delays were observed in numerical simulations of three scenarios of CO2 removal: (i) starting at a trough of CO2 uptake, lasting for about half a cycle (ca. 12 h in vivo); (ii) with the identical starting phase, but lasting for 1.5 cycles (ca. 36 h); and (iii) starting while CO2 increases, lasting for half a cycle again. Applying the same protocols to leaves of K. daigremontiana in vivo did not induce the predicted phase shifts, i.e. after the end of the CO2 removal the perturbed rhythm adopted nearly the same phase as that of the control plant. Second, when leaves were exposed to a nitrogen atmosphere for three nights prior to onset of continuous light to prevent malate accumulation, a small, 4-h phase advance was observed instead of a delay, again contrary to the model-based expectations. Hence, vacuolar malic acid accumulation is ruled out as the central pacemaking process. This observation is in line with our earlier suggestion [T.P. Wyka, U. Lüttge (2003) J Exp Bot 54:1471-1479] that in extended continuous light, CO2 uptake switches gradually from a CAM-like to a C3-like mechanism, with oscillations of the two CO2 uptake systems being tightly coordinated. It appears that the circadian rhythm of gas exchange in this CAM plant emerges from one or several devices that are capable of generating temporal information in a robust manner, i.e. they are protected from even severe metabolic perturbations.