ON THE MECHANISM OF REINITIATION OF ENDOGENOUS CRASSULACEAN-ACID-METABOLISM RHYTHM BY TEMPERATURE-CHANGES

Under continuous light the endogenous Crassulacean acid metabolism (CA M) rhythm of Kalanchoe daigremontiana Hamet et Perrier de la Bathie di sappears at high (>29.0 degrees C) or low (<8.0 degrees C) temperature s. We investigated the reinitiation of rhythmicity when temperature wa s reduced from above the upper and increased from below the lower thre shold level via measurements of (a) short-term changes in carbon-isoto pe discrimination to illustrate shifts between C-3 and C-4 carboxylati on in vivo, and (b) the malate sensitivity of phosphoenolpyruvate carb oxylase (PEPC) in vitro. When the net CO2-exchange rhythm disappears a t both temperatures, the instantaneous discrimination indicates low PE PC activity. Leaf malate concentration and osmolarity attain high and low values at low and high temperatures, respectively. After small tem perature increases or reductions from the low and high temperatures, r espectively, the rhythm is reinitiated, with phases shifted by 180 deg rees relative to each other. This can be related to the contrasting lo w and high leaf malate concentrations due to direct inhibition of PEPC and possibly also of the phosphorylation of PEPC by malate. The exper imental results were satisfactorily simulated by a mathematical CAM-cy cle model, with temperature acting only on the passive efflux of malat e from the vacuole. We stress the important role of the tonoplast in m alate compartmentation and of malate itself for the reinitiation and g eneration of endogenous CAM rhythmicity.