Studies on carbon flow in Crassulacean acid metabolism during the initial light period

In the Crassulacean acid metabolism (CAM) plants Kalanchoë tubiflora and Sedum morganianum a shift in the pathways occurs by which external CO2 enters the metabolism during the initial light period (phase II of the diurnal CAM cycle). At the beginning of phase II, CO2 is fixed mainly by the C4 pathway; during late phase II, however, it is fixed mainly via the C3 pathway. The C3 pathway contributes to the phosphoenolpyruvate-carboxylase-mediated CO2 fixation by the provision of three-carbon skeletons. Since the shift in the carbon-flow pathway is delayed after a CO2-free night when malic-acid accumulation in the vacuoles is prevented, it is very likely that the amount of malic acid in the vacuole is integrated in the mechanism which controls CAM during the initial light period. A light-on signal at the beginning of phase II is not required to bring about the shifts in the carbon-flow pathways, as is shown by the reaction of plants to a prolonged dark period. A model of carbon flow during phase II is proposed.