The appearance of a new molecular species of phosphoenolpyruvate carboxylase (PEPC) and the rapid induction of CAM in Sedum telephium L.

Abstract. When detached leaves of Sedum telephium are incubated in the absence of water, a rapid switch from C₃ photosynthesis to CAM (as indicated by the onset of day-to-night fluctuations in titratable acidity. ΔH⁺) occurs within the first dark period. The C₃-CAM switch in intact plants occurs within 3 5d. Extractable activity of phospho enol pyruvate carboxylase (PEPC) increases five-fold in intact plants during CAM induction; however, during rapid CAM induction in detached leaves, there is only a very small increase in PEPC activity. Fractionation by anion exchange chromatography of crude extracts from leaves of intact plants subjected to water deficit shows that CAM induction is associated with the appearance of a molecular species of PEPC termed PEPC I. PEPC I is barely detectable in well-watered plants which are not performing CAM. The major form in these plants is termed PEPC II. In leaves from intact plants, there is a significant positive correlation between PEPC I activity and ΔH⁺ during a period of increasing water deficit. PEPC I exhibits day to night fluctuations in malate sensitivity, being less sensitive during the dark period. In contrast, PEPC II is more sensitive to inhibition by malate and has no day to night fluctuation in sensitivity. In detached leaves deprived of water, a small increase in PEPC I capacity is detected at the end of the first dark period (20 h after the start of treatment). The results suggest that PEPC I is required for attainment of maximum nocturnal malic acid synthesis. There is a significant correlation between leaf water status (relative water content), ΔH⁺, total PEPC and PEPC I activity suggesting that the internal water status of the plant may be a trigger for CAM induction. Abscisic acid applied to detached leaves does not cause nocturnal acidification.