The Path of Carbon in CAM Plants at Night

Abstract

Following demonstrations that malic acid synthesis in crassulacean acid metabolism (CAM) plants at night requires only the carboxylation of phosphoenolpyruvate, glycolysis has been presumed to supply this substrate. This study has examined the path of carbon in CAM plants at night to establish the involvement of intermediary pools of metabolites in other processes, and to define characteristics of the pathway which are likely to be involved in its regulation. In both Bryophyllum tubiflorum Barv. and Kalanchoe daigremontiana Hamet et Perrier, carbon loss from starch was insufficient to account for malic acid gains. Quantitative examination of the pool sizes of various carbohydrates and malic acid during the dark showed that starch was only part of a larger glucan pool which was the carbon source material for phosphoenolpyruvate supply. Observations of the utilization of 14C-labelled glucan in a CAM plant in the dark supported the role of this compound as primary carbon donor for acid synthesis. The mainstream of carbon flow was shown not to pass through the free sugar pools, and it was also proposed that malic and citric acids were able to exchange carbon via tricarboxylic acid cycle reactions. Examination of the influence of temperature on the glucan-malic acid relationship suggested that the depressed malic acid levels accumulated after nights at higher temperature were due to regulation of the reactions concerned with acid synthesis from glucan.