Photorespiratory Enzyme Activities in C3and CAM Forms of the Facultative CAM Plant,Mesembryanthemum crystallimumL.

The extent to which photorespiration occurs in Crassulacean Acid Metabolism (CAM) plants has received limited attention. No comparative studies of C3 and CAM development have been made. To address this problem, activities of several photorespiratory enzymes were measured in a facultative CAM plant—Mesembryanthemum crystallinum L.—during induction of CAM by water stress (NaCl-treatment). Salt-treatment over a 22 d period produced a progressive change in metabolism from C3 to CAM. This was confirmed by (I) changes in gas exchange from C3 fixation to the characteristic CAM pattern of nocturnal CO2 uptake; (2) increases in did acid fluctuation and (3) a 30-fold increase in phosphoenol pyruvate (PEP) carboxylase activity. In contrast, no significant changes were observed in the activities of glycollate oxidase (GO), NAD-dependent-hydroxypyruvate reductase (HPR), glutamine synthetase (GS) or glutamate dehydrogenase (GDH) when plants were induced into CAM. Ion exchange chromatography on DEAE Sephacel detected only one GS isoenzyme (GS2, chloroplastic) at all stages of CAM induction, Western blot analysis, however, detected an additional, although minor, band of GS1 (cytosolic), in C3 plants, which disappeared following CAM induction. Our results show that, after development of CAM, these plants still retain the capacity to photorespire. This may be an essential requirement of CAM plants growing in habitats with variable rainfall. When water availability is high, stomata may open in the light allowing ribulose bisphosphate carboxylase oxygenase (Rubisco) activity and photorespiration to occur. The inherent capacity to photorespire would allow plants to survive variable periods of rainfall.