Ammonia Fixation via Glutamine Synthetase and Glutamate Synthase in the CAM Plant Cissus quadrangularis L

Succulent stems of Cissus quadrangularis L. (Vitaceae) contain glutamine synthetase, glutamate synthase, and glutamate dehydrogenase. The CO₂ and water gas exchanges of detached internodes were typical for Crassulacean acid metabolism plants. During three physiological phases, e.g. in the dark, in the early illumination period after stomata closure, and during the late light phase with the stomata wide open, ¹⁵NH₄Cl was injected into the central pith of stem sections. The kinetics of ¹⁵N labeling in glutamate and glutamine suggested that glutamine synthetase was involved in the initial ammonia fixation. In the presence of methionine sulfoximine, an inhibitor of glutamine synthetase, the incorporation of ¹⁵N derived from ¹⁵NH₄Cl was almost completely inhibited. Injections of amido-¹⁵N glutamine demonstrated a potential for ¹⁵N transfer from the amido group of glutamine into glutamate which was suppressed by the glutamate synthase inhibitor, azaserine. The evidence indicates that glutamine synthetase and glutamate synthase could assimilate ammonia and cycle nitrogen during all phases of Crassulacean acid metabolism.