A mutant line, RPr79/2, of barley (Hordeum vulgareL. cv. Maris Mink) has been isolated that has an apparent defect in photorespiratory nitrogen metabolism. The metabolism of14C-labelled glutamine, glutamate and 2-oxoglutarate indicates that the mutant has a greatly reduced ability to synthesise glutamate, especially in air, although in-vitro enzyme analysis indicates the presence of wild-type activities of glutamine synthetase (EC 6.3.1.2) glutamate synthase (EC 1.4.7.1 and EC 1.4.1.14) and glutamate dehydrogenase (EC 1.4.1.2). Several characteristics of RPr79/2 are very similar to those described for glutamate-synthase-deficient barley andArabidopsis thalianamutants, including the pattern of labelling following fixation of14CO2, and the rapid rise in glutamine content and fall in glutamate in leaves on transfer to air. The CO2-fixation rate in RPr79/2 declines much more slowly on transfer from 1 O2to air than do the rates in glutamate-synthase-deficient plants, and RPr79/2 plants do not die in air unless the temperature and irradiance are high. Analysis of (glutamine+NH3+2-oxoglutarate)-dependent O2evolution by isolated chloroplasts shows that chloroplasts from RPr79/2 require a fivefold greater concentration of 2-oxoglutarate than does the wild-type for maximum activity. The levels of 2-oxoglutarate in illuminated leaves of RPr79/2 in air are sevenfold higher than in Maris Mink. It is suggested that RPr79/2 is defective in chloroplast dicarboxylate transport.
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