CONTROL OF PHOTOSYNTHESIS IN BARLEY MUTANTS WITH REDUCED ACTIVITIES OF GLUTAMINE-SYNTHETASE AND GLUTAMATE SYNTHASE .3. ASPECTS OF GLYOXYLATE METABOLISM AND EFFECTS OF GLYOXYLATE ON THE ACTIVATION STATE OF RIBULOSE-1,5-BISPHOSPHATE CARBOXYLASE-OXYGENASE

Heterozygous mutants of barley (Hordeum vulgare L. cv. Maris Mink) wit h decreased activities of chloroplastic glutamine synthetase (GS) betw een 97 and 47% of the wild type and ferredoxin-dependent glutamate syn thase (Fd-GOGAT) down to 64% of the wild type have been used to study aspects of glyoxylate metabolism and the effect of glyoxylate on the a ctivation state of ribulose-1,5-bisphosphate carboxylase/oxygenase (Ru bisco) in vivo. In the leaf, the extractable activities of serine:glyo xylate aminotransferase decreased with a decrease in GS whereas activi ties of glutamate and alanine:glyoxylate aminotransferase increased, p ointing to a re-direction of amino donors from serine to glutamate and alanine. Under ambient conditions, the leaf contents of glutamate and alanine declined continuously with a decrease in GS, in parallel with the decrease in total amino acids. Glycine, serine and asparagine con tents decreased with a decrease in GS to approximately 70% of the wild type, but increased again with a further decrease in GS. At high irra diances and at low CO2 concentrations, glyoxylate contents exhibited a pronounced minimum between 60% and 80% GS. With a further decrease in GS, glyoxylate contents recovered and approached values similar to th e wild type. The activation state of Rubisco showed a negative correla tion with glyoxylate contents, indicating that a decrease in GS feeds back on the first step of carbon assimilation and photorespiration. Th e activation state of stromal fructose-1,6-bisphosphatase was unaffect ed by a decrease in GS or Fd-GOGAT, whereas the activation state of NA DP-dependent malate dehydrogenase changed in a complex manner. The CO2 - photocompensation point, Gamma, was appreciably increased in mutant s with 47% GS. 'Mitochondrial respiration' in the light (R(d)) was red uced with a decrease in GS. Relative rates of CO2 release into CO2-fre e air between the wild type and the 47%-GS mutant correlated with dete rminations of Gamma. These data are consistent with the view that whe n GS is decreased there is an increased oxidative decarboxylation of g lyoxylate resulting from a decreased availability of amino donors for the transamination of glyoxylate to glycine, and that when GS activiti es are lower than 70% of the wild type an additional mechanism operate s to reduce the photorespiratory loss of ammonia.