CONTROL OF PHOTOSYNTHESIS IN BARLEY LEAVES WITH REDUCED ACTIVITIES OFGLUTAMINE-SYNTHETASE OR GLUTAMATE SYNTHASE .1. PLANT CHARACTERISTICS AND CHANGES IN NITRATE, AMMONIUM AND AMINO-ACIDS

Wild-type and mutant plants of barley (Hordeum vulgare L. cv. Maris Mi nk) lacking activities of chloroplastic glutamine synthetase (GS) and of ferredoxin-dependent glutamate synthase (Fd-GOGAT) were crossed to generate heterozygous plants. Crosses of the F-2 generation containing GS activities between 47% and 97% of the wild-type and Fd-GOGAT activ ities down to 63% of the wild-type have been selected to study the con trol of both enzymes on photorespiratory carbon and nitrogen metabolis m. There were no major pleiotropic effects. Decreased GS had a small i mpact on leaf protein and the total activity of ribulose-1,5-bisphosph ate carboxylase-oxygenase (Rubisco). The activation state of Rubisco w as unaffected in air, but a decrease in GS influenced the activation s tate of Rubisco in low CO2. In illuminated leaves, the amino-acid cont ent decreased with decreasing GS, while the content of ammonium rose, showing that even small reductions in GS limit ammonium re-assimilatio n and may bring about a loss of nitrogen from the plants, and hence a reduction in protein and Rubisco. Leaf amino-acid contents were restor ed, and ammonium and nitrate contents decreased, by leaving plants in the dark for 24 h. The ratios of serine to glycine decreased with a de crease in GS when plants were kept at moderate photon flux densities i n air, suggesting a possible feedback on glycine decarboxylation. This effect was absent in high light and low CO2. Under these conditions a mmonium contents exhibited an optimum and aminoacid contents a minimum at a GS activity of 65% of the wild-type, suggesting an inhibition of ammonium release in mutants with less than 65% GS. The leaf contents of glutamate, glutamine, aspartate, asparagine, and alanine largely fo llowed changes in the total amino-acid contents determined under diffe rent environmental conditions. Decreased Fd-GOGAT resulted in a decrea se in leaf protein, chlorophyll, Rubisco and nitrate contents. Chlorop hyll alb ratios and specific leaf fresh weight were lower than in the wild-type. Leaf ammonium contents were similar to the wild-type and to tal leaf amino-acid contents were only affected in low CO2 at high pho ton flux densities, but mutants with decreased Fd-GOGAT accumulated gl utamine and contained less glutamate.