Assimilation of Nitrogen in Mutants Lacking Enzymes of the Glutamate Synthase Cycle

N labelling was used to investigate the pathway of nitrogen assimilation in photorespiratory mutants of barley (Hordeum vulgare cv. Maris Mink), in which the leaves have low levels of glutamine synthetase (GS) or glutamate synthase, key enzymes of ammonia assimilation. These plants grew normally when maintained in high C02, but the deletions were lethal when photorespiration was initiated by transfer to air. Enzyme levels in roots were much less affected, compared to leaves, and assimilation of labelled nitrate into amino acids of the root showed very little difference between wild type and mutants. Organic nitrogen was exported from roots in the xylem sap mainly as glutamine, levels of which were somewhat reduced in the GS deficient mutant and enhanced in the glutamate synthase deficient mutant. In the leaf, the major effect was seen in the glutamate synthase mutant, which had an extremely limited capacity to utilize the imported glutamine and amino acid synthesis was greatly restricted. This was confirmed by the supply of (15N)-glutamine directly to leaves. Leaves of the GS-deficient mutant assimilated ammonia at about 75% the rate found for the wild type, and this was almost completely eliminated by addition of the inhibitor methionine sulphoximine. Root enzymes, together with residual levels of the deleted enzymes in the leaves, have sufficient capacity for ammonia assimilation, through the glutamate synthase cycle, to provide adequate input of nitrogen for normal growth of the mutants, if photorespiratory ammonia production is suppressed.