Nitrogen Metabolism of Mutants of Barley Lacking Key Enzymes of Ammonia Assimilation

Abstract

The photorespiratory cycle as outlined in Figure 1 was originally proposed by Keys et al. (1978) (see Givan et al. 1988, for a review). Ammonia is released in the mitochondria following the conversion of glycine to serine at a rate ten times that generated by the reduction of nitrate. It is now considered probable that the majority of the ammonia passes directly into the chloroplast where it is converted to glutamine by the chloroplastic isoenzyme of glutamine synthetase (GS) (Lea et al. 1990 a,b). In most C₃ plants the cytosolic form of GS is not present in sufficient amounts to carry out the high rates of assimilation required (McNally et al. 1983). ¹⁵N-labelling studies have clearly shown that the glutamine is converted to glutamate (woo et al. 1982, 1987 a; Rhodes et al. 1989) by ferredoxin-dependent glutamate synthase which is localized solely in the chloroplast (Wallsgrove et al. 1983).