A study of photorespiratory ammonia production in the C-4 plant Amaranthus edulis, using mutants with altered photosynthetic capacities

Previous studies have indicated that the rate of photorespiration in C-4 plants is low or negligible. In this study, wild-type and mutant leaves of the C-4 plant Amaranthus edulis were treated with the glutamine synthetase inhibitor, phosphinothricin and the glycine decarboxylase inhibitor, aminoacetonitrile, at different concentrations of CO2. The time course of ammonia accumulation in leaves of the wild type was compared with a mutant lacking phosphoenolpyruvate carboxylase activity (EC 4.1.1.31), and with three different mutants that accumulated glycine. The increase in the concentration of ammonia in the leaves, stimulated by the treatments was used as a measurement of the rate of photorespiration in C-4 plants. The application of glutamine and glycine maintained the rate of photorespiratory ammonia production for a longer period in the wild type, and increased the rate in a mutant lacking phosphoenolpyruvate carboxylase suggesting that there was a lack of amino donors in these plants. The calculated rate of photorespiration in Amaranthus edulis wild-type leaves when the supply of amino donors was enough to maintain the photorespiratory nitrogen flow, accounted for approximately 6% of the total net photosynthetic CO2 assimilation rate. In a mutant lacking phosphoenolpyruvate carboxylase, however, this rate increased to 48%, when glutamine was fed to the leaf, a value higher than that found in some C-3 plants. In mutants of Amaranthus edulis that accumulated glycine, the rate of photorespiration was reduced to 3% of the total net CO2 assimilation rate. The rate of ammonia produced during photorespiration was 60% of the total produced by all metabolic reactions in the leaves. The data suggests that photorespiration is an active process in C-4 plants, which can play an important role in photosynthetic metabolism in these plants.