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 le aves of the C-4 plant Amaranthus edulis were treated with the glutamin e 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 co mpared with a mutant lacking phosphoenolpyruvate carboxylase activity (EC 4.1.1.31), and with three different mutants that accumulated glyci ne. The increase in the concentration of ammonia in the leaves, stimul ated by the treatments was used as a measurement of the rate of photor espiration in C-4 plants. The application of glutamine and glycine mai ntained the rate of photorespiratory ammonia production for a longer p eriod in the wild type, and increased the rate in a mutant lacking pho sphoenolpyruvate carboxylase suggesting that there was a lack of amino donors in these plants. The calculated rate of photorespiration in Am aranthus edulis wild-type leaves when the supply of amino donors was e nough to maintain the photorespiratory nitrogen flow, accounted for ap proximately 6% of the total net photosynthetic CO2 assimilation rate. In a mutant lacking phosphoenolpyruvate carboxylase, however, this rat e 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 t hat 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 metaboli c reactions in the leaves. The data suggests that photorespiration is an active process in C-4 plants, which can play an important role in p hotosynthetic metabolism in these plants.