The Effect of Phosphinothricin on the Assimilation of Ammonia in Plants

Phosphinothricin, Methionine Sulfoximine, Glutamine Synthetase, Am m onia Accum ulation, Photosynthetic Rate The effects o f DL-phosphinothricin and L-methionine sulfoxim ine on the enzymes o f nitrogen assimilation were studied. Furthermore we investigated the accumulation o f am m onia and the photosynthetic activity after the treatment o f mustard plants with phosphinothricin. Phosphino­ thricin was a specific and very strong inhibitor o f glutamine synthetase. Major differences, however, were found between the phosphinothricin affinity o f the leaf enzyme and that o f the root o f mustard plants. The leaf enzyme was 50% inhibited at a concentration o f 104 M phosphinothricin (pl5 0 = 4), whereas the root enzyme already showed the same effect at a concentration of 2 x 1 0 _ 5 m (p l5 0 = 4.7). In addition Kx values o f about 0.03 m M for the leaf enzyme and 0.002 mM for the root enzyme respectively were determined. Phosphinothricin treatment o f plants caused an ammonia accumulation in tissues. The accum ulation was light dependent. At the beginning o f the light period the major sources o f am m onia accum ulation could be the nitrogen assimilation as well as catabolic processes o f nitrogen com pounds. A clear contribution of photorespiration was only found when higher concentrations o f am m onia were reached. The application o f phosphinothricin induced a strong reduction o f C 0 2 assim ilation.