The effects of ᴅʟ-phosphinothricin and L-methionine sulfoximine on the enzymes of nitrogen assimilation were studied. Furthermore we investigated the accumulation of ammonia and the photosynthetic activity after the treatment of mustard plants with phosphinothricin. Phosphino-thricin was a specific and very strong inhibitor of glutamine synthetase. Major differences, however, were found between the phosphinothricin affinity of the leaf enzyme and that of the root of mustard plants. The leaf enzyme was 50% inhibited at a concentration of 10-4 m phosphinothricin (pI50 = 4), whereas the root enzyme already showed the same effect at a concentration of 2 × 10-5m (pI50 = 4.7). In addition Ki values of about 0.03 mм for the leaf enzyme and 0.002 mм for the root enzyme respectively were determined. Phosphinothricin treatment of plants caused an ammonia accumulation in tissues. The accumulation was light dependent. At the beginning of the light period the major sources of ammonia accumulation could be the nitrogen assimilation as well as catabolic processes of nitrogen compounds. A clear contribution of photorespiration was only found when higher concentrations of ammonia were reached. The application of phosphinothricin induced a strong reduction of CO2 assimilation.
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