The Effect of Phosphinothricin (Glufosinate) on Photosynthesis I. Inhibition of Photosynthesis and Accumulation of Ammonia

Phosphinothricin (glufosinate) is an irreversible inhibitor of glutamine synthetase in plants. En­zyme inhibition results in substantial light-dependent accumulation of ammonia, inhibition of photosynthesis and ultimately the death of the plants. First of all the metabolic processes in which NH3 is liberated were established. Comparative investigations under photorespiratory and non- photorespiratory conditions with Sinapis alba plants revealed that about 60% of the accumulated NH3 derives from photorespiration. Trials with PPT and KNO3 showed that nitrate assimilation is insignificant as a source of NH3, and so the remaining 40% is accumulated in catabolic or anabolic processes. Under photorespiratory conditions CO2 fixation after treatment with 1 mᴍ PPT finally fell below the compensation point, and this was independent of the addition of nitrate. In non- photorespiratory conditions, by contrast, photosynthesis continued at a level of over 80% for several hours in entire plants and in severed leaves, although at the same time substantial amounts of ammonia had accumulated.