The Effect of Phosphinothricin (Glufosinate) on Photosynthesis II. The Causes of Inhibition of Photosynthesis

It was shown in the previous study that phosphinothricin (glufosinate) causes an accumulation of ammonia and inhibition of photosynthesis. The extent to which there is a connection between these two processes is now investigated in the present study. First of all. the role of NH₃ per se in the impairment of photosynthesis was to be clarified. For this purpose, the inhibition of photosyn­thesis was investigated in relation to exogenously applied ammonia in chloroplasts, protoplasts and entire leaves. The comparison with the experimental results in leaves in which the ammonia was formed endogenously (by action of phosphinothricin) shows that the ammonia toxicity at least cannot be solely responsible for the inhibition of photosynthesis. This is confirmed by the finding that photosynthesis is maintained under non-photorespiratory conditions, although considerable amounts of NH₃ are concentrated at the same time in the plants. A process connected with photorespiration appears to play the essential role.

Simultaneous administration of phosphinothricin and glutamine largely prevented the impair­ment of photosynthesis. This indicates that the main cause for the inhibition of photosynthesis by phosphinothricin is based on a depletion of glutamine. Three subsequent reactions which may cause the inhibition of photosynthesis are discussed: 1) an inhibition of protein biosynthesis, 2) a toxic accumulation of glyoxylate in the photorespiratory cycle and 3) a deficiency of intermediates of the Calvin cycle in consequence of the interrupted photorespiration.