Abstract
Summary Glufosinate (phosphinothricin) irreversibly inhibits glutamine synthetase and leads to a great decrease in the amino acids glutamine, glutamate, aspartate, serine, glycine and alanine. Due to the lack of glutamate and serine, the transamination of glyoxylate into glycine in the course of photorespiration cannot take place. The inhibition of this part of the photorespiratory process plays the essential role with respect to the photosynthesis inhibition caused by PPT under atmospheric conditions. After addition of different photorespiration or Calvin cycle intermediates to phosphinothricin no decrease in photosynthesis inhibition can be measured. This suggests that photosynthesis inhibition is not induced by a depletion of Calvin cycle intermediates. A drastic C02-release can be measured after phosphinothricin treatment so that no C02 compensation point can be adjusted. As in the case of PPT a great C02-release can be measured when using aminooxyacetate, which inhibits the transamination of glyoxylate into glycine in photorespiration. Glyoxylate, which cannot convert following phosphinothricin treatment, oxidizes non-enzymatically to C02 and formate. Tests on the effect of glyoxylate or P-glycolate on ribulose-1,5-bisphosphate carboxylase activity show that the enzyme activity in crude leave extract can only be inhibited when high concentrations of glyoxylate or P-glycolate are added. However, in intact spinach chloroplasts the enzyme activity can be inhibited by using much lower concentrations of the compounds. This may indicate that the ribulose-1,5-bisphosphate carboxylase activase is inhibited, which means that photosynthesis cannot occur after the application of phosphinothricin.
Published Version
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