An increase free ammonia content was confirmed in Japanese barnyard millet (Echinochloa utilis OHWI), crabgrass (Digitaria adscendens HENR.), livid amaranth (Amaranthus lividus L.) and purple nutsedge (Cyperus rotundus L.) treated with bialaphos, L-2-amino-4-[(hydroxy)(methyl)phosphinoyl]butyryl-L-alanyl-L-alanine. The ammonia content reached about 30 to 100 times higher than the control's in 24 to 48 hr after the treatment. The increment was more marked in shoots than in roots of the barnyard millet, and more in leaves than in stems of livid amaranth. The accumulation was not momentary, but persistent until the death of the plant. Such a remarkable accumulation of ammonia was not observed in plants treated with any other herbicide. Therefore, the ammonia accumulation is considered to be a particular action caused by bialaphos. The close correlation between the free ammonia content and the herbicidal activity indicated that the toxicity of accumulated ammonia is a primary factor to the herbicidal activity of bialaphos.
Bialaphos, L-2-amino-4-[(hydroxy)(methyl)phosphinoyl]butyryl-L-alanyl-L-alanine, at a concentration of 0.3 to 3 ms did not inhibit glutamine synthetase (L-glutamine: ammonia ligase (ADP), EC 6. 3. 1. 2, GS) extracted from shoots of Japanese barnyard millet (Echinochloa utilis OHWI). However, low GS activity was observed in the shoots treated with bialaphos. On the other hand, L-2-amino-4-[(hydroxy) (methyl)phosphinoyl]butyric acid (L-AMPB), a metabolite of bialaphos, inhibited GS both after extraction and in the shoot. The results suggest that bialaphos was metabolized into L-AMPB in plants and inhibited GS. The quantitative analysis of free amino acids in the bialaphos treated shoots showed a remarkable decrease in glutamine content 48 hr after the treatment. An exogenous application of glutamine increased the glutamine to some extent in the bialaphos-treated shoots, but did not antagonize bialaphos in the herbicidal activity. Thus, it appears that a decrease of glutamine content by the herbicide was not a main factor to induce the herbicidal activity.
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