Greenhouse experiments were conducted to determine the effects of glufosinate on ammonium accumulation and photosynthetic inhibition in Palmer amaranth. Glufosinate applied at 410 g ha−1 reduced glutamine synthetase activity and enhanced ammonium content 30 min after treatment. Glufosinate application resulted in rapid inhibitions of the photosynthetic rate and stomatal conductance during the first 2 h after treatment (HAT), whereas the ammonium concentration increased over the same time period. Ammonium content 6 HAT was 22 times higher in treated plants than in untreated plants, whereas the photosynthetic rate of treated plants decreased by 63%. At 24 HAT, the ammonium content was 53 times higher in treated plants than in untreated plants; however, no further inhibition of photosynthesis occurred. Photosynthetic inhibition in Palmer amaranth coincided with the rapid accumulation of ammonium and decrease in stomatal conductance shortly after glufosinate application.
