Glutamine synthetase activity and ammonium accumulation is influenced by time of glufosinate application

Abstract

Time of glufosinate application impacts weed control. Although leaf angle of velvetleaf significantly contributes to the time of day (TOD) effect, it is not the sole reason for reduced efficacy. Absorption and translocation of glufosinate, glutamine synthetase (GS) activity, and ammonium accumulation were investigated as possible physiological components of the TOD effect. Absorption and translocation were not associated with a decrease in glufosinate efficacy. GS activity, ammonium accumulation, and plant biomass were influenced by the time of glufosinate application. Velvetleaf GS activity quickly diminished during the light period and remained high during the dark period following glufosinate treatment. GS activity was higher in plants treated at 10:00 pm than in those treated at 2:00 pm 12 HAT. With little GS inhibition during the dark period, ammonium accumulation occurred only during the light period. At 72 HAT, ammonium concentration was nearly 1.5-fold greater in plants treated at 2:00 pm versus those treated at 10:00 pm with 160 g/ha glufosinate. Plant biomass of plants treated with 160 g/ha glufosinate at 10:00 pm was greater than those treated at 2:00 pm at 72 HAT. Increasing the application rate from 160 to 320 g/ha removed these differences in GS activity, ammonium accumulation, and plant biomass. These results demonstrate that GS activity and ammonium accumulation are physiological processes involved in the TOD effect.