Effects of soil-applied glufosinate-ammonium on tomato plant growth and ammonium accumulation

Glufosinate-ammonium (GLA) is a postemergence herbicide applied as a foliar spray, but if applied to roots in hydroponics, GLA was herbicidal to tomato (Lycopersicon esculentum Mill.). In the present research, the efficacy of GLA directly applied into soils was compared with foliar application. The effect of degradation time in soil on efficacy also was studied. These studies were conducted in a greenhouse with tomato (Heinz 1439) in pot culture. In one experiment, GLA was applied into soil at concentrations ranging from 0 to 15 mg/kg. Symptoms of yellowing and cupping of leaves occurred within 5 days after treatment (DAT) with GLA addition above 1.5 mg/kg. At 5 DAT, biomass suppression was not evident with any GLA treatment, but GLA at 1.5 mg/kg soil was herbicidal, as measured by NH4+ concentration in plant tissue. With the increase of GLA from 0 to 15 mg/kg, shoot NH4+ concentration increased from near 0 to 2.95 mg N/g biomass. To compare the effects of foliar and soil applications of GLA, GLA was brushed onto foliage or was poured and washed into soil for an application of 0-3 mg/plant. At 3 DAT, plants treated with 1.5 or 3 mg foliar GLA showed symptoms of yellowing, cupping, and wilting on top leaves, whereas 5 days were required for symptoms to develop at the same amounts applied to soil. Plant growth was retarded slightly by GLA applied at more than 1.5 mg/plant. To study the effects of degradation time in soil on efficacy, GLA was added into soil at 0-3 mg/kg before tomato seedlings were transplanted and was allowed to incubate for 5-20 days. At 4 days after transplanting into these media, regardless of time of degradation, plants receiving GLA applications above 0.25 mg/kg showed toxicity symptoms of yellowing, cupping, and wilting of top leaves. The growth of all GLA-treated plants was inhibited to about half of the growth of plants not receiving GLA. All GLA treatments led to NH4+ accumulation in tomato shoots, and this increase was several times that of plants receiving no GLA, ranging from 0.09 mg N/g fresh biomass with no GLA treatment to between 1.9 and 2.6 mg N/g biomass with GLA added. All elevated NH4+ concentrations were herbicidal. With only 5 days allowed for GLA to degrade in soil, the tomato shoot NH4+ accumulation was higher than in soil with longer degradation times. With an increase in the amount of soil-applied GLA up to 1 mg/kg, the NH4+ concentration in plant tissue increased. However, if GLA was applied at more than 1 mg/kg, a further increase in NH4+ concentration in plant tissue was not detected. This study suggests that soil-applied GLA is herbicidal and that 20 days is not long enough for GLA added into soil to degrade to a nonharmful concentration.