Herbicidal Actions of Root-Applied Glufosinate Ammonium on Tomato Plants

Abstract

The herbicidal action of foliar applications of glufosinate-ammonium (GLA) is due to toxic accumulation of unassimilated NH4+ in leaves; however, the effects of root-applied GLA on NH4+ accumulation and plant growth are unknown. In a dose-response hydroponics experiment, tomato (Lycopersicon esculentum Mill.) plants were grown in nitrate-based solutions with GLA added at 0, 6, 12, 25, or 50 mg·L-1. To observe plant responses to an exogenous NH4+ source with herbicide-induced responses, plants were grown in an NH4+-based solution without GLA addition. At 6 days after treatment (DAT), GLA in solution at 25 mg·L-1 produced partial leaf wilting, chlorosis, and necrosis of foliage, and at 50 mg·L-1, plants were fully wilted and necrotic. Ammonium (NH4+-N) concentration in shoots at 6 DAT increased from 0 to 6 mg·g-1 fresh weight with increasing GLA in the nutrient solution. Ethylene evolution doubled (from 4 to 8 nL·g-1·h-1, fresh weight) with increases in GLA from 0 to 25 mg·L-1 but declined with apparent plant death with GLA at 50 mg·L-1. Other treatments, including NH4+ nutrition, did not induce toxicity symptoms in leaves or give increases in NH4+ accumulation or ethylene evolution during the 6 days of the experiment. In a time-course experiment, tomato plants treated with GLA at 25 mg·L-1 were chlorotic at 4 DAT. Ethylene evolution (fresh weight basis) rose from an initial rate of 2.6 nL·g-1·h-1 to 8.3 nL·g-1·h-1 after 4 days. At 9 DAT, all plants receiving this treatment died. In the time-course experiment, an exogenous NH4+ treatment caused a slight inhibition in shoot fresh weight relative to NO3- nutrition with no GLA but caused no visible symptoms and only slight enhancements in NH4+ accumulation and ethylene evolution over the 9-day period. Following GLA treatment, NH4+ accumulated in the shoots and increased sharply with time, whereas exogenous NH4+ led to NH4+ accumulation primarily in roots. Results suggest that GLA was absorbed by roots and translocated to shoots, where it initiated accumulation of NH4+ and ethylene evolution as indications of herbicidal action. Chemical name used: glufosinate-ammonium, GLA.