Low doses of glyphosate change the responses of soyabean to subsequent glyphosate treatments

Many herbicides promote plant growth at doses well below the recommended application rate (hormesis). The objectives of this study were to evaluate glyphosate-induced hormesis in soyabean (Glycine max) and determine whether pre-treating soyabean seedlings with low doses of glyphosate would affect their response to subsequent glyphosate treatments. Seven doses (1.8-720g a.e. ha(-1)) of glyphosate were applied to 3-week-old seedlings, and the effects on the electron transport rate (ETR), metabolite (shikimate, benzoate, salicylate, AMPA, phenylalanine, tyrosine and tryptophan) levels and dry weight were determined. The lowest dose stimulated ETR and increased biomass the most. Benzoate levels increased 203% with 3.6g a.e. ha(-1) glyphosate. Salicylate content and tyrosine content were unaffected, whereas phenylalanine and tryptophan levels were increased by 60 and 80%, respectively, at 7.2g a.e. ha(-1). Dose-response curves for these three amino acids were typical for hormesis. In another experiment that was replicated twice, soyabean plants were pre-treated with low doses of glyphosate (1.8, 3.6 or 7.2g a.e. ha(-1)) and treated with a second application of glyphosate (1.8, 3.6, 7.2, 36, 180 or 720g a.e. ha(-1)) 14days later. For total seedling dry weight, a 3.6 and 7.2g a.e. ha(-1) glyphosate dose preconditioned the soyabean seedlings to have greater growth stimulation by a later glyphosate treatment than plants with no preconditioning glyphosate exposure. Optimal hormetic doses were generally higher with pre-treated plants than plants that had not been exposed to glyphosate. Thus, pre-exposure to low doses of glyphosate can change the hormetic response to later low-dose exposures.