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Title: Low doses of glyphosate change the response of soybean to later glyphosate exposures

Author
item SILVA, FERDINANDO - Sao Paulo State University (UNESP)
item Duke, Stephen
item Dayan, Franck
item VELINI, EDIVALDO - Sao Paulo State University (UNESP)

Submitted to: Weed Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/23/2015
Publication Date: 12/10/2015
Citation: Silva, F.M., Duke, S.O., Dayan, F.E., Velini, E.D. 2015. Low doses of glyphosate change the response of soybean to later glyphosate exposures. Weed Research. 56:124-136. DOI: 10.1111/wre.12189

Interpretive Summary: The stimulatory effect of low doses of toxic substances is known as hormesis. Many herbicides that cause severe injury to plants at recommended rates, promote growth or have other stimulatory effects at very low doses. The objective of this study was to evaluate glyphosate-induced hormesis in soybean (Glycine max (L.) Merr.) and to determine whether pre-treating soybean seedlings with low doses of glyphosate would affect their response to subsequent glyphosate treatments. Seven doses (1.8 – 720 g ae ha-1) of glyphosate were applied to 3-week-old soybean 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. The levels of benzoate, a metabolite associated with the shikimate pathway, increased 203% with 3.6 g ae ha-1 glyphosate. There were no significant effects on salicylate, phenylalanine, tryptophan and tyrosine content, although the dose response curves for the three amino acids were typical for hormesis. In another experiment, soybean plants in the V2 stage were pretreated with low doses of glyphosate (1.8, 3.6 or 7.2 g ae ha-1), and treated with a second application of glyphosate (1.8, 3.6, 7.2, 36, 180 720 g ae ha-1) 14 days later. A 3.6 g ae ha-1 glyphosate dose preconditioned the plants to have greater growth stimulation by a later glyphosate treatment than plants with no pre-conditioning exposure. A 7.2 g ae ha-1 pre-conditioning treatment increased the dose causing maximal hormesis in the second exposure to the herbicide. Thus, pre-exposure to low doses of glyphosate can change the hormetic response to later low doses.

Technical Abstract: The stimulatory effect of low doses of toxic substances is known as hormesis. Many herbicides that cause severe injury to plants at recommended rates, promote growth or have other stimulatory effects at very low doses. The objective of this study was to evaluate glyphosate-induced hormesis in soybean (Glycine max (L.) Merr.) and to determine whether pre-treating soybean seedlings with low doses of glyphosate would affect their response to subsequent glyphosate treatments. Seven doses (1.8 – 720 g ae ha-1) of glyphosate were applied to 3-week-old soybean 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. The levels of benzoate, a metabolite associated with the shikimate pathway, increased 203% with 3.6 g ae ha-1 glyphosate. There were no significant effects on salicylate, phenylalanine, tryptophan and tyrosine content, although the dose response curves for the three amino acids were typical for hormesis. In another experiment, soybean plants in the V2 stage were pretreated with low doses of glyphosate (1.8, 3.6 or 7.2 g ae ha-1), and treated with a second application of glyphosate (1.8, 3.6, 7.2, 36, 180 720 g ae ha-1) 14 days later. A 3.6 g ae ha-1 glyphosate dose preconditioned the plants to have greater growth stimulation by a later glyphosate treatment than plants with no pre-conditioning exposure. A 7.2 g ae ha-1 pre-conditioning treatment increased the dose causing maximal hormesis in the second exposure to the herbicide. Thus, pre-exposure to low doses of glyphosate can change the hormetic response to later low doses.