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We studied the physiological mechanisms involved in the deleterious effects of a glyphosate-based herbicide (Factor® 540) on photosynthesis and related physiological processes of willow (Salix miyabeana cultivar SX64) plants. Sixty-day-old plants grown under greenhouse conditions were sprayed with different rates (0, 1.4, 2.1, and 2.8 kg a.e ha-1) of the commercial glyphosate formulated salt Factor® 540. Evaluations were performed at 0, 6, 24, 48, and 72 h after herbicide exposure. We established that the herbicide decreases chlorophyll, carotenoid and plastoquinone contents, and promotes changes in the photosynthetic apparatus leading to decreased photochemistry which results in hydrogen peroxide (H2O2) accumulation. H2O2 accumulation triggers proline production which can be associated with oxidative protection, NADP+ recovery and shikimate pathway stimulation. Ascorbate peroxidase and glutathione peroxidase appeared to be the main peroxidases involved in the H2O2 scavenging. In addition to promoting decreases of the activity of the antioxidant enzymes, the herbicide induced decreases in ascorbate pool. For the first time, a glyphosate-based herbicide mode of action interconnecting its effects on shikimate pathway, photosynthetic process and oxidative events in plants were presented.

HighlightsPhosphate increased glyphosate uptake and decreased its toxicity in willows PO 4 3-concentrations ≥ 200 mg l -1 doubled glyphosate uptake by willow roots PO 4 3-concentrations ≥ 200 mg l -1 increased antioxidant system activity PO 4 3-maintained photosynthesis rates by inducing reactive oxygen species scavenging 3 Abstract Phosphate (PO 4 3-) has been shown to increase glyphosate uptake by willow, a plant species known for its phytoremediation potential. However, it remains unclear if this stimulation of glyphosate uptake can result in an elevated glyphosate toxicity to plants (which could prevent the use of willows in glyphosate-remediation programs). Consequently, we studied the effects of PO 4 3-on glyphosate uptake and toxicity in a fast growing willow cultivar (Salix miyabeana SX64). Plants were grown in hydroponic solution with a combination of glyphosate (0, 0.001, 0.065 and 1 mg l -1 ) and PO 4 3-(0, 200 and 400 mg l -1 ). We demonstrated that PO 4 3-fertilization greatly increased glyphosate uptake by roots and its translocation to leaves, which resulted in increased shikimate concentration in leaves. In addition to its deleterious effects in photosynthesis, glyphosate induced oxidative stress through hydrogen peroxide accumulation.Although it has increased glyphosate accumulation, PO 4 3-fertilization attenuated the herbicide's deleterious effects by increasing the activity of antioxidant systems and alleviating glyphosateinduced oxidative stress. Our results indicate that in addition to the glyphosate uptake, PO 4 3-is involved in glyphosate toxicity in willow by preventing glyphosate induced oxidative stress.

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