Phytotoxic activity of root absorbed glyphosate in corn seedlings (Zea mays L.)

Growth chamber experiments were conducted in order to study the absorption, translocation and activity of glyphosate when applied to roots with aqueous solution avoiding any glyphosate - substrate interaction. Corn seedlings at the first leaf stage were set up in individual graduated cylinders containing different solutions of ¹⁴C-glyphosate (0-30 mg ae kg^-1). After 26 h of root exposure, plants were transferred to fresh nutrient solution and grown for the next 5 days. After harvest, plants were separated into seed, root, mesocotyle, coleoptile, cotyledon, first leaf and all new leaves (apex), and quantified ¹⁴C radioactivity contained in each part. Glyphosate uptake was only 11% of the theoretical mass flow into the plant. The amount of glyphosate translocated from roots was positively correlated with plant uptake (P < 0.01). Total plant fresh weight presented a logistic response to glyphosate amounts, including a growth stimulant effect (hormesis), when plants absorbed less than 0.6 μg.The treated plants presented a normal pattern of glyphosate allocation, with the apex the principal sink, accumulating more than 38% of mobilized glyphosate. When corn plants absorbed more than 0.6 μg they showed a decrease in growth. The relatively high glyphosate quantities allocated in the new leaves showed the relevance of the symplastic pathway in the translocation process for root absorbed glyphosate.