Differential phytotoxicity of glyphosate in maize seedlings following applications to roots or shoot

The transport and differential phytotoxicity of glyphosate was investigated in maize seedlings following application of the herbicide to either roots or shoots. One-leaf maize seedlings (Zea mays L.) were maintained in graduated cylinders (250 mL) containing nutrient solution. Half of the test plants were placed in cylinders (100 mL) containing different C-14-glyphosate concentrations; the remainder received foliar appliation of C-14-glyphosate. After 26 It, the roots and the treated leaves were washed with distilled water, and the plants placed again in cylinders (250 mL) containing fresh nutrient solution for 5 days. Plants were weighed, and split into root, seed, cotyledon, coleoptile, mesocotyl, first leaf and apex. The recovery of 14 C-glyphosate was over 86%. For both application treatments, the shoot apex was the major sink of the mobilized glyphosate (47.9 +/- 2.93% for root absorption and 45.8 +/- 2.91% for foliar absorption). Expressed on a tissue fresh weight basis, approximately 0.26 mug a.e. g(-1) of glyphosate in the apex produced a 50% reduction of plant fresh weight (ED50) when the herbicide was applied to the root. However, the ED50 following foliar absorption was only 0.042 mug a.e. g(-1) in the apex, thus maize seedlings were much more sensitive to foliar application of the herbicide.