SURFACTANT-INCREASED GLYPHOSATE UPTAKE INTO PLASMA-MEMBRANE VESICLES ISOLATED FROM COMMON LAMBSQUARTERS LEAVES

Plasma membrane vesicles were isolated from mature leaves of lambsquar ters (Chenopodium album L.) to investigate whether this membrane is a barrier to glyphosate uptake and whether surfactants possess different ial abilities to enhance glyphosate permeability. Amino acids represen ting several structural classes showed Delta pH-dependent transport, i ndicating that the proteins necessary for active, proton-coupled amino acid transport were present and functional. Glyphosate uptake was ver y low compared to the acidic amino acid glutamate, indicating that gly phosate is not utilizing an endogenous amino acid carrier to enter the leaf cells and that the plasma membrane appears to be a significant b arrier to cellular uptake. In addition, glyphosate flux was much lower than that measured for either bentazon or atrazine, both lipid-permea ble herbicides that diffuse through the bilayer. Glyphosate uptake was stimulated by 0.01% (v:v) MON 0818, the cationic surfactant used in t he commercial formulation of this herbicide for foliar application. Th is concentration of surfactant did not disrupt the integrity of the pl asma membrane vesicles, as evidenced by the stability of imposed pH gr adients and active amino acid transport. Nonionic surfactants that dis rupt the cuticle but that do not promote glyphosate toxicity in the fi eld also increased glyphosate transport into the membrane vesicles. Th us, no correlation was observed between whole plant toxicity and surfa ctant-aided uptake. Current data suggest that surfactant efficacy may be the result of charged surfactants' ability to diffuse away from the cuticle into the subtending apoplastic space, where they act directly on the plasma membrane to increase glyphosate uptake.