Uptake and accumulation of the herbicide bentazon by cultured plant cells

Cellular absorption of the herbicide bentazon, a weak acid with pK_a 3.45, was investigated using suspension-cultured cells of velvetleaf (Abutilon theophrasti Medic.). Bentazon accumulated rapidly to concentrations approximately four times that of the external medium. Bentazon accumulation against a concentration gradient was not due to its conversion to metabolites, partitioning into lipids, or binding onto cellular constituents. Bentazon uptake was related linearly to the external bentazon concentration, implying that movement of the herbicide into cells was not carrier-mediated. Bentazon was able to diffuse freely and extensively out of the cells, indicating that bentazon can readily diffuse across cell membranes. Potassium cyanide and carbonyl cyanide m-chlorophenyl hydrazone inhibited bentazon accumulation as did nitrogen gas when bubbled through the uptake medium. Absorption was pH-dependent with the greatest amount of bentazon accumulating at acidic external pH. Calculations indicated that conversion of uncharged bentazon to bentazon anion in the cytoplasm accounts for cellular accumulation of bentazon. These results provide evidence that bentazon is absorbed across membranes via simple diffusion and that bentazon accumulates in plant cells via an energy-dependent, ion-trapping mechanism which results in bentazon accumulation in the cytoplasm.