Uptake and Accumulation of the Herbicide Bentazon by Cultured Plant Cells

Abstract

Cellular absorption of the herbicide bentazon, a weak acid with pK. 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 extemal bentazon concentration, implying that movement of the herbicide into cells was not carriermediated. 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 extemal 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.A herbicide must be absorbed by plant cells and accumulate at the site of action before it causes phytotoxicity. In general, nonfacilitated diffusion is the primary mechanism by which herbicides move across plant cell membranes (10), although active or carrier-mediated transport of a few herbicides into plant cells has been documented (5, 17). Natural, plant growth regulating chemicals (1) and secondary plant products (13) are transported across membranes by a combination of passive and active processes. Knowledge of the mechanisms involved in the assymetric distribution of herbicides across membranes may explain relationships between herbicide concentration and phytotoxicity.

Surfactant-Increased Glyphosate Uptake into Plasma Membrane Vesicles Isolated from Common Lambsquarters Leaves

“…Glyphosate uptake was also very low in comparison to other herbicidal compounds (Table 11). Atrazine rapidly diffuses across the membrane because of its highly nonpolar character (Price and Balke, 1982), and bentazon accumulates in the cytoplasm of cells by a weak acid, ion-trapping mechanism (Sterling et al, 1990). Our results, coupled with the findings of Haderlie et al (1977) and Richard and Slife (1979), suggest the plasma membrane is a significant barrier to glyphosate uptake.…”

In Vivo 31P-Nuclear Magnetic Resonance Studies of Glyphosate Uptake, Vacuolar Sequestration, and Tonoplast Pump Activity in Glyphosate-Resistant Horseweed

“…The herbicide bentazon, for example, was reported to experience a 10-fold improved uptake at pH 4.6 versus 6.6 (Sterling et al, 1990). This was attributed to increased passive transport across the plant cell membrane for the uncharged molecule present at pH 4.6, followed by deprotonation of bentazon at the higher intracellular pH.…”

Metabolism of the Folate Precursor p-Aminobenzoate in Plants

“…1B). As a hydrophobic weak acid, pABA can cross membranes by simple diffusion according to pH and concentration gradients (5). However, pABA has been shown to occur in plants mainly as its glucose ester (pABA-Glc), which is hydrophilic and almost certainly cannot readily diffuse across membranes (6).…”