Herbicide entry into plant cells is needed for a herbicide to reach its target site of action. Plant cells absorb herbicides by either passive or active processes. Most herbicides are absorbed by plant cells via passive absorption (diffusion). Neutral, lipophilic herbicides rapidly diffuse across the lipid bilayer of plant membranes down a concentration gradient to reach concentrations in the cell equal to concentrations in the external medium. Weak acid herbicides, when in their lipophilic, ionic form, also passively diffuse into plant cells. However, many weak acid herbicides reach cellular concentrations greater than external concentrations due to ion trapping, where the less lipophilic anion accumulates in alkaline compartments of the plant cell.
In addition, experimental evidence supports active, carrier-mediated transport of some herbicides into plant cells. Herbicides can also accumulate in plant cells to concentrations greater than equilibrium concentrations by other mechanisms ?including herbicide metabolism, binding to cellular constituents or partitioning into lipids. However, these accumulation mechanisms probably do not contribute to phytotoxicity.
Suggested reading for advanced learners:
Sterling, T.M. 1994. Mechanisms of herbicide absorption across plant membranes and accumulation in plant cells. Weed Sci. 42:263-276.