Retention times needed for oil soluble herbicides are as short as 1 hr. Environmental parameters that favor cuticular penetration water soluble herbicides include high humidity, adequate soil moisture, and warm temperatures. Minimum kill can be expected when plants are under water stress, low humidity, and cool temperatures. Variable weather conditions account for much of the inconsistency encountered with the performance of foliage applied herbicides. Movement of herbicide across the cell membrane into the cytoplasm Herbicides move into cells by diffusion, ion trapping, or with co-transport with hydrogen ions through protein carriers located on the plasma membrane.
Most herbicides cross membranes by simple diffusion.
Metabolic energy is not required for this movement. Lipophilic and neutral herbicides diffuse more freely across the plasma membrane than hydrophilic or ionized herbicides. Unless altered in some manner, herbicides that diffuse into the cytoplasm diffuse back out and are not retained in the phloem long enough for subsequent symplastic translocation. Some herbicides diffuse across the plasma membrane and are concentrated within the sieve cell cytoplasm. The ionized herbicide is trapped since diffusion of ions back out is greatly retarded.