Adjuvants are traditionally thought to exert their main effect on the
cuticle or spray droplet to enhance foliar-applied herbicide penetrati
on. However, considerable evidence exists indicating that the plasma m
embrane is a barrier to intracellular penetration of herbicides and a
site of action for adjuvants. Surfactants may penetrate through the cu
ticle and into the region of the plasma membrane. Insertion into the m
embrane causes a general 'loosening' to provide greater penetration by
highly polar herbicides such as glyphosate. Weak acid herbicides typi
cally have a lipophilic moiety and, therefore, can move more easily th
rough the membrane but the rate and accumulation is dependent on pH co
nditions across the membrane. Ammonium salts have been shown to affect
the pH of the apoplast in a manner which allows faster penetration an
d greater accumulation of weak acid herbicides. Examination and unders
tanding of the plasma membrane as a barrier to herbicide penetration w
ill aid in defining the mechanisms of adjuvant action and improve the
efficiency of agrochemical use.