The plasma membrane as a barrier to herbicide penetration and site for adjuvant action

Adjuvants are traditionally thought to exert their main effect on the cuticle or spray droplet to enhance foliar‐applied herbicide penetration. However, considerable evidence exists indicating that the plasma membrane is a barrier to intracellular penetration of herbicides and a site of action for adjuvants. Surfactants may penetrate through the cuticle and into the region of the plasma membrane. Insertion into the membrane causes a general “ loosening” to provide greater penetration by highly polar herbicides such as glyphosate. Weak acid herbicides typically have a lipophilic moiety and, therefore, can move more easily through the membrane but the rate and accumulation is dependent on pH conditions across the membrane. Ammonium salts have been shown to affect the pH of the apoplast in a manner which allows faster penetration and greater accumulation of weak acid herbicides. Examination and understanding of the plasma membrane as a barrier to herbicide penetration will aid in defining the mechanisms of adjuvant action and improve the efficiency of agrochemical use.