Cuticle penetration

It is known for years that cuticle penetration may be improved in this case by adding appropriate surfactants to the active ingredient, and by obtaining unstable lipophilic derivatives of these substances, as is the case of the acidic pesticides (glyphosate, 2,4-D...for instance), which are transformed to lipophilic esters reemitting the free acid inside the cells (Figure 1). 

The barrier to penetration of water and solutes across the cuticular membrane occurs at the air-cuticle interface. This solid-state barrier is composed of cutin and waxes, and the latter are presumed to be relatively inaccessible to polar compounds. The determining factor in cuticle penetration by herbicides appears to involve the HLB of the compound and its ability to partition from air into the nonpolar wax components and thence into the more polar regions within the basal regions of the cuticle. 

The characteristics of cuticle penetration by nonpolar (lipophilic) chemicals have been investigated in depth by Schonherr and his co-workers, with particular reference to accumulation in plant cuticles, permeation across plant cuticles, and desorption from plant cuticles. ... 

Hamburg and McCall have examined the factors affecting the rate of cuticle penetration of yellow foxtail [Setaria glauca (L.) Beauv.] by a series of aryloxyphenoxypropionate herbicides. They selected this family... 

Their results demonstrated the importance of structure (and adjuvant) in controlling the penetration mechanism. Cuticle penetration by haloxyfop, haloxyfop-methyl, haloxyfop-ethyl, and fluazifop-butyl without COC followed BP kinetics, being impeded by an initial fast penetration stage. With the exception of haloxyfop, the addition of COC to the formulation simplified the penetration kinetics to SP. The BP behavior in the absence of COC may be due to the relatively rapid penetration of Ortho X-77, leaving the polar pyridinyloxy ester on the leaf surface. These polar esters may require the assistance of the surfactant and/or COC to facilitate cuticle penetration. In contrast, the behavior of quizalofop-methyl and diclofop-methyl was SP in the presence or absence of COC. 

The mechanism by which surfactants facilitate cuticle penetration is uncertain. A number of studies involving the use of radiolabeled surfactants have helped to clarify their possible role. Penetration and distribution studies with [ CJ-Tween 20, and [ CJ-Tween showed very limited foliar... 

Uptake of the lipophilic compounds atrazine and DDT, however, was increased by surfactants in inverse relationship to their HLB, suggesting copenetration with the surfactant. The water solubilities of these relatively lipophilic compounds were markedly increased by surfactants, particularly at intermediate HLBs. Stevens and Bukovac concluded that surfactants with short E chains would be surfactants of choice to maximize cuticle penetration of nonpolar a.i., while those with long E chains would be preferable for more hydrophilic a.i. " ... 

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