Polar compounds cuticle penetration

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 importance of the cuticle as a barrier to penetration of herbicides, especially polar compounds, is well documented. Among a range of factors, the physicochemical characteristics of the epicuticular waxes may be of... 

A problem in trying to use these results in developing potential pest control agents is that none of these compounds is likely to penetrate either the cuticle or the CNS of insects effectively, since all are fully ionized at physiological pH. Derivatization of the polar groups would be one possible solution to this problem. The formamidines and imidazolines generally do not suffer from this problem since they have pKas which are lower than those of the related phenylethylamines and thus are at least partly in the free base form at pH 7. 

Penetration into insects is greatly influenced by the manner in which the compound is presented. Classic studies by Treherne (19) indicated that when the toxicant is supplied to detached insect cuticle in aqueous solution penetration decreased with increasing polarity, explicable on the assumption that partition into and passage through the lipoidal epicuticle is the rate determining step. In contrast several studies (20 - 22) have shown that when the toxicant is dissolved... 

Five amides, 50(n=4,6), 55, 5 and 6(n=6), isolated from P. guineense were investigated by Candy and co-workers [10] with respect to their knockdown and lethal activity against adult houseflies, as well as the rates of recovery of M. domestica from the effects of sublethal doses of the amides. As with other insecticides, rapid and potent knockdown activity on topical application of the amides depends, apart from the intrinsic toxicities of the individual compounds, on their ability to penetrate the cuticle and relevant membranes en route to the site of action. Attempts to correlate knockdown activity in the P. guineense amide series with polarity on this basis seemed to show some inconsistencies, perhaps as a result of steric factors [10,102]. The rate of recovery from the effects of sub-lethal doses of insecticides is associated, inter alia, with processes that lead to removal of the toxicant, so that recovery from treatment with Piper amides embodying the methylenedioxyphenyl group, which retards the action of mixed function oxidases, is slower than recovery from the effects of the less stable aliphatic amides. 

The physicochemical properties of compounds affect not only the transport of compounds in plants but also their penetration through the leaf cuticle. This topic is discussed in Chapter 8, and here only brief comment is made on the implications of penetration for subsequent transport. The influence of physicochemical properties on the movement of compounds across cuticles seems to be rather similar to that observed for membranes, in that compounds of intermediate lipophilicity (log Ko values of 1 to 3) appear to penetrate the most rapidly. Uptake of more polar and more lipophilic compounds can, however, be greatly increased by the use of appropriate surfactants. Acidic compounds are taken up relatively slowly, presumably because of poor penetration of the anions in consequence, they are often applied as esters which enter the plant rapidly and are then metabolized to the active acid. Thus, herbicides of a wide range of structures and physical properties are applied to plant foliage the use of surfactants and adjuvants increases the uptake of chemical into the leaf and hence its availability for transport via xylem and phloem but does not otherwise influence transport patterns. 

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