Uptake and Transport Following Foliar Application

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. 

The same general principles determine xylem translocation of herbicides whether they are foliage applied or soil applied. Provided that the barrier of cuticular penetration can be largely overcome by appropriate choice of formulation, application to foliage is a more reliable way of getting herbicides into the plant. This route avoids the losses caused by sorption to soil, and it bypasses the endodermal barrier surrounding the vascular tissues in the root. Once through the cuticle and into the leaf tissue. 

For many years, it was assumed that the failure of many compounds to move via phloem was due to their failure to enter phloem sieve tubes. Tyree et then postulated that most compounds can, in fact, enter the phloem quite freely however, only those compounds well retained in phloem are able to move appreciable distances in the phloem sieve tubes. Compounds escaping quickly will be swept upward by the much greater flow of the xylem stream. This elegant concept has become generally accepted and is known as the intermediate permeability theory.  

The efficiency of transport in phloem is now regarded as being determined by two processes. The first of these is the extent to which compounds are accumulated in phloem cells relative to the mesophyll cells, for example, by the ion-trap effect in the case of weak acids (Section 9.2.3.3). The second factor is the extent to which compounds are retained in phloem cells during transport. 

Distribution Patterns within Plants and Their Relationship to Herbicide Properties 

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