VII. XENOBIOTIC TRANSPORT
Understanding of the transport of xenobiotics is essential to the judicious development and use of sys-
temic pesticides. An example of failure to use such information was the program in which solutions of
benomyl (Benlate) were injected into trunks of elm trees in an attempt to control Dutch elm disease. Ben-
late is soluble at the pH of xylem (5.0–6.0) but essentially insoluble at sieve tube pH (~8.0). As a result,
the Benlate was translocated through xylem to leaves, providing some temporary protection to those
structures. However, no protection was provided to any part of the plant supplied by the phloem, i.e., roots
and even newly developed xylem.
The first extensive study of xenobiotic translocation was done by Crafts [304]. He was able to
demonstrate whether a material was transported through the xylem, phloem, neither, or both. It has been
generally assumed that once a xenobiotic has entered the translocation system, it moves passively with
the flow, which is driven by processes independent of the xenobiotic. In addition, the xenobiotic must not
leak from the translocation stream too easily or it will not travel far.
An important mechanism for phloem translocation of xenobiotics is ion trapping of weak acids (Ref.
305, cited in Ref. 306). The basis of ion trapping is as follows: a weak acid is protonated in the lower pH
of the apoplast and ionized in the higher pH of sieve tubes. The protonated form readily penetrates the
lipid portion of membranes. Once in the sieve tube, however, the ionized form would not easily penetrate
the plasmalemma of sieve tubes (i.e., is ion trapped) and, therefore, is carried to the sink with the flow.
Tyree et al. [306] proposed a model for phloem mobility of substances that does not ionize. This
model is based on the idea that there is an optimal membrane permeability for translocation of a xenobi-
440 HENDRIXFigure 2 Theoretical distribution of xenobiotic in sieve tube of a “linearized” plant 0.3 m. Concentration of
xenobiotic in the sieve tube is plotted against distance as a fraction of the concentration in the source leaf
apoplast. Source is assumed to be 0.05 m long. In this calculation, the sieve tube radius times sap velocity, rV,
is 1.5 10 ^9 m sec^1. Curve R is for the optimum permeability, P, of about 2 10 ^9 m sec^1. In curve B, P
is 10 times larger, 2 10 ^8 m sec^1. Dashed line extending from curve A shows how concentration would de-
cline if Vin the root remained constant instead of decreasing. (From Ref. 306.)