Nonionized compounds plant transport

Briggs et examined the translocation of 20 compounds in barley and observed that transport was optimal for compounds of log 1.8 at this point, compounds crossed the membrane barrier at up to about 90% the efficiency of water (i.e., TSCF of 0.9). More polar and more lipophilic compounds were less well transported (Figure 9.5a), despite the fact that the lipophilic compounds were present at high concentrations in the roots. It was concluded that compounds of intermediate lipophilicity are best able to cross plant membranes this relationship has also been observed with animal membranes. Values of TSCF taken from the literature for other nonionized compounds studied in a range of plant species fitted the same Gaussian relationship reasonably well (Figure 9.5b). 

The pathways of uptake and translocation of herbicides by plants appear to be largely controlled by the physicochemical properties of the herbicides. Bromilow and co-workers have produced a simple diagram (Figure 9.8) indicating the properties required by nonionized compounds and weak acids for the various types of transport. It should be noted that the divisions between the categories are somewhat arbitrary and that even... 

All compounds with log less than 4 exhibit at least some movement in the xylem, and nonionized compounds (and probably cations also) of log less than 0 together with weak acids additionally move in phloem. While the accumulation of weak acids in plant compartments such as phloem is an energy-dependent process, there is no good evidence that specific carriers are used in the long-distance transport of current herbicides. 

Kleier, using similar concepts, has produced a computer model that predicts phloem transport of xenobiotics based upon their physicochemical properties. Topp et Ryan et and Boersma et have attempted to assess the uptake by plants from soil of nonionized organic chemicals based upon their physicochemical properties and persistence. From our present understanding of the transport in plants of herbicides and other pesticides, it should be possible to predict approximately the behavior of new or putative herbicidal molecules. With due account taken of the complicating factors such as availability in soil, penetration across leaf cuticles, and metabolism, this approach should be applicable to compounds used on a wide range of crop and weed species in a variety of situations. 

---