The uptake by barley roots from nutrient solution and subsequent transport to shoots of two series of amine bases were measured over 6 to 72 h. The compounds were chosen to span systematically ranges of lipophilicity (assessed using 1-octanol/water partition coefficients, and pKa that would K ow ) include commercial pesticide amines. In a series of six substituted phenethylamines, strong bases with pKa D 9Γ5, all the compounds were strongly taken up by roots from solutions of pH 8Γ0 ; uptake declined substantially as the pH was lowered to 5Γ0, especially for the compounds of intermediate lipophilicity (log 2 to 3). This uptake could be ascribed to three processes : (i) accumulation K ow of the cation inside the root cells due to the negative charge on the plasmalemma, as given by the Nernst equation and important only for the polar compounds which have low permeation rates through membranes ; (ii) accumulation into the vacuole by ion-trapping, which was the dominant process at high pH for all compounds and at all pH values for the compounds of intermediate lipophilicity ; (iii) partitioning on to the root solids, substantial only for the most lipophilic compounds. Translocation to shoots was proportional to uptake by roots, this ratio being independent of external pH for each compound and being optimal for the compounds of intermediate lipophilicity. Such proportionality was also observed in a series of three weaker bases of intermediate lipophilicity, in which compounds of pKa 7Γ4 to 8Γ0 were also well taken up and translocated whereas the very weak base 4-ethylaniline (pKa 5Γ03) was much less so. Tests with quaternised pyridines conΓrmed that organic cations move only slowly through membranes. The observed behaviour of the amines could be modelled reasonably well assuming that transport within the plant was dominated by movement across membranes of the non-ionised species, and this appeared to be true even for the most lipophilic phenethylamine 4Γ67) studied, though (log K ow its long-distance movement would be as the protonated species.
1998 Society ( of Chemical Industry