Physicochemical factors affecting the uptake by roots and translocation to shoots of amine bases in barley

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, K-ow) and pKa that would include commercial pesticide amines. In a series of six substituted phenethylamines, strong bases with pKa similar to 9.5, all the compounds were strongly taken up by roots from solutions of pH 8.0; uptake declined substantially as the pH was lowered to 5.0, especially for the compounds of intermediate lipophilicity (log k(ow) 2 to 3). This uptake could be ascribed to three processes: (i) accumulation 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 7.4 to 8.0 were also well taken up and translocated whereas the very weak base 4-ethylaniline (pKa 5.03) was much less so. Tests with quaternised pyridines confirmed 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 (log K-ow 4.67) studied, though its long-distance movement would be as the protonated species. (C) 1998 Society of Chemical Industry