Phloem translocation of pyridinecarboxylic acids and related imidazolinone herbicides in Ricinus communis

The movement of [C-14]pyridinecarboxylic acids was investigated in the castor bean plant Ricinus communis L. after application by petiole injection. The three isomeric unsubstituted acids appeared in similar concentrations in phloem sap taken from the upper main stem over a period of 6 h after application, but nicotinic acid occurred at higher concentrations than its isomers in phloem sap from the lower stem indicating better retention in the phloem sieve tubes. After 24 h, nicotinic acid had accumulated to a greater extent in the phloem sinks than had picolinic or isonicotinic acids. Phloem transport of substituted analogues, including dihalopyridinecarboxylic acids and imidazolinone herbicides, was measured in short-term tests, and attempts made to relate such transport to the lipophilicity and pK(a) of the compounds. Analogues of the imidazolinone, imazapyr, were less well transported than imazapyr itself, and were also much less well taken up by barley roots from nutrient solution compared to the herbicidal imidazolinone, and this could explain their low herbicidal efficacy despite a high intrinsic activity against the target enzyme acetolactate synthase.