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 in jection. The three isomeric unsubstituted acids appeared in similar co ncentrations in phloem sap taken from the upper main stem over a perio d of 6 h after application, but nicotinic acid occurred at higher conc entrations than its isomers in phloem sap from the lower stem indicati ng better retention in the phloem sieve tubes. After 24 h, nicotinic a cid had accumulated to a greater extent in the phloem sinks than had p icolinic or isonicotinic acids. Phloem transport of substituted analog ues, including dihalopyridinecarboxylic acids and imidazolinone herbic ides, was measured in short-term tests, and attempts made to relate su ch transport to the lipophilicity and pK(a) of the compounds. Analogue s of the imidazolinone, imazapyr, were less well transported than imaz apyr itself, and were also much less well taken up by barley roots fro m 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.