The herbicide aclonifen: The complex theoretical bases of sunflower tolerance

Aclonifen is a diphenylether herbicide in which one ring is unsubstituted and the other is NH₂-1, Cl-2, NO₂-6 substituted. This substitution, especially NH₂-1, is a particular feature in the diphenylether herbicidal family. In contrast with other herbicides of the same family (acifluorfen, oxyfluorfen, bifenox) this compound is not only acting through a phytotoxic protoporphyrin IX accumulation but also through an inhibition of carotenoid biosynthesis. Both biochemical targets are associated to chloroplast structuration in young seedlings under light and the pre-emergence or preplant treatment is consequently an appropriate agronomic strategy. Numerous weeds such as Alopecurus sp., Sinapis sp., Brassica sp., Chenopodium sp. are very sensitive to this herbicide but sunflower culture appears very tolerant. Our results suggest that this tolerance has a complex origin including (1) low root uptake, (2) conjugation in the roots probably with glutathione and (3) low xylem transfer from root to shoot. The low root uptake is associated to aclonifen crystallization on the root surface at high concentrations. The combination of this physico-chemical effect with a sufficient root conjugation rate can explain both the very low amounts of aclonifen or derivatives found in the aerial parts and the strong sunflower tolerance.