The role of waxes in the uptake of metolachlor into sorghum in relation to the protectant CGA 43089

Summary The grass weed herbicide metolachlor (2‐chloro‐N‐[2‐ethyl‐6‐methylphenyl]‐N‐[2‐methoxy‐1‐methylethyl]acetamide) which is especially effective against wild millets, inhibits the formation of epicuticular waxes on sorghum leaves. The metolachlor protectant CGA 43089 [α ‐ (cyanomethoximino) ‐ benzacetonitrile] prevents the depletion of the waxes on the leaves of metolachlor‐treated sorghum plants, as demonstrated by scanning electron microscopy. This alteration of the plant surface polymers also changes their permeability to the herbicide. 14C‐metolachlor uptake into isolated coleoptiles and first leaves of sorghum which had been pretreated with the herbicide was increased. Incubation with added protectant reduced the uptake of 14C‐metolachlor. It is postulated that the modifications caused by metolachlor and its protectant to sorghum surface structures influence the action of the herbicide in two ways: The selectivity observed against sorghum and millet grasses could occur because of an increased uptake of metolachlor through cuticles which are particularly sensitive to the structural changes caused by the herbicide, since the composition of the plant waxes is very species‐specific. The loss of cuticular integrity is prevented by the protectant CGA 43089, which greatly reduces penetration of metolachlor.