IDENTIFICATION OF 2 MECHANISMS OF SULFONYLUREA RESISTANCE WITHIN ONE POPULATION OF RIGID RYEGRASS (LOLIUM-RIGIDUM) USING A SELECTIVE GERMINATION MEDIUM

A biotype of rigid ryegrass (Lolium rigidum Gaudin biotype VLR69) resi stant to some ALS inhibitors was characterized to determine the mechan isms of resistance to the sulfonylurea herbicides. The biotype had a h igh level of resistance to chlorsulfuron (20x) and triasulfuron (25x), and an intermediate level of resistance to imazaquin (7x) and sulfome turon (7.5x) but exhibited a low level of resistance to imazapyr (2.5x ). At 60 to 90 g ai ha-1 sulfometuron, 4% of the population survived w ithout apparent herbicidal effect. The same response to sulfometuron w as also observed when seeds of the resistant biotype VLR69 were germin ated on agar containing sulfometuron. At 27 nM sulfometuron, 4% of the seeds germinated and grew normally while the growth of the bulk of th e population was retarded. This differential response to sulfometuron at the seedling stage allowed selection of sulfometuron-resistant indi viduals from the population. Activity of ALS extracted from these sulf ometuron-resistant plants was less sensitive to inhibition by chlorsul furon than ALS extracted from plants considered sulfometuron susceptib le in the same system. Unselected plants from the VLR69 population wer e able to detoxify chlorsulfuron more rapidly than susceptible VLR1 pl ants. It is apparent that there are at least two mechanisms of sulfony lurea resistance in the resistant biotype VLR69 which occur at differe nt frequencies within the population. These data show that more than o ne mechanism or resistance may develop in response to herbicide select ion pressure and that the resulting populations are not necessarily ho mogeneous.