Dintroaniline Herbicide Resistance in Rigid Ryegrass (Lolium rigidum)

Thirteen biotypes of rigid ryegrass were screened for trifluralin resistance. From these, the two most resistant biotypes, SLR 31 and SLR 10, were chosen for further studies involving exposure to other dinitroanilines, mitosis-inhibiting herbicides and 14 C-trifluralin. SLR 31, and SLR 10 exhibited an approximate 10-fold reduced sensitivity to trifluralin in comparison to susceptible biotypes. Resistance to five other dinitroaniline herbicides was observed, with reduced sensitivity varying from 32-fold for ethalfluralin to 2.5-fold for isopropalin. The resistance in rigid ryegrass to other herbicides and drugs that affect mitosis were tested. Resistance comparable to that of trifluralin was recorded for the herbicides terbutol and DCPA, while low levels of cross-resistance to amiprophosmethyl was present. Trifluralin affected mitotic indices at a much lower level in the susceptible biotypes than in the resistant biotypes. No differences in the uptake and translocation of 14 C-trifluralin were observed between resistant and susceptible biotypes. Most of the 14 C detected in the plant material was in the root tissue. A small level of 14 C was detected in the seeds, and no substantial increases were noted in coleoptile tissue. The resistance spectra in SLR 31 and SLR 10 were phenotypically similar to those occurring in an intermediate trifluralin-resistant goosegrass and trifluralin-resistant green foxtail.